UNIT I

INTRODUCTION

For Many Years Electronic Instruments Have Been EasilyIdentifiEd Products. Although They Ranged In
Size And Functionality, They All Tended To Be BoxShaped ObjectsWith A Control Panel And A Display.
StandAlone Electronic Instruments Are Very Powerful, Expensive And Designed ToPerform One Or More
SpecifiC Tasks DefiNed By The Vendor. However, The User Generally Cannot Extend Or Customize Them.
The Knobs And Buttons On The Instrument, The BuiltIn Circuitry, And The Functions Available To The
User, All Of These Are SpecifiC To The Nature Of The Instrument. In Addition, Special Technology And
Costly Components Must Be Developed To Build These Instruments, Making Them Very Expensive And
Hard To Adapt. Widespread Adoption Of The Pc Over The Past Twenty Years Has Given Rise To A New
Way For Scientists And EngiNeers To Measure And Automate The World Around Them. One Major
Development Resulting From The Ubiquity Of The Pc Is The Concept Of Virtual Instrumentation. A Virtual
Instrument Consists Of An IndustryStandard Computer Or Workstation Equipped With O_TheShelf
Application SoftWare, Cost EffEctive Hardware Such As PlugIn Boards, And Driver Software| Which
Together Perform The Functions Of Traditional Instruments. Today Virtual Instrumentation Is Coming Of
Age, With Engineers And Scientists Using Virtual Instruments In Literally Hundreds Of Thousands Of
AppliCations Around The Globe, Resulting In Faster Application Development, Higher Quality Products
And Lower Costs.

Virtual Instruments Represent A Fundamental Shift From Traditional Hardware Centred Instrumentation
SysTems Towards Software Centred Systems That Exploit The Computing Power, Productivity, Display
And Connectivity Capabilities Of Popular Desktop Computers And Work staTions. Although Pc And
Integrated Circuit Technologies ExpeRiencedSignifiCant Advances In The Past Two Decades, It Is
measurement data. For others, a virtual instrument is a computer equipped with software for a variety of
uses including drivers for various peripherals, as well as analogue to digital and digital to analogue
converters, representing an alternative to expensive conventional instruments with analogue displays and
electronics. Both views are more or less correct. Acquisition of data by a computer can be achieved in
various ways and for this reason the understanding of the architecture of the measuring instrument becomes
important. A virtual instrument can be defined as an integration of sensors by a PC equipped with specific
data acquisition hardware and software to permit measurement data acquisition, processing and display. A
virtual instrument can replace the traditional front panel equipped with buttons and display by a virtual front

panel on a PC monitor.

Computer and Display

The computer and the display are the heart of virtual instrument systems. These systems are typically based
on a personal computer or workstation with a high resolution monitor, a keyboard, and a mouse. It is
important for the chosen computer to meet the system requirements specified by the instrumentation
software packages. Rapid technological advancements of PC technology have greatly enhanced virtual
instrumentation. Moving rom DOS to Windows gave to PC users the graphical user interface and made 32bit
software available for building virtual instruments. The advances in processor performance supplied the
power needed to bring new applications within the scope of virtual instrumentation. Faster bus architectures
(such as PCI) have eliminated the traditional data transfer bottleneck of older buses(ISA). The future of
virtual instrumentation is tightly coupled with PC technology.

3.2 Software

If the computer is the heart of the virtual instrument systems, the software is their brain. The software
uniquely denes the functionality and personality of the virtual instrument system. Most software is designed
to run on industry standard operating systems on personal computers and workstations. Software
implemented can be divided into several levels, which can be described in a hierarchical order.

Register level software

Register level software requires the knowledge of inner register structure of the device (DAQ board, RS 232
instrument, GPIB instrument or VXI module) for entering the bit combination taken from the instruction
manual in order to program measurement functions of the device. It is the hardest way in programming. The
resulting program is strongly hardware dependent and it is rarely executable on systems with different
hardware. area (panel). Logic low is easily established with a point and drag action list. Test Point takes
advantage of every Microsoft Windows features. Measurement Studio | is a measurement tool for data
acquisition, analysis, visualization and Internet connectivity. This development tool helps you build your test
system by integrating into your existing Microsoft compiler. Measurement Studio provides a collection of
controls and classes designed for building virtual instrumentation systems inside Visual Basic or Visual C++.
With Measurement Studio you can configure plugin data acquisition boards, GPIB instruments, and serial
devices from property pages without writing any code. With user interface components you can conure real
time 2Dand 3D graphs, knobs, meters, gauges, dials, tanks, thermometers, binary switches, and LEDs. With
powerful Internet components, you can share live measurement data among applications via the Internet.

SCPI | Standard commands for programmable Instruments

SCPI is not a software tool as are former systems, but it is an elective aid enabling easy standardized control
of programmable instruments. SCPI decreases development time and increases a readability of test
programs. SCPI provides an easy understandable command set, guarantees a well defined instrument
behavior under all conditions, which prevents unexpected instrument behavior. Although IEEE 488.2 is used
as basis of SCPI, it defines programming commands that we can use with any type of hardware or
communication link. It has an open structure. The SCPI Consortium continues in adding commands and
functionality to the SCPI standard .Real time and embedded control has been long the domain of specialized
programs. Advances in industry standard technologies including more reliable operating systems, more
powerful processors and computer based real time engineering tools are introducing new levels of control
and determinism to virtual instrumentation. This presents new opportunities for scientists to take on
increasingly sophisticated real time and embedded development. Software scales across development on
the PC into development in real time and embedded applications. Scientists and engineers can move into
new application areas without a steep learning curve because the software itself evolves to incorporate
emerging computer technologies.

Interconnect Buses

Four types of interconnect buses dominate the industry: the serial connection (serial port), the GPIB, the PC
bus and VXI bus. Serial port. Serial communication based on RS232standard is the simplest way of using a
computer in measurement applications and control of instruments. Serial communication is readily available
via the serial port of any PC and it is limited in data transmission rate and distance (up to 19:2 Kbytes/sec,
recently 115 Kbytes/sec,

known as mainframes". Mainframes include power supplies, air cooling equipment and backplane
communication for the modules. The VXI bus is unique in that it combines a computer backplane based on
the VME bus for high speed communication and users a quality EMC environment that allows high
performance instrumentation similar to that found in GPIB. As a result, much more compact measuring
systems can be built. There are three ways to communicate between the computer and the VXI bus
instruments.

a) The rest method is by using GPIB. In this case, a GPIB to VX I bus converter module is plugged into the
VXI us mainframe and a standard interface cable connects it and the GPIB interface card in the computer.
The advantages and disadvantages of this technique are very similar to a pure GPIB design. This system
tends to be easy to program, but data speeds are limited to GPIB speeds. However, because the internal
data speeds within the VXI bus mainframe can exceed10 Mbytes/s, often a high speed application is solved
by local high speed acquisition and processing occurring within the mainframe and high level results transfer
to the computer over GPIB. Figure 5 shows an example of VXI bus system using GPIB.

b) The second technique is to use a higher speed interconnect bus between the VXI bus mainframe and the
computer. The most common implementation of this is high speed exible cable interface known as MXI bus
.As in GPIB, an MXI bus interface card and software are installed on the computer and a cable attaches it to
an MXI bus to VXI bus converter module in the VXI bus mainframe. MXI bus is essentially an implementation
of the VXI bus on aexible cable. This means that the

to the use of such systems. As in the case of large and complex plants, a structured networked
measurement system can be adopted by scaling its use to the geographical area. The geographical process
to be monitored and controlled is partitioned into cells that can be dealt with by a single processing unit or a
group of locally connected units.

Geographically distributed units are connected by a geographical computer network into a distributed
measurement system. In this case communication delays usually cannot be neglected. This is even more
relevant if the traffic in the computer network is not negligible due to the number of computers connected and
the amount of communications, especially if a public computer network is used to realize the
interconnections among the measuring processing units. It seems that in the near future local network
(LAN)can be considered as a kind of measurement bus, from the viewpoint of measurement and control
systems. Atypical example of such a system including various virtual instruments is presented in Fig. 8. It
can be considered as arst step to a wider, Internet based technology .In the last few years a surprisingly
rapid growth of fast and reliable communication networks has allowed an easy interchange of information
and commands between computers both connected to local networks and connected to faraway site of wide
area networks (WAN), such asthe Internet. Thus, network services and programmable instrumentation now
permit the development of measurement laboratories distributed on a wide geographical area and
simultaneously available to several users variously located in the territory.

Common Internet based software can be used to provide easy data migration between various
communication to the emergence of virtual instruments, which are substantially different from their physical
ancestors. Virtual instruments are manifested in different forms ranging from graphical instrument panels to
complete instrument systems. Modular instrumentation building blocks are becoming more prevalent in the
industry and are allowing users to develop capabilities unattainable using traditional instrument architectures.
Despite these changes however, the measurement paradigm remains unaltered. This might be the proper
platform for the new development. The trend in virtual instrumentation increasingly integrates the
measurement systems into more complex monitoring and control systems distributed over different (possibly
geographically distant) locations. The remote instrumentation control is becoming popular since the networks
have become reliable and worldwide and almost every new instrument embeds programmable capabilities.
The past has shown that unless proper standards are available, diversification due to adhoc solutions will
slow the progress in the field. Thus, it seems a proper challenge for the future to start thinking of
standardization of virtual instrumentation and distributed measurement systems.

THE CONCEPTION OFVIRTUAL INSTRUMENT

Usually instrumentation manufacturers provide specific functions to given architecture and fixed interfaces
for measuring devices, and thus limit the application domain of these devices. In actual use much time is
required for adjusting the measuring range and for saving and documenting the results.

The advent of microprocessors in the measurement and instrumentation fields produced rapid modifications
of measuring device technology, soon followed by the appearance of computer based measurement
techniques.

Conceptual model of early computerized instrumentation is given in Fig. 1.A single user controls the system,
which runs exclusively on a piece of hardware. There is a single control structure, which is formed by the
combination of the user and the program that controls the multiple devices attached to the instrumentation
bus. The main challenges are the device coupling and the programming models.

The measurement consists of three parts, as shown in Fig. 2, acquisition of measurement data or signals,
conditioning and processing of analysis of measurement signals and presentation of data. The concept of
virtual instrument is frequently used in industrial measurement practice, but not always with precisely the
same meaning. For some people, virtual instruments are based on standard computers and represent
systems for storage, processing and presentation of means of integration of the display, control and
centralization of complex measurement systems.

Industrial instrumentation applications, however, require high rates, long distances, and multi vendor
instrument connectivity based on open industrial network protocols. In order to construct a virtual instrument
it is necessary to combine the hardware and software element is which should perform data acquisition and
control, data processing and data presentation in a different way to take maximum advantage of the PC. It
seems that in the future the restrictions of instruments will move more and more from hardware. Such a
general conception of virtual instrumentation is presented in Fig. 3.The vendor of virtual instrument can use
the serial communication based on RS232 standard or the parallel communication based on GPIB standard
(known also as HPIB, IEEE 488.12 or IEC 625.12), PC bus, or VXI bus (VME extension for Instrumentation).
The main categories of virtual instruments:
a) Graphical front panel on the computer screen to control the modules or instruments
a1) controlled module is plug in DAQ board,
a2) controlled instrument is based on GPIB board,
a3) controlled instrument is connected via serial port,
a4) controlled instrument is VXI board (or system).
b) Graphical front panel with no physical instruments at all connected to the computer. Instead, the computer
acquires and analyses the data from _les or from other computers on a network, or it may even calculate its
data mathematically to simulate a physical process or event rather than acquiring actual real world data.
To the PC connections according to point a) the following process measuring devices are attached:
1. Sensors
2.GPIB instruments
3. Serial instruments
4.VXI instruments
This structure is a result of international standardization allowing more freedom in using boards and
instruments from various manufactures. The main representative features of virtual instruments describing
their functionality are following:
1. Enhancing traditional instrument functionality with computers;
2.Opening the architecture of instruments;
3.Widespread recognition and adoption of virtual instrument software development frameworks.

BASIC COMPONENTS OF VIRTUAL INSTRUMENTS

The basic components of all virtual instruments include a computer and a display, the virtual instrument
software, a bus structure (that connects the computer with the instrument hardware) and the instrument
hardware.
Driver level software

One of the most important components in measurement systems today is the device driver software. Device
drivers perform the actual communication and control of the instrument hardware in the system. They
provided medium level easy to use programming model that enables complete access to complex
measurement capabilities of the instrument In the past programmers spent a significant amount of time
writing this software from scratch for each instrument of the system. Today, instrument drivers are delivered
as modular, other shelf components to be used in application programs. Several leading companies
formed(in 1988) the Interchangeable Virtual Instrument (IVI) Foundation. The IVI Foundation was formed to
establish formal standards for instrument drivers and to address the limitations of the former approaches.

High-level tool software

Currently the most popular way of programming is based on the high level tool software. With easy to use
integrated development tools, design engineers can quickly create, configure and display measurements in a
user friendly form, during product design, and verification. The most known, popular tools are as follows:_
Lab VIEW (Laboratory Virtual Instrument Engineering Workbench) | is a highly productive graphical pro
gamming language for building data acquisition and instrumentation systems. To specify the system
functionality one intuitively assembles block diagrams | a natural design notation for engineers. Its tight
integration with measurement hardware facilities rapid development of data acquisition, analysis and
presentation of solutions. Lab Windows /CVI (C for Virtual Instrumentation) |is a Windows based, interactive
ANSI C programming environment designed for building virtual instrumentation applications. It delivers a
draganddrop editor for building user interfaces, a complete ANSI C environment for building test program
logic, and a collection of automated code generation tools, as well as utilities for building automated test
systems and monitoring applications of laboratory experiments. The main power of CVI lies in
the set of libraries. HP VEE (Hewlett-Packard's Visual Engineering Environment) | allows graphical
programming for instrumentation applications. It is a kind of Visual Engineering Environment, an iconic
programming language for solving engineering problems. It also provides an opportunity to gather, analyze
and display data without conventional(text based) programming .Test Point | is a Windows based object
oriented software package that contains extensive GPIB instrument and DAQ board support. It contains a
novel state of the art user interface that is easy to use. Objects, called\stocks" are selected and dragged with
a mouse to a work and 15 m) and it allows only one device to be connected to a PC.GPIB. It was the rest
industry standard bus for connecting computers with instrumentation. A major advantage of GPIB is that the
interface can be embedded on there are of a standard instrument. This allows dual use of the instrument: as
a standalone manual instrument or as a computer controlled instrument. Because of this feature, there are a
wide variety of high performance GPIB instruments to choose from. The GPIB operas exiblecable that
connects a GPIB interface card in the computer to up to 15 instruments over a distance of up to twenty
meters. The interface card comes with software that allows transmission of commands to an instrument and
reading of results. Each GPIB instrument comes with a documented list of commands for initiating each
function. Typically, there is no additional software delivered with the instrument. GPIB has a maximum data
rate of 1 Mbytes/s and typical data transfers are between 100and 250 Kbytes/s. It depends on the response
of the measured subject. PC bus. With the rapid acceptance of the IBM personal computer in test and
measurement applications, there has been a corresponding growth of plug in instrumentation cards that are
inserted into spare slots. However, high accuracy instruments require significant circuit board space to
achieve their intended precision. Because of the limited printed circuit board space and close proximity to
sources of electromagnetic interference, PC
bus instruments tend to be of lower performance than GPIB instruments but also of lower cost. Many are
simple ADCs, DACs, and digital I/O cards. PC bus instrumentation is best suited for creating small,
inexpensive acquisition systems where the performance is not of paramount importance. Since these cards
plug directly into the computer backplane and contain no embedded command interpreter as found in GPIB
instruments, personal computer plug in cards are nearly always delivered with driver software so that they
can be operated from a personal computer. This software may or may not be compatible with other virtual
instrument software packages, so it is recommended to check with the vendors beforehand. Most data
acquisition boards are multifunctional, ie they accept both analogue and digital signals. These plug in data
acquisition boards gain wider and wider acceptance due to their low price and high edibility obtained from the
associated software.

VXI bus. In the late eighties, the VME extension for Instrumentation (VXI) standard allowed communication
among units with transfer over 20 Mbytes/second between VXI systems. VXI instruments are installed in
arrack and are controlled by, and communicate directly with, a VXI computer. These VXI instruments do not
have buttons or switches for direct local control and do not have local display typical in traditional
instruments.

It is an open system instrument architecture that combines many of the advantages of GPIB and computer
backplane buses. VXI bus instruments are plug in modules that are inserted into specially designed card

cages

conversions to VXI bus are simple and fast, bringing MXI bus performance within a factor of 2 or so of native
VXI bus speeds. The advantage of MXI bus is that it allows the use of the shelf computers to communicate
with VXI bus instruments at a speed consider ably higher than GPIB. A disadvantage is that the MXI bus
cable can be thick and unwieldy, and there is some loss of data transfer bandwidth due to the con version.
Figure 6 shows an example VXI bus system using MXI bus.

c) The third way is to insert powerful VXI bus computers directly into the VXI bus mainframe. VXI bus
computers tend to be repackaged versions of industry standard personal computers and workstations that
run industry standard operating systems and software. The advantage of this technique is that it preserves
the full communications performance of VXI bus. The disadvantage is that the choice of VXI bus computers
will always be a subset of the choice of standard industry computers. VXI bus computer technology will
typically lag behind the performance of the industry as a whole, offer fewer alternative configurations and be
priced at a premium due to its lower volume. Figure 7 shows an example VXI bus system using an
embedded computer.

3.4 Instrument Hardware The preceding subsection on interfaces also touches on the attributes found in
each of the respective instrument hardware products. One note is worth to be repeated: Virtual
instrumentation never eliminates the instrument hardware completely. To measure the real world there will
always be some sort of measurement hardware, sensor, transducer and conditioning circuit, but the physical
form factor of this instrumentation may continue to evolve.
DISTRIBUTED MEASUREMENT SYSTEMS

The present trend in interconnected measurement systems is to extend the area covered by the
interconnected systems in the geographical scale. This sets a further limit pathways. Multicomputer
processing systems are effective in creating complex systems by overcoming limitations of a single computer
concerned with the overall computing power or the number of signals to be acquired and processed.
Standard software languages such as C and Java can be used with other shelf development tools to
implement the embedded network node applications and the web based applications respectively. Internet
based TCP/I P protocols, Ethernet technology and/or Data Sockets can be used to design the networking
infrastructure, Fig. 9.DataSocket is a software technology for Windows that makes sharing all measurements
across a network (remote Web and FTP sites) as easy as writing information to able. It uses URLs to
address data by the same way we use URL in a Web browser to specify Web pages. Data Socket included
with any software tool is ideal when someone wishes to complete control over the distribution of the
measurement but does not want to learn the intri cacies of the TCP/IP data transfer protocols.
In all types of networked and distributed measurement systems presented above, real time operation and
condistributed measurement system one can take remote measurements, distribute a program's execution,
or publish measurement data over the Internet. The evolved hardware and software technologies provide
users with the tools they need for easy building of a powerful distributed system. By publishing your
measurement or automation application over the Internet real time data can be viewed by users on remote
computers. With application development environments Web servers are available so you can publish a user
interface to the Internet. Without any additional programming you can publish your front panel as a Web
page so users across the Internet can view these panels running within any standard Web browser.
Applications have one or more measurement nodes physically separated from the computer that is
controlling them and collecting data. Remote measurement applications often require high speed streaming
of data and several clients connected to a single measurement. For streaming measurement data across a
network Data Socket provides you with an easy to use interface. Using Data Socket you can easily stream
any kind of measurement data across a local area network or the Internet to several client programs. Both
Web servers and Data Socket provide a simple and convenient way to publish your measurement data.

UNIT II
DATA ACQUISTITION IN VI

DATA ACQUISITION BOARD AND CONFIGURATION SOFTWARE

The ATMIO16DE10 DAQ board utilized belongs to the family of enhanced multiple function
input/output boards [8, 9]. It has digital as well as analog input/output capabilities. It has
a total of 16 analog input channels, which can be used in, single ended and differential modes, all
software selectable. All 16 channels can be used in single ended mode only if all the signals have
a common ground. Otherwise, only eight analog input channels are available. In the differential
mode, input voltage range is 0v to 1v. It has two twelve bit analog output channels, whose output
range is 5v to +5v or from 0v to 10v, software selectable, with a current of 5mA to +5mA. The
sampling rate is 100 k samples/sec. There are, a total of thirty two digital input/output channels.
It is also equipped with 24bit, 20 MHz counter/timers. Digital channels are compatible with
TTL/CMOS. The hardware configuration of the DAQ board was studied and the required
channels were configured for this application. The corresponding gains and input voltages were
selected.
SIGNAL CONDITIONER (SCX)

Signal conditioning is necessary to step down the voltages or to shield the signals from
noise and distortion. The actual voltage and current measured have to be stepped down and
isolated to protect the board. The circuit diagram of the conditioning circuitry, thus developed is
shown in Fig. 2. The maximum generated voltage was 125v. The voltage divider was used to
obtain 1v across the 440k resistor. As the generator voltage varies from 0 to 125v, the voltage
across 440k varies from 0v to 1v. The two 63mA fuses protect the board against any large
currents due to high voltage. Without the availability of other current sensing devices, in order to
get a reference voltage for the current, a variable resistor was used in series with the load. The
resistor was

adjusted to yield a 1v drop across it when maximum load current flows in the generator circuit.
Two fuses protect the board against excessive currents.

Motor speed was sensed using a photo tachometer, whose output is a proportional analog
signal (0 1 V) for interfacing directly to the NIDAQ board. The motor torque was measured
using a Hampden load cell, with its analog output voltage used for the interface. Finally, before
connecting the signal conditioning hardware to the board, the voltage at the output of the signal
conditioning board was checked to make sure that they are less than one volt over the complete
range of input signals.

Lab VIEW STUDENT EDITION

Lab VIEW is an abbreviation for Laboratory Virtual Instrument (VI) Engineering

Workbench. It is a powerful instrumentation and analysis software system that runs on PCs.

Lab VIEW was first developed in 1983 by National Instruments, since then it has become a
standard in the program development applications much like C or BASIC development systems.
However, Lab VIEW departs from the sequential nature of traditional programming languages
and features easy to use graphical programming environment called "G" [1014].

Lab VIEW allows design of the front panel or user interface and the block diagram or
graphical code. The front panel is interactive because it simulates the panel of a physical
instrument. It can contain indicators and controls such as graphs, charts, switches, numeric
displays, control knobs and various other kind of controls. The indicators are program outputs
whereas controls are user inputs. User input is through the keyboard and mouse while program
output is displayed on the screen of the monitor.

The block diagram or graphical code constructed in the LabVIEW's graphical

programming language, ‗G‘ is the source code of the VI‘s. It is the actual executable program.
The block diagram contains lower level VI‘s, built in functions, program execution control
structures and constants. Wires are drawn to connect the required objects together, indicating the
flow of data in a block diagram. Every object on the front panel has a corresponding terminal on
the block diagram that facilitates the flow of data between the user and the program.
In windows, when the LabVIEW icon is opened, the tools to build the front panel of the
VI are available through the menu bar. The tool bar can be changed into run mode palette or edit
mode palette by clicking on the run/edit mode icon. Some of the helpful features of the front
panel and the block diagram of LabVIEW are:
A click on the run button enables the VI to run and that on the stop button enables its stop.
The run button is broken if the VI is not compiled correctly or due to an illegal connection. This
prevents making serious mistakes and wasting time on debugging later on.
The execution highlighting button causes the VI to highlight the flow of data as it passes
through the diagram. When execution highlighting is on, the intermediate data values that would
otherwise not appear can even be seen.
Operating tool is used to change the values in controls, click on/off buttons and knobs.
Positioning tool is used to select, delete, move, and resize objects etc.
Wiring tool is used only in the block diagram. It is used to wire terminals together.
The coloring tool enables selection of several colors to the front panel or block diagram.

FRONT PANEL

Familiarity with the different functions and menus of LabVIEW is necessary before
beginning to build the front panel. As per the initial objective of the project, three parametersmotor
speed, generator voltage and current had to be displayed as graphs and as numeric displays. The
following procedure was used to devise the front panel and the block diagram. Using the
positioning tool, with click on the right mouse button and from the pop up window selections of the
choice of five different types of graphs, a Waveform Chart was chosen for the real time display of
data. Once the chart was on the panel, it was duplicated to have a total of three graphs on the front
panel. Numeric displays of the variables were set up from the pop up menu on the respective
graphs. The data acquisition system requires two knobs to control the acquisition rate and the
number of samples per second. The program execution to acquire the data from the system in
operation is controlled by an on/off switch. An optional LED serves for visual indication of on/off
status. The completed preliminary front panel of the project is shown in the Fig. 3.

BLOCK DIAGRAM

The block diagram is the screen where the source code for the VI is developed. In the
block diagram, there is a terminal for every object created in the front panel. First the data has to
be read in a continuous manner or in real time. Data input is at three different channels of the
DAQ board, so a channel number is specified for each parameter. Once acquired, the data is in
the range of 0 to 1v. So scaling has to be done to bring back data to their original scale before
displaying the signals on the graph. Also each channel has to adopt a different scaling factor as
variables‘ ranges are different.
In the block diagram environment from the pop up menu for functions, if the Data acquisition
option is selected, it in turn opens another window showing several analog and digital input output
VI's. There are four VI's for analog input. Two for sampling channel‘s and two for acquiring
waveforms. Due to entirely different variables, AI (Analog input) Acquire Waveforms(the third
option) option was chosen for individual channels so as to treat them individually and condition
them before displaying on the graphs. The eight terminals of the chosen VI icon have to be
configured in order to attribute meaningful data acquisition and display. Three of the waveform
acquire VI‘s were used on the block diagram, one for each parameter to be read. In order for the VI
to function, it needs the required values for each terminal. Since the differential connections at the
DAQ board input were only used, it was not required to use high limit and low limit terminals on
the VI. The terminals for sampling rate and number of samples are to be varied at the same time for
all the three inputs. Thus these are connected to the terminals of two control knobs. The two control
knobs are the ones selected on the front panel. The device and channel number entries correspond to
1 and 0 in this particular case. This readies the VI for further processing and display. The same
process is repeated with the other two AI Acquire Waveform VI‘s, with the only difference being
the channel numbers.
To view the displayed waveforms in actual ranges, these signals require appropriate calibration. The
latter is achieved by using the arithmetic or logic function features. For the chosen channels, from
the mathematics option, the multiplication function was selected to provide the required
multiplication factor. This factor can be fine tuned by calibration with a physical instrument, for
greater accuracy. With the system now capable of acquiring data, rescale it and display it on the
graph, it however, is not set up to carry out this task in a continuous manner. In order to display the
waveforms in real time, the system must function continuously. To accomplish this, once again
from the pop up functions menu, selection of the Struts & Constants option allows selection of the
while loop as one appropriate for this system. A while loop executes the program or diagram inside
it as long as the Boolean value wired to its conditional terminal is true. It checks the conditional
terminal value at the end of each iteration. If the value is true, another iteration occurs. The whole
diagram is enclosed inside the loop and the conditional terminal of the loop is connected to the
terminal of the Boolean switch as shown in the system block diagram in Fig. 4. This is the switch
on the front panel that turns the whole system on or off and which gives control to the user. The
execution highlighting function enables debugging and removing bad connections. The answers to
some of the questions that arose when configuring the NIDAQ were found by browsing the
National Instruments Internet site. Now the system was ready for real time data acquisition and
instrumentation. The front panel and block diagram screens can be flipped back and forth by
pressing < ctrl > and < f > key simultaneously or by selecting ' show panel / Show window ' option
from ' windows ' in the top menu bar. This feature provides immediate information on the display
configuration and the programmed block diagram.

To add to the capabilities of the system, it was desired to save the acquired data as an Excel file for
further manipulation and hard copy. This is also very useful if the characteristic curves of the
generator are to be drawn later. The Write to Spread Sheet File VI enabled this feature. The details
of setting up the different inputs of this VI can be found in reference [5]. Ina later step, the
procedure to automatically obtain the hardcopy of the data screen was achieved. As a further step in
the development, the front panel and the block diagram of the Lab VIEW were both extended to
incorporate the display of the motor torque variable and also the display of motor torque vs. the
speed and the generator terminal voltage vs. the load current. The latter, accomplished with
additional programming is illustrated in the block diagram Fig. 5. The experiment section contains
the display of the front panel devised for this block diagram.
EXPERIMENTS ON MOTORGENERATOR SETUP

In order to run the system, in the Lab VIEW environment, from the file menu the VI that
needs to be run is opened. With the motor generator set off, the on/off switch is enabled to turn
on the VI. On the top tool bar, the run button is clicked ON. The LED next to the on/off button
comes ON indicating that the system is on. The motor is turned on and brought to its normal
speed of 1800 rpm. The RPM vs. time graph as well as the digital display as shown in Fig. 6.
start responding to the increasing speed of the motor. Then the generator field is supplied and an
observation of the generated voltage vs. time graph and the digital display is made. For the safety
of the computer and board, the output of the SCX board with generator at rated voltage is
checked to make sure the voltage at the board terminals is not more than 1 volt.

A resistive load is connected across the generator terminals. With increasing steps of
load, the current display similarly shows a graph of the load current vs. time and also its digital
display. The four graphs can be seen on one screen in real time, and change in one parameter
clearly shows the effect on others.

Detailed study of the graphs can be done once the VI is turned off. This is achieved by
turning off the on/off push button on the front panel. Using the tools on the tool bar, each graph
can be zoomed in or zoomed out, rewound to look at previous values etc. The data saved on
spreadsheet files can be viewed in Excel or any other spreadsheet environment.
SCOPE OF FUTURE WORK

Partial results of this project served a senior undergraduate student fulfils the requirements of his
EET degree in winter 1997. The project as developed was utilized as a demonstration laboratory in
the Division‘s electric machines and drives course in the winter and fall semesters in 1997. It also
has been used in three occasions as a display during Engineering Open house and Metro Detroit
area public school visits, to highlight the scope of computer use in practical systems. The latter part
of the development is currently being utilized to observe and analyze the performance of a machine
tool system. The monitoring and display of the lathe motor torque as a function of the depth of cut
of the tool is the main objective of the system. The testing of this system is awaiting the torque
monitor fixture setup. The project is further to include the signals from a load monitoring system,
for which separate front panel and block diagrams are being devised. This is part of a NSF funded
Greenfield Coalition‘s Manufacturing Engineering curriculum development project for which the
first author is the principal investigator. The future plans for the development include i) devising the
Lab VIEW based instrumentation system as a real time visual controller rather than only as data
indicator ii) to serve as an example while seeking external funding for the electric machines and
drives laboratory future expansion to several stations iii) to continue to serve as a demonstration set
up in other activities of the Division, iv) to serve as an a station for any measurement and control
system
CONCLUSION

An instrumentation project utilizing the student edition of National Instruments LabVIEW and data
acquisition tools NIDAQ is presented. Pertinent details about the methodology and the
configuration of hardware and the front panel are provided. Efficient usage of graphic programming
capabilities of LabVIEW are outlined. Screens showing the set up of the display of several
parameters of the motor generator system are enclosed along with the actual screens of real time
instrumentation. Procedures developed to save the acquired data as XL files are described. Current
utilization of the project and future scope of the project are provided.

UNIT –III

COMMUNICATION NETWORKED MODULES

INTRODUCTION

These days, practically every business, no matter how small uses computers to handle various
transactions and as business grows, they often need several people to input and process data
simultaneously and in order to achieve this, the earlier model of a single computer serving all the
organisations computational needs has been replaced by a model in which a number of separate but
interconnected computers do the job and this model is known as a Computer Netw9rk. By linking
individual computers over a network their productivity has been increased enormously. A most
distinguishing characteristic of a general computer network is that data can enter or leave at any
point and can be processed at any workstation. For example: A printer can be controlled from any
word processor at any computer on the network. This is an introductory unit where, you will be
learning about the basic concepts regarding Computer Networks. Here, you will learn about
Networks, different types of Networks, their applications, Network topology, Network protocols,
OSI Reference Mode!, TCP/IP Reference Model. We shall also examine some of the popular
computer networks like Novell network, ARPANET, Internet, and ATM networks. Towards t4e end
of this unit the concept of Delays in computer networks is also discussed.

OBJECTIVES

After going through this unit, you should be able to: understand the concept of computer networks
differentiate between different types of computer networks understand the different application of
networks compare the different network topologies signify the importance of network protocols
know the importance of using networked system understand the layered organization and
structuring of computer networks using OSI and TCPIIP reference model have a broad idea about
some of the popular networks like Novell network, ARPANET, INTERNET, ATM etc., and
understand the concept of delays.
3.0 MAIN CONTENT
What is a computer network?

Figure 1: A computer networked environment

A Computer network consists of two or more autonomous computers that are linked (connected)
together in order to: Share resources (files, printers, modems, fax machines). Share Application
software like MS Office. Allow Electronic communication. Increase productivity (makes it easier to
share data amongstusers). Figure 1 shows people working in a networked environment. The
Computers on a network may be linked through Cables, telephones lines, radio waves, satellites etc.
A Computer network includes, the network operating system in the client and server machines, the
cables, which connect different computers and all supporting hardware ill between such as bridges,
routers and switches. In wireless systems, antennas and towers are also part of the network.
Computer networks are generally classified according to their structure and the area they are
localised in as:

Local Area Network (LAN): The network that spans a relatively small area that is, in the single
building or campus is known as LAN.

Metropolitan Area Network (MAN): The type of computer network that is, designed for a city or
town is known as MAN.

Wide Area Network (WAN): A network that covers a large geographical area and covers different
cities, states and sometimes even countries, is known as WAN. The additional characteristics that
are also used to categorise different types of networks are:

Topology: Topology is the graphical arrangement of computer systems in a network. Common

topologies include a bus, star, ring, and mesh.

Protocol: The protocol defines a common set of rules which areused by computers on the network
that communicate between hardware and software entities. One of the most popular protocols for
LANs is the Ethernet. Another popular LAN protocol for PCs is the tokenring network.

Architecture: Networks can be broadly classified as using either a peertopeer or client/server

architecture.

3.2 Network Goals and Motivations

Before designing a computer network we should see that the designed network fulfils the basic
goals. We have seen that a computer network should satisfy a broad range of purposes and should
meet various requirements. One of the main goals of a computer network is to enable its users to
share resources, to provide low cost facilities and easy addition of new processing services. The
computer network thus, createsa global environment for its users and computers. Some of the basic
goals that a Computer network should satisfy are: Cost reduction by sharing hardware and software
resources. Provide high reliability by having multiple sources of supply. Provide an efficient means
of transport for large volumes of data among various locations (High throughput).

Provide inter process communication among users and processors. Reduction driving data transport.
Increase productivity by making it easier to share data amongst users. Repairs, upgrades,
expansions, and changes to the network should be performed with minimal impact on the majority
of network users. Standards and protocols should be supported to allow many types of equipment
from different vendors to share the network (Interpretability). Provide centralized/distributed
management and allocation of network resources like host processors, transmission facilities etc.

3.3 Classification of Networks

Depending on the transmission technology i.e., whether the network contains switching elements or
not, we have two types of networks: Broadcast networks. Point to point or Switched networks.

3.3.1 Broadcast Networks

Broadcast networks have a single communication channel that is shared by all the machines on the
network. In this type of network, short messages sent by any machine are received by all the
machines on the network. The packet contains an address field, which specifies for whom the
packet is intended. All the machines, upon receiving a packet check for the address field, if the
packet is intended for itself, it processes it and if not the packet is just ignored. Using Broadcast
networks, we can generally address a packet to all destinations (machines) by using a special code
in the address field. Such packets are received and processed by all machines on the network. This
mode of operation is known as ―Broadcasting‖. Some Broadcast networks also support transmission
to a subset of machines and this is known as ―Multicasting‖. One possible way to achieve
Multicasting is to reserve one bit to indicate multicasting and the remaining (nl) address bits contain
group number. Each machine can subscribe to any or all of the groups. Broadcast networks are
easily configured for geographically localised networks. Broadcast networks may be Static or
dynamic, depending on how the channel is allocated. In Static allocation, time is divided into
discrete intervals and using round robin method, each machine is allowed to broadcast only when its

LAN (Local Area Network)

Local Area Network is a computer network that spans over a relatively small area. Most LANs are
confined to a single building or group of buildings within a campus. However, one LAN can be
connected to other LANs over any distance via telephone lines and radio waves. A system of LANs
connected in this way is called a wide area network (WAN). Most LANs connect workstations and
personal computers. Each node(individual computer) in a LAN has its own CPU with which it
executes programs, but it is also able to access data and devices anywhere on the LAN. This means
that many users can share data as well as expensive devices, such as laser printers, fax machines etc.
Users can also use the LAN to communicate with each other, by sending email or engaging in chat
sessions. There are many different types of LANs, Ethernets being the most common for PCs. The
following characteristics differentiate one LAN from another:

Topology

The geometric arrangement of devices on the network. For example, devices can be arranged in a
ring or in a straight line.
Protocols

The rules and encoding specifications for sending data. The protocols also determine whether the
network uses peer to peer or client/server architecture.

Media

Devices can be connected by twisted pair wire, coaxial cables, or fiber optic cables. Some networks
communicate via radio waves hence, do not use any connecting media. LANs are capable of
transmitting data at very fast rates, much faster than data can be transmitted over a telephone line;
but the distances are limited, and there is also a limit on the number of computers that can be
attached to a single LAN.

The typical characteristics of a LAN are: Confined to small areas i.e., it connects several devices
over a distance of5to 10km. High speed. Most inexpensive equipment .Low error rates. Data and
hardware sharing between users owned by the user. Operates at speeds ranging from 10Mbps to
100Mbps. Nowadays 1000 Mbps are available.

3.3.2 Point toPoint or Switched Networks

Point to point or switched, networks are those in which there are many connections between
individual pairs of machines. In these networks, when a packet travels from source to destination it
may have to first visit one or more intermediate machines. Routing algorithms play an important
role in Point to point or Switched networks because often multiple routes of different lengths are
available An example of switched network is the international dialup telephone system.

The different types of Point to point or Switched networks are:

Circuit Switched Networks.
Packet Switched Networks.
In Switched network, the temporary connection is established from one point to another for either
the duration of the session (circuit switching) or for the transmission of one or more packets of data
(packet switching).
Circuit Switched Networks

Circuit Switched networks use a networking technology that provides a temporary, but dedicated
connection between two stations no matter how many switching devices are used in the data
transfer route. Circuit switching was originally developed for the analog based telephone system in
order to guarantee steady and consistent service for two people engaged in a phone conversation.
Analog circuit switching has given way to digital circuit switching, and the digital counterpart still
maintains the connection until broken (one side hangs up). This means bandwidth is continuously
reserved and ―silence is transmitted‖ just the same as digital audio in voice conversation.

Packet Switched Networks

Packet switched Networks use a networking technology that breaks up a message into smaller
packets for transmission and switches them to their required destination. Unlike circuit switching,
which requires a constant point to point circuit to be established, each packet in a packet switched
network contains a destination address. Thus, all packets in a single message do not have to travel
the same path. They can be dynamically routed over the network as lines become available or
unavailable. The destination computer reassembles the packets back into their proper sequence.
Packet switching efficiently handles messages of different lengths and priorities. By accounting for
packets sent, a public network can charge customers for only the data they transmit. Packet
switching has been widely used for data, but not for real time voice and video. However, this is
beginning to change. IP and ATM technologies are expected to enable packet switching to be used
for everything was X.25, which was defined when all circuits were digited and susceptible to noise.
Subsequent technologies, such as frame relay and SMDS were designed for today‘s almost error
free digital lines. ATM uses a cell switching technology that provides the bandwidth sharing
efficiency of packet switching with the guaranteed bandwidth of circuit switching. Highe level
protocols, such as TCPIIP, IPX/SPX and NetBIOS, are also packet based and are designed to ride
over packet switched topologies. Public packet switching networks may provide value added
services, such as protocol conversion and electronic mail.

Bus Topology

In Bus topology, all devices are connected to a central cable, called the bus or backbone. The bus
topology connects workstations using a single cable. Each workstation is connected to the next
workstation in a point to point fashion. All workstations connect to the same cable. Figure 2 shows
computers connected using Bus Topology. In this type of topology, if one workstation goes faulty
all workstations may be affected as all workstations share the same cable for the sending and
receiving of information. The cabling cost of bus systems is the least of all the different topologies.
Each end of the cable is terminated using a special terminator. The common implementation of this
topology is Ethernet. Here, message transmitted by one workstation is heard by all the other
workstations.
Advantages of Bus Topology
Installation is easy and cheap when compared to other topologies Connections are simple and this
topology is easy to use. Less cabling is required.
Disadvantages of Bus Topology
Used only in comparatively small networks. As all computers share the same bus, the performance
of the network deteriorates when we increase the number of computers beyond a certain limit. Fault
identification is difficult. A single fault in the cable stops all transmission.
3.4.2 Star Topology

Start topology uses a central hub through which, all components are connected. In a Star topology,
the central hub is the host computer, and at the end of each connection is a terminal as shown in
Figure 3.Nodes communicate across the network by passing data through the hub. A star network
uses a significant amount of cable as each terminal is wired back to the central hub, even if two
terminals are side by side but several hundred meters away from the host. The central hub makes all
routing decisions, and all other workstations can be simple. An advantage of the star topology is,
that failure, in one of the terminals does not affect any other terminal; however, failure of the
central hub affects all terminals. This type of topology is frequently used to connect terminals to a
large timesharing host computer.
Advantages of Star Topology
Installation and configuration of network is easy. Less expensive when compared to mesh topology.
Faults in the network can be easily traced. Expansion and modification of star network is easy.
Single computer failure does not affect the network. Supports multiple cable types like shielded
twisted pair cable, unshielded twisted pair cable, ordinary telephone cable etc.
Disadvantages of Star Topology
Failure in the central hub brings the entire network to a halt. More cabling is required in comparison
to tree or bus topology because each node is connected to the central hub.
3.4.3 Ring Topology

In Ring Topology all devices are connected to one another in the shape of a closed loop, so that
each device is connected directly to two other devices, one on either side of it, i.e., the ring topology
connects workstations in a closed loop, which is depicted in Figure 4 Each terminal is connected to
two other terminals (the next and the previous), with the last terminal being connected to the first.
Data is transmitted around the ring in one direction only; each station passing on the data to the next
station till it reaches its destination.

Information travels around the ring from one workstation to the next. Each packet of data sent on
the ring is prefixed by the address of the station to which it is being sent. When a packet of data
arrives, the workstation checks to see if the packet address is the same as its own, ifit is, it grabs the
data in the packet. If the packet does not belong to it, it sends the packet to the next workstation in
the ring. Faulty workstations can be isolated from the ring. When the workstation is powered on, it
connects itself to the ring. When power is off, it disconnects itself from the ring and allows the
information to bypass theworkstation. The common implementation of this topology is token ring.
A break in the ring causes the entire network to fail. Individual workstations can be isolated from
the ring.
Advantages of Ring Topology
Easy to install and modify the network. Fault isolation is simplified. Unlike Bus topology, there is
no signal loss in Ring topology because the tokens are data packets that are regenerated at each
node.
Disadvantages of Ring Topology
Adding or removing computers disrupts the entire network. A break in the ring can stop the
transmission in the entire network. Finding fault is difficult. Expensive when compared to other
topologies.
3.4.4 Tree Topology

Tree topology is a LAN topology in which only one route exists between any two nodes on the
network. The pattern of connection resembles a tree in which all branches spring from one root.
Figure 5 shows computers connected using Tree Topology. Tree topology is a hybrid topology, it is
similar to the star topology but the nodes are connected to the secondary hub, which in turn is
connected to the central hub. In this topology groups of star configured networks are connected to a
linear bus backbone.
Advantages of Tree Topology

Installation and configuration of network is easy. Less expensive when compared to mesh topology.
Faults in the network can be detected traced. The addition of the secondary hub allows more
devices to be attached to the central hub. Supports multiple cable types like shielded twisted pair
cable, unshielded twisted pair cable, ordinary telephone cable etc.

Disadvantages of Tree Topology

Failure in the central hub brings the entire network to a halt. More cabling is required when
compared to bus topology because each node is connected to the central hub.

3.4.5 Mesh Topology

Devices are connected with many redundant interconnections between network nodes. In a well
connected topology, every node has a connection to every other node in the network. The cable
requirements are high, but there are redundant paths built in. Failure in one of the computers does
not cause the network to break down, as they have alternative paths to other computers. Mesh
topologies are used in critical connection of host computers (typically telephone exchanges).
Alternate paths allow each computer to balance the load to other computer systems in the network
by using more than one of the connection paths available. A fully connected mesh network
therefore has n (n1)/2 physical channels to link n devices. To accommodate these, every device on
the network must have (nil) input/output ports.
Advantages of Mesh Topology

Use of dedicated links eliminates traffic problems. network. Point to point link makes fault isolation
easy. It is robust. Privacy between computers is maintained as messages travel along dedicated path.
Disadvantages of Mesh Topology

The amount of cabling required is high. A large number of I/O (input/output) ports are required.

3.4.6 Cellular Topology

Cellular topology, divides the area being serviced into cells. In wireless media each point transmits
in a certain geographical area called a cell, each cell represents a portion of the total network area.
Figure 7 shows computers using Cellular Topology. Devices that are present within the cell,
communicate through a central hub. Hubs in different cells are interconnected and hubs are
responsible for routing data across the network. They provide a complete network infrastructure.
Cellular topology is applicable only in case of wireless media that does not require cable
connection.
Advantages of Cellular Topology
If the hubs maintain a point to point link with devices, trouble shooting is easy. Hub to hub fault
tracking is more complicated, but allows simple fault isolation.
Disadvantages of Cellular Topology
When a hub fails, all devices serviced by the hub lose service (are affected).
OSI References Model

The Open System Interconnection (OSI) model is a set of protocols that attempt to define and
standardize the data communications process; we can say that it is a concept that describes how data
communications should take place. The OSI model was set by the International Standards
Organization(ISO) in 1984, and it is now considered the primary architectural model for inter
computer communications. The OSI model has the support of most major computer and network
vendors, many large customers, and most governments in different countries.

The Open Systems Interconnection (OS1) reference model describes how information from a
software application in one computer moves through a network medium to a software application in
another computer. The OSI reference model is a conceptual model composed of seven layers as
shown in Figure 9 each specifying particular network functions and into these layers are fitted the
protocol standards developed by the ISO and other standards bodies. The OSI model divides the
tasks involved with moving information between networked computers into seven smaller, more
manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers.
Each layer is reasonably self contained so that the tasks assigned to each layer can be implemented
independently. This enables the solutions offered by one layer to be updated without affecting the
other layers. The OSI model is modular. Each successive layer of the OSI model works with the one
above and below it. Although, each layer of the OSI model provides its own set of functions, it is
possible to group the layers into two distinct categories. The first four layers i.e., physical, data link,
network, and transport layer provide the end to end services necessary for the transfer of data
between two systems. These layers pr9.f1de the protocols associated with the communications
network used to link two computers together. Together, these are communication oriented. The top
three layers i.e., the application, presentation, and session layers provide the application services
required for the exchange of information. That is, they allow two applications, each running on a
different node of the network to interact with each other through the services provided by their
respective operating systems. Together, these are data processing oriented.

The following are the seven layers of the Open System Interconnection (OSI) reference model:
Layer 7 Application layer
Layer 6 Presentation layer
Layer 5 Session layer
Layer 4 Transport layer
Layer 3 Network layer
Layer 2 Data Link layer
Layer I Physical layer

Application Layer (Layer 7)

The Application layer is probably the most easily misunderstood layer of the model. This top layer
defines the language and syntax that programs use to communicate with other programs. The
application layer represents the purpose of communicating in the first place. For example, a
program in a client workstation uses commands to request data from a program in the server.
Common functions at this layer are opening, closing, reading and writing files, transferring files and
email messages, executing remote jobs and obtaining directory information about network resources
etc.
Presentation Layer (Layer 6)

The Presentation layer performs code conversion and data reformatting (syntax translation). It is the
translator of the network; it makes sure the data is in the correct form for the receiving application.
When data are transmitted between different types of computer systems, the presentation layer
negotiates and manages the way data are represented and encoded. For example, it provides a
common denominator between ASCII and EBCDIC machines as well as between different floating
point and binary formats. Sun‘s XDR and OSI‘s ASN.1 are two protocols used for this purpose.
This layer is also used for encryption and decryption. It also provides security features through
encryption and decryption.
Session Layer (Layer 5)

The Session layer decides when to turn communication on and off between two computers. It
provides the mechanism that controls the data exchange process and coordinates the interaction
(communication)between them in ail orderly manner. It sets up and clears communication channels
between two communicating components. It determines one way or two way communications and
manages the dialogue between both parties; for example, making sure that the previous request has
been fulfilled before the next one is sent. It also marks significant parts of the transmitted data with
checkpoints to allow for fast recovery in the event of a connection failure.

Transport Layer (Layer 4)

The transport layer is responsible for overall end to nd validity and integrity of the transmission i.e.,
it ensures that data is successfully sent and received between two computers. The lower data link
layer (layer 2) is only responsible for delivering packets from one node to another. Thus, if a packet
gets lost in a router somewhere in the enterprise Internet, the transport layer will detect that. It
ensures that if a 12MB file is sent, the full 12MB is received. If data is sent incorrectly, this layer
has the responsibility of asking for retransmission of the data. Specifically, it provides a network in
dependent, reliable message independent, reliable message interchange service to the top three
application oriented layers. This layer acts as an interface between the bottom and top three layers.
By providing the session layer (layer 5) with a reliable message transfer service, it hides the detailed
operation of the underlying network from the session layer.

Network Layer (Layer 3)

The network layer establishes the route between the sending and receiving stations. The unit of data
at the network layer is called a packet. It provides network routing and flow and congestion
functions across computer network interface.
It makes a decision as to where to route the packet based on information and calculations from other
routers, or according to static entries in the routing table. It examines network addresses in the data
instead of physical addresses seen in the Data Link layer. The Network layer establishes, maintains,
and terminates logical and/or physical connections. The network layer is responsible for translating
logical addresses, or names, into physical addresses The main device found at the Network layer is
a router
Data link Layer (Layer 2)

The data link layer groups the bits that we see on the Physical layer into Frames. It is primarily
responsible for error free delivery of data on a hop. The Data link layer is split into two sub ayers
i.e., the Logical Link Control (LLC) and Media Access Control (MAC). The Data Link layer
handles the physical transfer, framing (the assembly of data into a single unit or block), flow control
and error Control functions (and retransmission in the event of an error) over a single transmission
link; it is responsible for getting the data packaged and onto the network cable. The data link layer
provides the network layer (layer 3) reliable information transfer capabilities.

Physical Layer (Layer 1)

The data units on this layer are called bits. This layer defines the mechanical and electrical
definition of the network medium (cable) and network hardware. This includes how data is
impressed onto the cable and retrieved from it. The physical layer is responsible for passing bits
onto and receiving them from the connecting medium. This layer gives the data link layer (layer 2)
its ability to transport a stream of serial data bits between two communicating systems; it conveys
the bits that moves along the cable. It is responsible for ensuring that the raw bits get from one place
to another, no matter what shape they are in, and deals with the mechanical and electrical
characteristics of the cable. This layer has no understanding 01 the meaning of the bits, but deals
with the electrical and mechanical characteristics of the signals and signaling methods. The main
network device found the Physical layer is a repeater. The purpose of a repeater (as the name
suggests) is simply to receive the digital signal, reform it, and retransmit the signal. This has the
effect of increasing the maximum length of a network, which would not be possible due to signal
deterioration if, a repeater were not available. The repeater, simply regenerates cleaner digital signal
so it doesn‘t have to understand anything about the information it is transmitting, and processing on
the repeater is nonexistent. An example of the Physical layer is RS232.Each layer, with the
exception of the physical layer, adds information to the data as it travels from the Application layer
down to the physical layer. This extra information is called a header. The physical layer does not
append a header to information because it is concerned with sending and receiving information on
the individual bit level. We see that the data for each layer consists of the header and data of the
next higher layer. Because the data format is different at each layer, different terms are commonly
used to name the data package at each level. Figure 10 summarizes these terms layer by layer.
OSI Protocols

The OSI model provides a conceptual framework for communication between computers, but the
model itself is not a method of communication. Actual communication is made possible by using
communication protocols. In the context of data networking, a protocol is a formal set of rules and
conventions that governs how computers exchange information over a network medium. A protocol
implements the functions of one or more of the OSI layers. A wide variety of communication
protocols exist, but all tend to fall into one of the following groups: LAN protocols, WAN
protocols, network protocols, and routing protocols. LAN protocols operate at the network and data
link layers of the OSI model and define communication over the various LAN media. WAN
protocols operate at the lowest three layers of the OSI model and define communication over the
various wide area media.

Routing protocols are network layer protocols that are responsible for path determination and traffic
switching. Finally, network protocols are the various upper layer protocols that exist in a given
protocol suite.

Information being transferred from a software application in one computer system to software
application in another must pass through each of the OSI layers. Each layer communicates with
three other OSI layers i.e., the layer directly above it, the layer directly below it, and its peer layer
ill other networked systems. If, for example, in Figure 10, a software application in System A has
information to transmit to a software application in System B, the application program in System
Awill pass its information to the application layer (Layer 7) of System A. The application layer then
passes the information to the presentation layer (Layer 6); the presentation layer reformats the data
if required such that B can understand it. The formatted data is passed to the session layer (Layer 5),
which in turn requests for connection establishment between session layers of A and B, it then
passes the data to the transport layer. The transport layer breaks the data into smaller units called
segments and sends them to the Network layer. The Network layer selects the route for transmission
and if, required breaks the data packets further. These data packets are then sent to the Data link
layer that is responsible for encapsulating the data packets into data frames. The Data link layer also
adds source and destination addresses with error checks to each frame, for the hop. The data frames
are finally transmitted to the physical layer. In the physical layer, the data is in the fond of a stream
of bits and this is placed on the physical network medium and is sent across the medium to System
B. B receives the bits at its physical layer and passes them on to the Data link layer, which verifies
that no error has occurred. The Network layer ensures that the route selected for transmission is
reliable, and passes the data to the Transport layer. The function of the Transport layer is to
reassemble the data packets into the file being transferred and then, pass it on to the session layer.
The session layer confirms that the transfer is complete, and if so, the session is terminated. The
data is then passed to the Presentation layer, which major may not reformat it to suit the
environment of B and sends it to the Application layer. Finally the Application layer of System B
passes the information to the recipient Application program to complete the communication
process.

Interaction between different layers of OSI model

A given layer in the OSI layers generally communicates with three other OSI layers: the layer
directly above it, the layer directly below it, and its Peer layer in another networked computer
system. The data link layer in System A, for example, communicates with the network layer of
System A, the physical layer of System A, and the data link layer in System B.

Services provided by OSI layers

One OSI layer communicates with another layer to make use of the services provided by the second
layer. The services provided by adjacent layers help a given OSI layer communicate with its peer
layer in other computer systems. Three basic elements are involved inlayer services: the service
user, the service provider, and the service access point (SAP).

In this context, the service user is the OSI layer .that requests services from an adjacent OSI layer.
The service provider is the OSI layer that provides services to service users. OSI layers can provide
services to multiple service users. The SAP is a conceptual location at which one OSI layer can
request the services of another OSI layer.

OSI Model Layers and Information Exchange

The seven OSI layers use various forms of control information to communicate with their peer
layers in other computer systems. This control information consists of specific requests and
instructions that are exchanged between peer OSI layers. Control information typically takes one of
two forms: headers and trailers. Headers are pretended to data that has been passed down from
upper layers. Trailers are appended to data that has been passed down from upper layers. Headers,
trailers, and data are relative concepts, depending on the layer that analyses the information unit. At
the network layer, an information unit, for example, consists of a Layer 3 header and data. At the
data link layer, however, all the information passed down by the network layer (the Layer 3 header
and the data) is treated as data. In other words, the data portion of an information unit at a given
OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is
known as encapsulation.
3.6.2 TCP/IP Reference Model

TCP/IP stands for Transmission Control Protocol I Internet Protocol. It is a protocol suite used by
most communications software. TCP/IP is a robust and proven technology that was first tested in
the early I 980s on ARPA Net, the U.S. military‘s Advanced Research Projects Agency network,
and the world‘s first packet switched network. TCP/IP was designed as an open protocol that would
enable all types of computers to transmit data to each other via a common communications
language. TCP/IP is a layered protocol similar to the ones used in all the other major networking
architectures, including IBM‘s SNA, Windows‘ NetBIOS, Apple‘s AppleTalk, Novell‘s NetWare
and Digital‘s Decent. The different layers of the TCP/IP reference model are shown in Figure13.
Layering means that after an application initiates the communications, the message (data) to be
transmitted is passed through a number of stages or layers until it actually moves out onto the wire.
The data are packaged with a different header at each layer.

At the receiving end, the corresponding programs at each protocol layer unpack the data, moving it
―back up the stack‖ to the receiving application. TCP/IP is composed of two major parts: TCP
(Transmission Control Protocol) at the transport layer and IP (Internet Protocol) at the network
layer. TCP is a connection oriented protocol that passes its data to IP, which is a connectionless one.
TCP sets up a connection at both ends and guarantees reliable delivery of the full message sent.
TCP tests for errors and requests retransmission if necessary, because IP does not An alternative
protocol to TCP within the TCP/IP suite is UDP (User Datagram Protocol), which does not
guarantee delivery. Like IP, it is also connectionless, but very useful for real time voice and video,
where it doesn‘t matter if a few packets get lost.

Layers of TCP/IP reference model

Application Layer (Layer 4)

The top layer of the protocol stack is the application layer. It refers to the programs that initiate
communication in the first place. TCP/IP includes several application layer protocols for mail, file
transfer, remote access, authentication and name resolution. These protocols are embodied in
programs that operate at the top layer just as any custom made or packaged client/server application
would. There are many Application Layer protocols and new protocols are always being developed.
The most widely known Application Layer protocols are those used for the exchange of user
information, some of them are:
The HyperText Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of
the World Wide Web.
The File Transfer Protocol (FTP) is used for interactive file transfer.
The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and
attachments.
Telnet, is a terminal emulation protocol, and, is used for remote login to network hosts.Other
Application Layer protocols that help in the management of TCP/IP networks are:
The Domain Name System (DNS), which, is used to resolve a host name to an IP address.
The Simple Network Management Protocol (SNMP)

which is used between network management consoles and network devices (routers, bridges, and
intelligent hubs) to collect and exchange network management information. Examples of
Application Layer interfaces for TCP/IP applications are Windows Sockets and NetBIOS. Windows
Sockets provides a standard application programming interface (API) under the Microsoft Windows
operating system. NetBIOS is an industry standard interface for accessing protocol services such as
sessions, data grams, and name resolution.

Transport Layer (Layer 3)

The Transport Layer (also known as the HosttoHost Transport Layer) is responsible for providing
the Application Layer with session and datagram communication services. TCP/IP does not contain
Presentation and Session layers, the services are performed if required, but they are not part of the
formal TCP/IP stack. For example, Layer 6 (Presentation Layer) is where dataconversion (ASCII to
EBCDIC, floating point to binary, etc,) and encryption/decryption is performed, Layer 5 is the
Session Layer, which is performed in layer 4 in TCP/IP, Thus, we jump from layer 7 of OS I down
to layer 4 of TCP/IP.

From Application to Transport Layer, the application delivers its data to the communications
system by passing a Stream of data bytes to the transport layer along with the socket of the
destination machine. The core protocols of the Transport Layer are TCP and the User Datagram
Protocol (UDP).

TCP: TCP provides a one to one, connection oriented, reliable communications service. TCP is
responsible for the establishment of a TCP connection, the sequencing and acknowledgment of
packets sent, and the recovery of packets lost during transmission.
UDP: UDP provides a one to one or one to many, connectionless, unreliable communications
service. UDP is used when the amount of data to be transferred is small (such as the data that would
fit into a single packet), when the overhead of establishing a TCP connection is not desired, or when
the applications or upper layer protocols provide reliable delivery. The transport Layer encompasses
the responsibilities of the OSI Transport Layer and some of the responsibilities of the OSI Session
Layer.
Internet Layer (Layer 2)
The internet layer handles the transfer of information across multiple networks through the use of
gateways and routers. The internet layer corresponds to the part of the ORI network layer that is
concerned with the transfer of packets between machines that are connected to different networks. It
deals with the routing of packets across these networks as well as with the control of congestion. A
key aspect of the internet layer is the definition of globally unique addresses for machines that are
attached to the Internet.

The Internet layer provides a single service namely, best effort connectionless packet transfer. IP
packets are exchanged between routers without a connection setup; the packets are ‗touted
independently and so they may traverse different paths. For this reason, IP packets are also called
data rams. The connectionless approach makes the system robust; that is, if failures occur in the
network, the packets are routed around the points of failure; hence, there is no need to set up
connections, The gateways that interconnect the intermediate networks may discard packets when
congestion occurs. The responsibility for recovery from these losses is I passed on to the Transport
Layer, The core protocols of the Internet Layer are IP. ARP,ICMP, and IGMP.
The Internet Protocol (IP) is a routable protocol responsible for IP addressing and the
fragmentation and reassembly of packets.
The Address Resolution Protocol (ARP) is responsible for the resolution of the Internet Layer
address to the Network Interface Layer address, such as a hardware address.
The Internet Control Message Protocol (ICMP) is responsible for providing diagnostic functions
and reporting errors or conditions regarding the delivery of IP packets.
The Internet Group Management Protocol (IGMP) is responsible for the management of IP
multicast groups. The Internet Layer is analogous to the Network layer of the OSI model.

Link/Physical Layer (Layer I)

The Link/Physical Layer (also called the Network Access Layer) is responsible for placing TCP/IP
packets on the network medium and receiving TCP/IP packets of the network medium. TCP/IP was
designed
to be independent of the network access method, frame format, and types. This includes LAN
technologies such as Ethernet or Token Ring and WAN technologies such as X.25 or Frame Relay.
Independence
from any specific network technology gives TCP/IP the ability to be adapted to new technologies
such as Asynchronous Transfer Mode (ATM). The Network Interface Layer encompasses the Data
Link and Physical layers of the OSI Model. Note, that the Internet Layer does not take advantage of
sequencing and acknowledgement services that may be present in the Data Link Layer. An
unreliable Network Interface Layer is assumed, and reliable communications through session
establishment and the sequencing and acknowledgement of packets is the responsibility of the
Transport Layer.

Comparison between OSI and TCP/IP reference model

Both OSI and TCP/IP reference models are based on the concept of a stack of protocols. The
functionality of the layers is almost similar. In both models the layers are there to provide an end o
end network independent transport service to processes wishing to communicate with each other.
The Two models have many differences. An obvious difference between the two models is the
number of layers: the OSI model has seven layers and the TCP/IP has .four layers. Both have (inter)
network, transport, and application layers, but the other layers are different. OSI uses strict layering,
resulting in vertical layers whereas TCP/IP uses loose layering resulting in horizontal layers. The
OSI model supports both connectionless and connection oriented communication in the network
layer, but only connection oriented communication at the transport layer. The TCP/IP model has
only one mode in network layer (connectionless), but supports both modes in the transport layer.
With the TCP/IP model, replacing IP by a substantially different protocol would be virtually
impossible, thus, defeating one of the main purposes of having layered protocols in the first place.
The OSI reference model was devised before the OSI protocols were designed. The OSI model was
not biased toward one particular set of protocols, which made it quite general. The drawback of this
ordering is that the designers did not have much experience with the subject, and did not have a
good idea of the type of functionality to put in a layer.
With TCP/IP the reverse was true: the protocols carne first and the model was really just a
description of the existing protocols. There was no problem with the protocols fitting the model.
The only drawback was that the model did not fit any other protocol stacks.

All layers are not roughly, of equal size and complexity. In practice, the session layer and
presentation layer are absent from many existing architectures. Some functions like addressing,
flow control, retransmission are duplicated at each layer, resulting in deteriorated performance. The
initial specification of the OSI model ignored the connectionless model, thus, leaving much of the
LANs behind.
Some of the drawbacks of TCP/IP model are:
TCP/IP model does not clearly distinguish between the concepts of service, interface, and protocol.
TCP/IP model is not a general model and therefore it cannot be used to describe any protocol other
than TCP/IP.TCP/IP model does not distinguish or even mention the Physicalor the Data link layer.
A proper model should include both these layers as separate.
 UNIT IV

REAL TIME CONTROL

Current mode data transmission is the preferred technique in many environments, particularly in
industrial applications. Most systems employ the familiar 2wire, 4–20mA current loop, in which a
single twisted pair cable supplies power to the module and carries the output signal as well.

The 3wire interface is less common but allows the delivery of more power to the module
electronics. A 2wire system provides only 4mA at the line voltage (the remaining 16mA carries the
signal).
Current loops offer several advantages over voltage mode output transducers:
– They do not require a precise or stable supply voltage.
– Their insensitivity to IR drops makes them suitable for long distances.
– A 2wire twistedpair cable offers very good noise immunity.
– The 4mA of current required for information transfer serves two purposes:

it can furnish power to a remote module, and it provides a distinction between zero (4 mA) and no
information (no current flow).In a 2wire, 4–20mA current loop, supply current for the sensor
electronics must not exceed the maximum available, which is 4mA (the remaining 16mAcarries the
signal). Because a 3wire current loop is easily derived from the2wire version, the following
discussion focuses on the 2wire version.

Need for a Current Loop

The 4–20mA current loop shown in Fig. 1 is a common method of transmitting sensor information
in many industrial process monitoring applications. A sensor is a device used to measure physical
parameters such as temperature, pressure, speed, liquid flow rates, etc. Transmitting sensory
information through a current loop is particularly useful when the information has to be sent to a
remote location over long distances (1,000 ft, or more). The loop‘s operation is straightforward: a
sensor‘s output voltage is first converted to a proportional current, with 4mA normally representing
the sensor‘s zero level output, and 20mA representing the sensor‘s full scale output. Then, a
receiver at the remote end converts the 4–20mA current back into a voltage which in turn can be
further processed by a computer or display module. However, transmitting a sensor‘s output as a
voltage over long distances has several drawbacks. Unless very high input impedance devices are
used, transmitting voltages over long distances produces correspondingly lower voltages at the
receiving end due wiring and interconnect resistances. However, high impedance instruments can
be sensitive to noise pickup since the lengthy signal carrying wires often run in close proximity to
other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but
their high cost may be prohibitive when long distances are involved. Sending a current over long
distances produces voltage losses proportional to the wiring‘s length. However, these voltage losses
also known as loop drops‖ do not reduce the 4–20mA current as long as the transmitter an loop
supply can compensate for these drops. The magnitude of the current inthe loop is not affected by
voltage drops in the system wiring since all of the current (i.e., electrons) originating at the negative
(−) terminal of the loop power supply has to return back to its positive (+) terminal
 Fig1: Typical components in a loop

Fig2: 420 mA transmitter

Basic 2wire Circuit

A voltage output should be converted to current when configuring a ratio metric 4–20mA circuit,
because current mode applications require a 4mA offset and 16mA span. This section presents the
circuit details and results obtained from a current loop configuration based on the MAX1459 sensor
signal conditioner. In principle, a voltage regulator is to be added, which converts the 10–32V loop
voltage to a fixed 5V for operating the MAX1459. Figure shows the circuitry required for
implementing a standard ratio metric version of the MAX1459 circuit. The voltage regulator can be
any low cost device whose quiescent current is sufficiently low for the 4mA budget.

Advantages of 4–20mA Current Loop

Current loops offer four major advantages such as:

– Long distance transmission without amplitude loss
– Detection of offline sensors, broken transmission lines, and other failures
– Inexpensive 2wire cables
– Lower EMI sensitivity
RS 232C/RS 485

The RS232/485 port sequentially sends and receives bytes of information one bit at a time.
Although this method is slower than parallel communication, which allows the transmission of an
entire byte at once, it is simpler and can be used over longer distances because of lower power
consumption. For example, the IEEE 488 standard for parallel communication states that the cable
length between the equipments should not exceed 20m total, with a maximum distance of 2m
between any two devices. RS232/485 cabling ,however, can extend 1,200m or greater. Typically,
RS232/485 is used to transmit American Standard Code for Information Interchange (ASCII) data.
Although National Instruments serial hardware can transmit 7bit as wells 8bit data, most
applications use only 7bit data. Seven bit ASCII can represent the English alphabet, decimal
numbers, and common punctuation marks. It is a standard protocol that virtually all hardware and
software understand. Serial communication is mainly using the three transmission lines:

(1) ground, (2) transmit, and (3) receive. Because RS232/485 communications asynchronous, the serial
port can transmit data on one line while receiving data on another. Other lines such as the handshaking
lines are not required. The important serial characteristics are baud rate, data bits, stop bits, and parity.
To communicate between a serial instrument and a serial port on computer, these parameters must
match. The RS232 port, or ANSI/EIA232 port, is the serial connection found on Biocompatible PCs. It
is used for many purposes, such as connecting amuse, printer, or modem, as well as industrial
instrumentation. The RS232protocol can have only one device connected to each port. The RS485
(EIA485 Standard) protocol can have 32 devices connected to each port. With this enhanced multidrug
capability the user can create networks of devices connected to a single RS485 serial port. Noise
immunity and multi drop capability make RS485, the choice in industrial applications requiring many
distributed devices networked to a PC or other controller for data collection.USB was designed
primarily to connect peripheral devices to PCs, including keyboards, scanners, and disk drives.RS232
(Recommended standard232) is a standard interface approved by the Electronic Industries Association
(EIA) for connecting serial devices. In other words, RS232 is a long established standard that describes
the physical interface and protocol for relatively low
speed serial data
 Fig 4: RS232c DB 25 pinout

Signal Descriptions
– TxD – This pin carries data from the computer to the serial device

– RXD – This pin carries data from the serial device to the computer

– DTR signals – DTR is used by the computer to signal that it is ready to communicate with the
serial device like modem. In other words, DTR indicates the modem that the DTE (computer) is
ON.

– DSR – Similarly to DTR, Data set ready (DSR) is an indication from the modem that it is ON.

– DCD – Data carrier detect (DCD) indicates that carrier for the transmit data is ON.

– RTS – This pin is used to request clearance to send data to a modem.

– CTS – This pin is used by the serial device to acknowledge the computers RTS signal. In most
situations, RTS and CTS are constantly on throughout the communication session.

– Clock signals (TC, RC, and XTC) – The clock signals are only used for synchronous
communications. The modem or DSU extracts the clock from the data stream and provides a steady
clock signal to the DTE. The transmit and receive clock signals need not have to be the same, or
even at the same baud rate.
– CD – CD stands for Carrier detect. Carrier detect is used by a modem to signal that it has a made
a connection with another modem, or has detected a carrier tone. In other words, this is used by the
modem to signal that a carrier signal has been received from a remote modem.

RI – RI stands for ring indicator. A modem toggles (keystroke) the state of this line when an
incoming call rings in the phone. In other words, this is used by an auto answer modem to signal the
receipt of a telephone ring signal. The carrier detect (CD) and the ring indicator (RI) lines are only
available in connections to a modem. Because most modems transmit status information to a PC
when either a carrier signal is detected

RS485 Serial Communication

RS485 is an EIA standard for multipoint communications. It supports several types of connectors,
including DB9 and DB37. RS485 is similar to RS422 but can support more nodes per line. RS485
meets the requirements for truly multipoint communications network, and the standard specifies up
to32 drivers and 32 receivers on a single (2wire) bus. With the introduction of ―automatic‖ repeaters
and high impedance drivers/receivers this ―limitation "can be extended to hundreds or even
thousands of nodes on a network. The RS485 and RS422 standards have much in common, and are
often confused for that reason. RS485, which specifies bidirectional, half duplex data transmission,
is the only EIA standard that allows multiple receivers and drivers in ―bus‖ configurations. RS422,
on the other hand, specifies a single, unidirectional driver with multiple receivers.

GPIB

The purpose of this section is to provide guidance and understanding of the General Purpose
Interface Bus (GPIB) bus to new GPIB bus users or to provide more information on using the GPIB
bus‘s features.GPIB Data Acquisition and Control Module provides analog and digital signals for
controlling virtually any kind of a device and the capability toread back analog voltages, digital
signals, and temperatures. The 4867 Data Acquisition and Control module is an IEEE488.2
compatible device and has standard Commands for Programmable Instruments (SCPI) command
parser that accepts SCPI and short form commands for ease of programming. Applications include
device control, GPIB interfacing and data logging. The 4867is housed in a small 7 in.×7 in.

Fig 5: Block Diagram

Controllers have the ability to send commands, to talk data onto the bus and to listen to data from
devices. Devices can have talk and listen capability. Control can be passed from the active
controller (Controller in charge) to any device with controller capability. One controller in the
system is defined as the System Controller and it is the initial controller in charge (CIC).Devices are
normally addressable and have a way to set their address. Each device has a primary address
between 0 and 30. Address 31 is the unlisted outtalk address.

Types of GPIB Messages

GPIB devices communicate with other GPIB devices by sending device dependent messages and
interface messages through the interface system. Device dependent messages, often called data or
data messages, contain device specific information, such as programming instructions,
measurement results, machine status, and data files. Interface messages manage the bus. Usually
called commands or command messages, interface messages perform such functions as initializing
thebus.
Physical Bus Structure
GPIB is a 24 conductor as shown in Fig 6Physically, the GPIB interface system consists of 16
lowtrue signal lines and eight ground return or shielddrainlines. The 16 signal lines, discussed later,
are grouped into data lines(eight), handshake lines (three), and interface management lines (five).

Data Lines
The eight data lines, DIO1 through DIO8, carry both data and command messages. The state of the
Attention (ATN) line determines whether the information is data or commands. All commands and
most data use the 7bitASCII or ISO code set, in which case the eighth bit, DIO8, is either un usedor
used for parity.

Handshake Lines
Three lines asynchronously control the transfer of message bytes between devices. The process is
called a 3wire interlocked handshake. It guarantees that message bytes on the data lines are sent and
received without transmission error.

NRFD (not ready for data) – Indicates when a device is ready or not ready to receive a message
byte. The line is driven by all devices when receiving commands by listeners when receiving data
messages, and by the talker when enabling the HS488 protocol.

– NDAC (not data accepted) – Indicates when a device has or has not accepted a message byte. The
line is driven by all devices when receiving commands, and by Listeners when receiving data
messages.

– DAV (data valid) – Tells when the signals on the data lines are stable(valid) and can be accepted
safely by devices. The Controller drives DAV when sending commands, and the Talker drives DAV
when sending data messages. Three of the lines are handshake lines, NRFD, NDAC, and DAV,
which transfer data from the talker to all devices who are addressed to listen. Thetalker drives the
DAV line; the listeners drive the NDAC and NRFD lines. The remaining five lines
are used to control the bus‘s operation.

– ATN (attention) is set true by the controller in charge while it is sending interface messages or
device addresses. ATN is false when the bus is transmitting data.

– EOI (end or identify) can be asserted to mark the last character of a message or asserted with the
ATN signal to conduct a parallel poll.
Fig :6 Bus structure of GPIB

USB

The USB is a medium speed serial data bus designed to carry relatively large amounts of data over
relatively short cables: up to about 5m long. It can support data rates of up to 12Mbs−1 (megabits
per second), which is fast enough for most PC peripherals such as scanners, printers, keyboards,
mice, joysticks, graphics tablets, lowers digital cameras, modems, digital speakers, low speed
CDROM and CD writer drives, external Zip disk drives, and soon. The USB is an addressable bus
system, with a 7bit address code so it can support up to 127 different devices or nodes. However, it
can have only one host. So a PC with its peripherals connected through the USB forms a star Local
Area Network (LAN).On the other hand any device connected to the USB can have a number of
other nodes connected to it in daisychain fashion, so it can also form the

Fig 7: USB Structure

Most hubs provide either four or seven downstream ports, or less. Another important feature of the
USB is that it is designed to allow hot swapping. Devices can be plugged into and unplugged from
the bus without having to turn the power off and on again, reboot the PC or even, manually starta
driver program. A new device can simply be connected to the USB, and the PCs operating system
should recognize it and automatically set up the necessary driver to service it.
Need for USB

The USB is host controlled. There can only be one host per bus. The specification in itself does not
support any form of multi master arrangement. This is aimed at and limited to single point to point
connections such as a mobile phone and personal organizer and not multiple hub, multiple device
desktop configurations. The USB host is responsible for undertaking all transactions and scheduling
bandwidth. Data can be sent by various transaction methods using a token based protocol. One of
the original intentions of USB was to reduce the amount of cabling at the back of PC. The idea
came from the Apple Desktop Bus, where the keyboard, mouse and some other peripherals could be
connected together (daisy chained) using the one cable .However, USB uses a tiered star topology,
similar to that of 10BaseTEthernet. This imposes the use of a hub somewhere, which adds to greater
expense, more boxes on desktop and more cables. However it is not as bad as it may seem. Many
devices have USB hubs integrated into them. For example, keyboard may contain a hub which is
connected to computer. Mouse another devices such as digital camera can be plugged easily into the
back of keyboard. Monitors are just another peripheral on a long list which commonly has inbuilt
hubs. This tiered star topology, rather than simply daisy chaining devices together has some
benefits. First power to each device can be monitored and even switched off if an over current
condition occurs without disrupting other USB devices. High, full and low speed devices can be
supported, with the hub filtering out high speed and full speed transactions so lower speed devices
do not receive them. Up to 127 devices can be connected to any one USB bus at any one given time.
To extent devices simply add another port/host. While earlier USB hosts had two ports, most
manufacturers have seen this as limiting and are starting to introduce 4 and 5 port host cards with an
internal port for hard disks etc. The early hosts had one USB controller and thus both ports shared
the same available USB bandwidth. As bandwidth requirements have increased, multiport cards
with two or more controllers allowing individual channels are used.

Table: Pin connections

USB cables use two different types of connectors:

– Type A. Plugs for the upstream end.
– Type B. Plugs for the downstream end.

PCMCIA
Personal Computer Memory Card International Association (PCMCIA) is an international standards
body and trade association founded in 1989developed a standard for small, credit card sized
devices, called PC Cards. Originally designed for adding memory to portable computers, the
PCMCIA standard has been expanded several times and is now suitable for many types of devices.
The inclusion of PCMCIA technology in PCs delivers a variety of benefits. Besides providing an
industry standard interface for third party cards (PC Cards), PCMCIA allows users to easily swap
cards in and out of a PC as needed, without having to deal with the allocation of system resources
for those devices. These useful features hot swapping and automatic configuration, as well as card
slot power management and other PCMCIA capabilities –are supported by a variety of software
components on the PCMCIA based PC. In most cases, the software aspect of PCMCIA remains
relatively transparent to the user. As the demand for notebook and laptop computers began sky
rocketing in the late 1980s, users realized that their expansion options were fairly limited. Mobile
machines were not designed to accept the wide array of available expansion cards that their desktop
counterparts could enjoy

Fig 8: Triggering Architecture

Features of PCMCIA
– One rear slot, access from rear of PC
– Accept Type I/II/III Cards
– Comply with PCI Local Bus Specification Rev.2.2
– Comply with 1995 PC Card Standard
– Extra compatible registers are mapped in memory
– Use T/I 1410 PCMCIA controller
– Support PC Card with Hot Insertion and Removal
– Support 5V or 5/3.3V 16bit PC Cards
– Support Burst Transfers to Maximize Data Throughput on both PCIBuses
– Supports Distributed DMA and PC/PCI DMA
Utilities of PCMCIA Card in the Networking Category

Under the networking category, typically PCMCIA slot supports 4 types

of cards such as:
– LAN card
– Wireless LAN card
– Modem card
– ATA flash disk card

VISA

Virtual Instrumentation Software Architecture (VISA) is a standard I/O language for

instrumentation programming. VISA by itself does not provide instrumentation programming
capability. VISA is a high-level API that calls into lower level drivers.
VISA is capable of controlling VXI, GPIB, or Serial instruments and makes the appropriate
driver calls depending on the type of instrument being used. When debugging VISA problems, it is
important to keep in mind that this hierarchy exists.

Fig9: VISA Architecture

The terminology related with VISA are explained as follows:

Resources. The most important objects in the VISA language are known as resources. Operations.
In object oriented terminology, the functions that can be used with an object are known as
operations Attributes. In addition to the operations that can be used with an object, the object has
variables associated with it that contain information related to the object. In VISA, these variables
are known as attributes. There is a default resource manager at the top of the VISA hierarchy that
can search for available resources or open sessions to them. Resources can be GPIB, serial message
based VXI, or register based VXI. The most common operations for message based instruments are
read and write.

Waveform generator

Standalone traditional instruments such as oscilloscopes and waveform generators are very
powerful, expensive, and designed to perform one or more specific tasks defined by the vendor.
However, the user generally cannot extender customize them. The knobs and buttons on the
instrument, the built in circuitry, and the functions available to the user, are all specific to the nature
of the instrument. In addition, special technology and costly components must be developed to build
these instruments, making them very expensive and slow to adapt.
UNITV

Fourier Transforms

LabVIEW and its analysis VI library provide a complete set of tools to perform Fourier and spectral
analysis. The Fast Fourier Transform (FFT) and Power Spectrum VIs are optimized, and their
outputs adhere to the standard DSP format.

FFT is a powerful signal analysis tool, applicable to a wide variety of fields including spectral
analysis, digital filtering, applied mechanics, acoustics, medical imaging, modal analysis, numerical
analysis, seismography, instrumentation, and communications.

The LabVIEW analysis VIs, located on the Signal Processing palette, maximize analysis
throughput in FFT related applications. This document discusses FFT properties, how to interpret
and display FFT results, and how to manipulate FFT and power spectrum results to extract useful
frequency information.

FFT Properties

The fast Fourier transform maps time domain functions into frequencydomain representations.
FFT is derived from the Fourier transform equation, which is

where x(t) is the time domain signal, X(f) is the FFT, and ft is the frequency to analyze. Similarly,
the discrete Fourier transform (DFT) maps discrete time sequences into discrete frequency
representations. DFT is given by the following equation
where x is the input sequence, X is the DFT, and n is the number of samples in both the discrete
time and the discrete frequency domains. Direct implementation of the DFT, as shown in equation
2, requires approximately n complex operations. However, computationally efficient algorithms can
require as little as n log2(n) operations.

An inherent DFT property is the following:

FFT Standard Output:

The output format of the FFT VI can now be described with X. If the total number of samples, n, of
the input sequence, x, is even, the format of the complex output sequence, X, and the interpretation
of the results are shown in the below table.
Table 1: Output Format for Even n, k = n ÷ 2
Array Element Interpretation
X[0] DC component

X[1] 1st harmonic or fundamental

X[2] 2nd harmonic
. .
. .
. .
X[k2] (k2)th harmonic
X[k1] (k1)th harmonic
X[k] = X[k] Nyquist harmonic
X[k+1]= X[n(k1)] = X[(k1)] (k1)th harmonic
.
.
.
X[n3] 3rd harmonic
X[n2] 2nd harmonic
X[n1] 1st harmonic

Fig 1 : Graph of table

If the total number of samples, n, of the input sequence, x, is odd, the format of the complex output
sequence, X, and the interpretation of the results are shown in the below table
Table:2Output Format for Odd n, k = (n1) ÷ 2
Array Element Interpretation
X[0] DC component

X[1] 1st harmonic or fundamental

X[2] 2nd harmonic
. .
. .
. .

X[k1] kth 1harmonic

X[k] kth harmonic
X[k+1]= X[nk] = X[k] kth kth harmonic
. X[k+2]= X[n(k1)] = X[(k1)] (k1)th harmonic
.
.
X[n3] 3rd harmonic
X[n2] 2nd harmonic
X[n1] 1st harmonic

Fig 2 : Graph of table 2

Standard Output
Standard output is the format for even and odd sized discrete time sequences, described in Tables 1
and 2 of this document. This format is convenient because it does not require any further data
manipulation. To graphically display the results of the FFT, wire the output arrays to the waveform
graph Fig : 2 Graph of table2. The FFT output is complex and requires two graphs to display all the
information.
 Fig 3 : Standard output by using
VI Power spectrum and Correlation:

The power spectrum reveals the existence, or the absence, of repetitive patterns and correlation
structures in a signal process. These structural patterns are important in a wide range of applications
such as data forecasting, signal coding, signal detection, radar, pattern recognition, and decision-
making systems. The most common method of spectral estimation is based on the fast Fourier
transform (FFT).

In particular, the total power is given by

One can show that Px(w) is real, even and positive. The autocorrelation can be recovered with the
inverse Fourier transform

Power spectrum – properties:

In particular, the total power is given by

The power spectrum is sometimes called spectral density because it is positive and the signal
power can always benormalized to r(0) =(2p)1 .

Example: Uncorrelated noise has a constant power spectrum

Hence it is also called white noise.

Effect of Filtering on Power Spectrum
A linear system with impulse response h[k]

Transforms the power spectrum as

Spectral Content
The power spectrum gives the spectral content of the data. To see that consider the power of a signal
after filtering with a narrow band pass filter around w 0.

Power spectrum properties

The power spectrum captures the spectral content of the sequence. It can be estimate directly from
the Fourier transform of the data.

Correlation methods
Correlation is a statistical technique that can show whether and how strongly pairs of variables are
related.

Correlation Coefficient

The main result of a correlation is called the correlation coefficient (or "r"). It ranges from 1.0 to
+1.0. The closer r is to +1 or 1, the more closely the two variables are related.

Cross and Autocorrelation

The cross correlation is defined as

Note that correlation is a convolution with opposite sign. It can be computed with the Fourier
transform.

The autocorrelation is defined as

For a sample of finite length N this is typically normalized. We call this the sample autocorrelation
Autocorrelation properties
The autocorrelation is symmetric.

The zero lag gives the total power of the signal

The autocorrelation has the power as an upperbound

Windowing & flittering

Windowing is the process of taking a small subset of a larger dataset, for processing and analysis. A
naive approach, the rectangular window, involves simply truncating the dataset before and after the
window, while not modifying the contents of the window at all. However this is a poor method of
windowing and causes power leakage.

Windowing of a simple waveform like cosωt causes its Fourier transform to develop nonzero values
(commonly called spectral leakage) at frequencies other than ω. The leakage tends to be worst
(highest) near ω and least at frequencies farthest from ω.

If the waveform under analysis comprises two sinusoids of different frequencies, leakage can
interfere with the ability to distinguish them spectrally. If their frequencies are dissimilar and one
component is weaker, then leakage from the stronger component can obscure the weaker one‘s
presence. But if the frequencies are similar, leakage can render them irresolvable even when the
sinusoids are of equal strength. The rectangular window has excellent resolution characteristics for
sinusoids of comparable strength, but it is a poor choice for sinusoids of disparate amplitudes. This
characteristic is sometimes described as low dynamic range.
At the other extreme of dynamic range are the windows with the poorest resolution and sensitivity,
which is the ability to reveal relatively weak sinusoids in the presence of additive random noise.
That is because the noise produces a stronger response with high dynamic ange windows than with
high resolution windows. Therefore, high dynamic range windows are most often justified in
wideband applications, where the spectrum being analyzed is expected to contain many different
components of various amplitudes.
In between the extremes are moderate windows, such as Hamming and Hann. They are commonly
used in narrowband applications, such as the spectrum of a telephone channel. In summary, spectral
analysis involves a tradeoff between resolving comparable strength components with similar
frequencies and resolving disparate strength components with dissimilar frequencies. That tradeoff
occurs when the window function is chosen.
Application in Process Control projects

Software for Remote ON/OFF Control Experiment

The software for the Remote ON/OFF controller lab experiment is developed using LabVIEW.

There are two distinct parts in the software:

1. ON/OFF controller server program and logic

2. Internet communication using DataSocket protocol

These parts will be explained in the following sections
Fig 4: ONOFF Temperature Control
ON/OFF Controller Server Program and Logic
The LabVIEW program on the server first reads the voltage from the analog input channel of the
DAQ board and converts it to a temperature using the equation

Tout = 6 Vin + 60

where Tout is the process temperature, and Vin is the input voltage from the DAQ board. The
desired Setpoint temperature value of the process is obtained from the client computer, and the
Error is determined using the equation

Error = Setpoint . Tout.

Based on the error and neutral zone High Limit and Low Limit, an ON/OFF controller logic is
implemented using LabVIEW. In this logic, Digital Output Channel is the output logic value
sent to the DAQ board which controls the fan through the SSR, Cooling Fan Indicators are the
LED ON/ OFF indicators on the front panel of the LabVIEW VI that show the state of the fan,
and Within Limits Indicator shows if the process is operating within the neutral zone. The
Cooling Fan ON Indicator is also defined as a local read only variable which is used in the logic
implementation.
The LabVIEW front panel of the server program VI
Fig 5: Server front panel

Temperature data acquisition system

The hardware for server workstation consists of a PC with Pentium III, 550 MHz processor, 128
Mb RAM, Network Interface Card (NIC), National Instruments DAQ board. The server software
includes Windows 98, NIDAQ driver, LabVIEW 5.1 and NIData Socket Manager. The designed
experiment is connected to the DAQ board. The server is assigned static IP address. The clients
could be any PC.s with NIC that can run a LabVIEW program. The objective of the experiment
is to maintain the temperature inside a wooden box at some desired set point value, within
neutral zone limits, using a two state controller mode. The wooden box is heated with a light
bulb. The temperature is measured using LM335 solid-state The hardware for server workstation
consists of a PC with Pentium III, 550 MHz processor, 128 Mb RAM, Network Interface Card
(NIC), National Instruments DAQ board. The server software includes Windows 98, NIDAQ
driver, Lab VIEW 5.1 and NIData Socket Manager. The designed experiment is connected to the
DAQ board. The server is assigned static IP address. The clients could be any PC.s with NIC
that can run a LabVIEW program. The objective of the experiment is to maintain the
temperature inside a wooden box at some desired set point value, within neutral zone limits,
using a two state controller mode. The wooden box is heated with a light bulb. The temperature
is measured using LM335 solid state
 Fig 6: Temperature data Acquisition

Oscilloscope
Open Windows oscilloscopes from the 5000, 6000, 7000, and 8000 series are Windowsbased
and contain scopespecific software for acquisition, connectivity, and control. By default, the
scope user interface runs Windowsbased application software that performs the following
scopespecific tasks:

• Presents the scope user interface

• Configures scope hardware based on commands from the user

and signal content
• Displays acquired signals and derivations of acquired signals

You can use LabVIEW to extend the feature list of your scope to
include the following tasks:
• Custom analysis and signal processing of acquired
signals
• Automation of scope related tasks and sequences of
tasks • A unique and customizable user interface

• Custom reports, including reports that you can publish live over
the Internet
•
Example:
Connect the function generator to the scope and put up the sample waveform. Increase the
amplitude to 2v and pull down the VISA box and click on GPIB for the oscilloscope. The
display of the wave is shown in the fig 7.

Fig 7: waveform acquired by oscilloscope

Digital Multimeter
Digital Multimeters (DMMs) are specialized in takingflexible, high resolution measurements

• Low to medium acquisition speeds

• High resolution

• Measures current or resistance in addition to voltage.

• Different form factors: PXI, PCIe, PCI, USBSoftware Support in NI LabVIEW and
SignalExpressThe NI 407x series of DMMs offer unique capabilities − Isolated Digitizer Mode
at up to 1.8 MS/s

− Industry Leading Accuracy

− 2year Calibration Cycle
• USB4065 & PCMCIA4050 for portable measurements

Example:

Fig 8 : Front Panel

Fig 9: Block diagram of multimeter

 Fig 10: Components of motion control
Motion controller: The motion controller acts as brain of the system by taking the desired target
positions and motion profiles and creating the trajectories for the motors to follow, but out putting a
±10 V signal for servomotors, or a step and direction pulses for stepper motors.

Amplifier or drive: Amplifiers (also called drives) take the commands from the controller and
generate the current required to drive or turn the motor.

Motor: Motors turn electrical energy into mechanical energy and produce the torque required move
to the desired target position.

Mechanical elements: Motors are designed to provide torque to some mechanics. These include
linear slides, robotic arms and special actuators.

Feedback device or position sensor: A position feedback device is not required for some motion
control applications (such as controlling stepper motors), but is vital for servomotors. The feedback
device, usually a quadrature encoder, senses the motor position and reports the result to the
controller.
Image Acquisition and Processing
Image analysis combines techniques that compute statistics and measurements based on the
graylevel intensities of the image pixels. Image processing contains information about lookup
tables, convolution kernels, spatial filters and grayscale morphology. The lookup table (LUT)
transformations are basic image processing functions that highlight details in areas containing
significant information at the expense of other areas. These functions include histogram
equalization, gamma corrections logarithmic corrections ,and exponential corrections. NIIMAQ is a
complete and robust API for image acquisition. Whether you are using LabVIEW,

Measurement Studio, Visual Basic, or Visual C++, NIIMAQ gives you high level control of
National Instruments image acquisition devices. NIIMAQ performs all of the computer and board
specific tasks, allowing straightforward image acquisition without register level programming.
NIIMAQis compatible with NIDAQ and all other National Instruments driver software for easily
integrating an imaging application into any National Instruments solution. NIIMAQ is included
with your hardware at no charge that you can call from your application programming environment.
These functions include routines for video configuration, image acquisition (continuous and
singleshot), memory buffer allocation, triggercontrol and board configuration. NIIMAQ performs
all
functionality required to acquire and saveimages. For image analysis functionality, refer to the
IMAQ Vision software analysis librarieswhich are discussed later in this course. NIIMAQ resolves
many of the complex issues betweenthe computer and IMAQ hardware internally, such as
programming interrupts and DMA controllers.

Fig 11: NI IMAQ functions

NIIMAQ and IMAQ Vision use five categories of functions to acquire and display images:
(2) Utility functions—Allocate and free memory used for storing images; begin and endimag
acquisition sessions

(3) Single buffer acquisition functions—Acquire images into a single buffer using the snap
and grab functions

(4) Multiple buffer acquisition functions—Acquire continuous images into multiple buffers
using the ring and sequence functions

(5) Display controls—Display images for processing

(6) Trigger functions—Link a vision function to an event external to the computer, such as
receiving a pulse to indicate the position of an item on an assembly line

Snaps and grabs are the most basic types of acquisitions. A snap is simply a snapshot, in which you
acquire a single image from the camera. A grab is more like a video, in which you acquire every
image that comes from the camera. The images in a grab are displayed successively ,producing a
full motion video, consisting of around 25 to 30 frames per second. IMAQ Snap, IMAQ Grab Setup
and IMAQ Grab Acquire are used to snap and grab images.
IMAGE PROCESSING TOOLS AND FUNCTIONS IN IMAQ VISION

Utility functions include VIs for image management and manipulation, file management, calibration
and region of interest processing. Image processing functions include VIs for analysis, color
processing, frequency processing, filtering, morphology, operations, and processing,
includingIMAQ Histogram, IMAQ Threshold and IMAQ Morphology. Machine vision VIs are
used for common inspection tasks such as checking for the presence or absence of parts in an image
or measuring dimensions in comparison to specifications. Some examples of the machine vision
Visage the caliper and coordinate system VIs.

Image processing is a time-consuming process, both in computer processor time and development
time. National Instruments has developed an application to accelerate the design time of a machine
vision application..IMAQVision Assistant allows even the first time vision developer to learn image
processing techniques and test inspection strategies. In addition, more experienced developers can
develop and explore vision algorithms faster with less programming.