Transducer Interfacing

Instrumentation & Measurement

 Instrumentation Systems
• Two or more instruments are used in
coordinated manner to perform an overall
measuring function, the resulting arrangement is
called Measuring System
• The interconnection of instrument or devices is
called Interfacing.
• There are three basic types of interfacing
– Analog to analog
– Analog to digital
– Digital to digital
 Analog Systems
• Analog signals are characterized by
continuous levels of voltage or current, rather
than discreet or digital
• The signal is conditioned to remove any
unwanted components, while presenting the
proper level to the measuring instrument
• Oscilloscopes, analog meters or analog chart
recorders can be used for displaying results.
• The oscilloscope displays high speed changes and
noise, while both the recorder and analog meter store
and display the short term average values.

• Following characteristics must be considered when

assembling and measuring system

– Signal Level Compatibility

– Impedance Matching
– Susceptibility to noise
– Proper Grounding
– Proper Shielding
– Power line conditioning
– Signal Conditioning
– Ambient Temperature
 Signal Conditioning
• Signal Conditioning usually involves following functions
– Filtering
• Used to reject unwanted component of a signal.
• By using a narrow band-reject, or notch , filter, individual
frequencies such as 60 Hz power-line interface (hum) can be
greatly suppressed or eliminated
– Offset/Level conversion
• Offset/Level conversion either changes the output signal of the
transducer from one voltage or current level to another, or from
one form to other.
• Using current level technique eliminates the effect of voltage drop
and impedance changes within the loop do not affect the signal
level
• Linearization
– When the output signal from a transducer is not a
linear function of the measured variable, it is often
desirable to linearize this relation before passed on to
instrument or recorder.
– For example, if the transducer has an exponential
response, its output must be passed through an
amplifier circuit that has logarithmic response
• Buffering
– Finally, buffering isolates the signal source from its
load.
– This serves both to make the transducer output
independent of load changes and to provide a form of
impedance matching
 Analog-To-Digital Systems
.
 The IEEE-488 Standard and Bus
• IEEE-488 is a short-range digital communications bus
specification. It was created for use with
automated test equipment in the late 1960s, and is
still in use for that purpose. IEEE-488 was created as
HP-IB (Hewlett-Packard Interface Bus), and is
commonly called GPIB (General Purpose Interface
Bus).
• In 1975, the IEEE standardized the bus as Standard
Digital Interface for Programmable Instrumentation,
IEEE-488
• IEEE-488 is an 8-bit, electrically parallel bus. The bus employs
sixteen signal lines — eight used for bi-directional data
transfer, three for handshake, and five for bus management
— plus eight ground return lines.

• IEEE-488 specifies a 24-pin Amphenol-designed micro ribbon

connector

• The IEEE-488 interface bus provides the capability to

interconnect up to 15 devices.

• Once connected together, any one device can transfer data to

one or more other devices on the bus.
• Every device connected to the bus must be able to perform at
least one of the following role: talker, listener, and controller.
 RS-232C Standard
• The RS-232C standard is an asynchronous serial
communication method.
• Serial means that the information is sent 1-bit at a time.
• Asynchronous means that no clock signal is sent with the
data. Each side uses its own clock and a start and stop bit.
Synchronous communication means that a clock signal is sent
in addition to a data signal (or interwoven with it)See
http://en.wikipedia.org/wiki/Asynchronous_serial_communic
ation
• The RS-232C standard works at the physical layer of the
communication standard. This is the lowest level and the one
that physically connects the devices.
• The communication is done through the serial port of the PC.
This is a male connector with 25 (old) or 9 (new) pins, in both
cases only 9 pins, at the most, are used.
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 Why Use a Serial Interface?
• There are several reasons:
– Serial cables can be longer (50 feet) than parallel
cables.
– Serial cables are cheaper than parallel cables, although
there is a cost of converting parallel data to the serial
connection.
– Infra Red are serial. Detecting 1 point of light isn't
easy, try detecting 8 or more points of light.
– Many microcontrollers use SCI (Serial Communication
Interface) to communicate with the outer world.
– SATA disks are serial.

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 The Connector
The pins abbreviations
are (numbers in
parentheses are the
25D pin numbers.
1. CD (8)
2. RD (Rx) (3)
3. TD (Tx) (2)
4. DTR (20)
5. SG (Ground) (7)
6. DSR (6)
7. RTS (4)
8. CTS (5)
9. RI (22)
15
 9D to 25D Conversion
.

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 DTE and DCE
• A DTE (Data Terminal Device) is usually a computer.
• A DCE (Data Communication Device) is usually a Modem. A
Cable connecting 2 PCs is also a Modem. It is called a NULL
Modem.
• The problem with communications is that the DTE speed is
much greater than the DCE speed. The communication
protocol bridges this gap.
• The DCE to DCE speed is also called the line speed.
• Maximum Modem speed today is 56K BPS (56K Bits Per
Second), this is the line speed. On the other hand the
maximum DTE/DCE speed is 115,200 BPS. This is the gap
software must bridge.

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 The Current Loop
• For digital serial communications, a digital current
loop is a communication interface that uses current
instead of voltage for signaling
• Current loops can be used over moderately long
distances (tens of kilometres, 5000feet), and can be
interfaced with optically isolated links
• Long before the RS-232 standard, current loops
were used to send digital data in serial form for
teleprinters. More than two teleprinters could be
connected on a single circuit allowing a simple form
of networking
• Analog current loops are used where a device must
be monitored or controlled remotely over a pair of
conductors. Only one current level can be present
at any time
• For industrial process control instruments, analog
4–20 mA and 10–50 mA current loops are commonly
used for analog signaling, with 4 mA representing the
lowest end of the range and 20 mA the highest.

• The key advantages of the current loop are that the

accuracy of the signal is not affected by voltage drop
in the interconnecting wiring, and that the loop can
supply operating power to the device.

• Even if there is significant electrical resistance in the

line, the current loop transmitter will maintain the
proper current, up to its maximum voltage capability
 Long Distance Data Transmission
• When data is required to transmit over a distance
greater than those permitted by Serial (50ft) or
Current Loop (5000 ft), telephone lines are usually
used.
• Since the telephone system can not handle the
digital signals directly, the digital signals must be
converted to audio tones at the sending ends and
back to the digital signal at the receiving end.
• This conversion is the function of the device called
Modem.