Programmable

Logic Controllers
Third Edition

Frank D. Petruzella
McGraw-Hill
 Chapter 2

PLC Hardware
Components
 I/O SECTION
Input and output (I/O) modules enable the PLC
to both sense and control a process.

The I/O section consists of an I/O rack and

individual I/O modules.
 Input Interface
Input interface modules accept signals from the
machine or process devices and convert them into
signals that can be used by the controller.

Process
The input interface allows
Input Module status information
regarding processes to be
communicated to the CPU.
Sensor
 Output Interface
Output interface modules convert controller signals
into external signals that can be used to control
the machine or process.

Pilot Light
External Output
From Output Module
The output interface allows
the CPU to communicate
operating signals to the
process devices under its
control.
 Logical Rack

A chassis is a physical hardware assembly that houses

devices such as I/O modules, processor modules, and
power supplies.
A logical rack is an addressable unit consisting of
128 input points and 128 output points.
 Logical Rack
A word in the output
image table file and its
corresponding word in
the input image table
file are called an I/O group. Input and output addresses
A rack uses 8 words
for the PLC-5 family of
in the input image table
controllers are in the octal
file and 8 words in the
numbering system (numbers
output image table file.
8 and 9 are not used).

A rack can contain a

maximum of 8 I/O
groups.
 Remote I/O Rack
One benefit of a PLC system is the ability to locate the I/O modules
near the field devices to minimize the amount of wiring required.

To communicate with the processor,

the remote rack uses a special
communications network.
I/O Module Addressing
The location of a module within a rack
and the terminal number of the module
to which an input or output device
is connected will determine the device’s
address.

Each input and output device must have

a specified address.

This address is used by the processor

to identify where the device is located
in order to monitor or control it.
 I/O Module Addressing

In general, the basic addressing elements include:

Type – the type determines if an input or

output is being addressed.

Slot – the slot number is the physical location

of the I/O module.

Word and Bit – the word and bit are used to

identify the actual terminal connection
in a particular I/O module.
PLC-5 Addressing
SLC 500 Addressing
 Discrete I/O Module Addressing

Input module in slot 0, terminal 0 Output module in slot 0, terminal 0

Input module in slot 0, terminal 5

Output module in slot 0, terminal 6
Analog I/O Module Addressing
 Symbolic Addressing

Symbolic addresses are real names or codes the programmer can substitute for
a logical address because they relate physically to the application.
They are a physical name convention for a location in the data table. In this
example, the symbolic addresses are LS_3 and pump_14, while the actual
addresses are I:3/3 and O:4/14, respectively.
Allen-Bradley ControlLogix controllers use a type of symbolic addressing format. Each
bit is referenced by a tag name that is used to identify the memory location in the
controller.
 Typical Combination I/O Module
Consists of a printed circuit board
and a terminal assembly. The printed
circuit board contains the electronic
circuitry used to interface the circuit
of the processor with that of the input
or output device.

It is designed to plug into a slot or connector in the

I/O rack, or directly into the processor.

The terminal assembly, which is attached to the front

edge of the printed circuit board, is used for making
field-wiring connections.
Typical Combination I/O Module

Status lights for each of the

inputs and outputs

Connections to the power supply

used to power the inputs and
outputs.

Terminals for each input and output

connection
Most modules have plug-on wiring terminal strips. The
terminal strip is plugged into the actual module.
If there is a problem with a module, the entire strip is
removed, a new module is inserted, and the terminal
strip is plugged into the new module.
 I/O Module Cards

I/O modules can be 8, 16, or 32 point cards.

The number refers to the number of inputs or outputs available.
The standard I/O module has eight inputs or outputs.
A high density module may have up to 32 inputs or outputs.

The advantage with the high-density module is that it is possible to install 32

inputs or outputs in one slot for greater space savings. The only
disadvantage is the high-density output modules cannot handle as much
current per output.
1. Which of the following allows status information
regarding processes to be communicated to
the CPU?

(a) Input interface module

(a) Output interface module

(a) Power supply

(a) Logical rack

2. A logical rack is an addressable unit consisting
of ______ input points and _____ output points.

(a) 8, 16

(a) 16, 16

(a) 34, 34

(a) 128, 128

 3. Locating the I/O modules near the field devices
will minimize the amount of wiring required.
(True/False)

4. Each input and output device must have

a specified address. (True/False)

5. Most modules have plug-on wiring terminal

strips. (True/False)
6. The standard I/O module has _____ inputs or
outputs.

(a) 128

(a) 64

(a) 32

(a) 8
7. Which of the following is not normally an
addressing element?

(a) If an input or output is being addressed.

(a) The slot in which the I/O module is located.

(a) The number of inputs or outputs available

(a) The terminal connection point on the

I/O module.
8. The location of a specific input or output field
device is identified by the processor by means of its
function. (True/False)

9. Status indicators are provided on each output of

an output module to indicate that the input
associated with the output is active. (True/False)

10. The location of a module within a rack and

the terminal number of the module to which an
input or output device is connected will
determine the device’s address. (True/False)
 Discrete I/O Modules

Discrete type I/O interface modules connects field

devices of the ON/OFF nature.

Discrete input modules

are used with field control
devices such as selector
switches, pushbuttons,
and limit switches.
 Discrete I/O Modules

Discrete type I/O interface modules connects field

devices of the ON/OFF nature.

Discrete output modules

are used with field load
devices such as lights,
small motors, solenoids,
and motor starters that
require simple ON/OFF
switching.
Combination I/O Module

The arrangement of the terminals,

status
Status
indicators, and power supply
connections
Indicators may vary.

I/O modules can have both input

and output connections in the same
physical module.
Input Connections
Power Supply
A module inserted into the wrong
Connections
slot could be damaged by improper
Output
voltages Connections
connected through the
wiring arm.
 Discrete I/O Modules
The classification of discrete I/O covers bit-orientated inputs and outputs. In this
type of input or output, each bit represents a complete information element in
itself.
Each discrete I/O module is powered by some field-supplied voltage source
of a specified value.
Common voltage ratings are:

Input Interfaces Output Interfaces

12 V ac/dc 12-48 V ac
24 V ac/dc 120 V ac
48 V ac/dc 230 V ac
120 V ac/dc 120 V ac
230 V ac/dc 230 V dc
5 V dc (TTL level) 5 V dc (TTL level)
24 V dc
 Discrete I/O Modules
The I/O modules receive their module voltage and
current from the power supply module through the
backplane of the rack enclosure into which they are
inserted. Power from this supply is used to power the
electronics, that reside on the I/O board. The relatively
higher currents required by the loads of an output
module are supplied by user-supplied power.
 AC Discrete Input Module
The input circuit is composed of two basic sections:
the power section and the logic section.

The power and logic sections are normally coupled

together with a circuit, which electrically separates
the two.
 AC Discrete Input Module Operation
When the pushbutton is closed,
120 VAC is applied to the bridge
rectifier through R1 and R2.

This produces low level DC voltage across the LED.

The resulting light switches the phototransistor into conduction and
the closed status of the pushbutton is communicated to the processor.
Input Module Tasks

Sense when a signal is received

from a sensor

Convert the input signal to the

correct voltage level for the
particular PLC

Isolate the PLC from

fluctuations in the input signal’s
voltage or current

Send a signal to the processor

indicating which sensor originated
the signal
 AC Discrete Output Module
The output circuit is composed of two basic sections:
the power section and the logic section, coupled by
an isolation circuit.

The output interface can be though of as a simple

electronic switch to which power is applied to
control the output device.
 AC Discrete Output Module Operation

The processor sets the output status

according to the logic program.

When the processor calls for This in turn switches the

an output, a voltage is applied Triac into conduction which,
across the LED of the isolator, in turn, turns on the lamp.
which switches the
phototransistor into conduction.
AC Discrete Output Module
Provided with LEDs that indicate
the status of each output.

Fuses are generally required for

each circuit. Some modules also
provide visual indicators for
fuse status.

Individual AC outputs are

usually limited to 1 or 2 amps.
The maximum current load
for any one module is also
specified.
 Interposing Relay Connection
Used for controlling larger loads such as motors.

The low current output

from the module operates
the relay coil, the contacts
of which operate the
motor starter.
 Types Of Discrete Output Modules
Are used to turn two-state devices either ON or OFF

Triac outputs can only be used only for control

of AC devices.

Transistor outputs can be used only for control

of DC devices.

Relay outputs can be used with AC or DC devices.

However they have a much slower switching
time compared to solid-state outputs.
 Sinking and Sourcing DC References
The design of DC field devices typically require that
they be used in a specific sinking or sourcing circuit,
depending on the internal circuitry of the device.

Sourcing device
with sinking
input module
circuit
 Sinking And Sourcing References
Sinking and sourcing references are terms used to
describe a current signal flow relationship between
DC field input and output devices in a control system
and their power supply.

Sourcing device
with sinking
output module
circuit
 Sinking And Sourcing References
Sinking device
with sourcing
input module
circuit

Field devices connected to the negative side (DC common)

of the field power supply are sinking field devices

Sinking device
with sourcing
output module
circuit
11. A switch or pushbutton is
a(n)___________ input.
(a) analog
(b) discrete
(c) power
(d) ROM
 12. I/O modules cannot have both input and
output connections in the same physical module.
(True/False)

13. 600 VAC is a common voltage rating for a

discrete input module. (True/False)

14. The relatively higher currents required by the

loads of an output module are supplied by
user-supplied power rather than the power supply
module. (True/False)
15. An opto-isolator is used to:
(a) rectify AC inputs
(b) control AC outputs
(c) electrically isolate the processor
circuitry from the inputs and outputs
(d) electrically isolate the inputs from the
outputs
16. The __________ module senses when a
signal is received from a sensor.
(a) input
(b) output
(c) communications
(d) power supply

17. The output interface can be thought of as

a(n) ________to which power is applied to
control the output device.
(a) amplifier
(b) rectifier
(c) switch
(d) filter
18. Fuses are generally required for each circuit of
the _________ module.
(a) input
(b) output
(c) processor
(d) power supply

19. Individual AC output module currents are

usually limited to ______ amps.
(a) 1 or 2 amps
(b) 10 or 20 amps
(c) 15 or 30 amps
(d) 50 or 100 amps
20. Which type output modules can be used
with AC or DC devices?
(a) Transistor
(b) Triac
(c) Integrated Circuit
(d) Relay
 Discrete Versus Analog Devices

Discrete devices are inputs ON/OFF

and outputs that have only toggle switch
two states: on and off.

Analog devices are inputs

Analog
and outputs that can have
control valve
an infinite number or states.
 Analog Input Interface Module

A varying low DC
voltage proportional
to the temperature
being monitored is
produced by the
thermocouple.

This voltage is amplified and digitized

by the analog input module and then
sent to the processor on command from
a program instruction.

There
Typicalare two basic
analog types
inputs mayofvary
analog
from
input
0 to 20modules available:
milliamperes, 4 tocurrent
20 milliamperes,
sensing and
or 0 to 10 voltage sensing.
volts
 Analog Output Interface Module
Both analog inputs and outputs are word-oriented.

The analog output

interface module
receives from the
processor digital
data, which are
converted into a
proportional voltage
or current to control
an analog field device.
 Analog I/O Control System

The valve is initially

open 100%.

As the fluid level in the tank

approaches the preset point,
the processor modifies the output,
which adjusts the valve to maintain
a set point.
Special I/O Modules
High-Speed Counter Module
Used to provide an interface for
applications requiring counter
speeds that surpass the capacity
of the PLC ladder program.
They have the electronics needed
to operate independently of the
processor.

A typical count rate is 0 to 75 kHz,

which means the module would be
able to count 75,000 pulses per
second.
Special I/O Modules

Thumb-Wheel Module

Allows the use of thumb-wheel

switches for feeding information
to the PLC to be used in the
control program.
 Special I/O Modules
Encoder-Counter Module

This module allows the user to read the signal from the
encoder on a real-time basis and stores this information
so it can be read later by the processor.
 Special I/O Modules
Stepper-Motor Module

This module provides pulse trains to a stepper-motor

translator, which enables control of a stepper motor.
 Special I/O Modules
BCD-Output Module

This module enables a PLC to operate devices that

require BCD-coded (binary coded decimal) signals
such as seven-segment displays.
 Intelligent I/O Modules
Have their own microprocessor on board that
can function in parallel with the PLC.

PID module is used in process control applications

that incorporate PID algorithms. The PID module
allows process control to take place outside the CPU.
 Intelligent I/O Modules

Communications Module

This module allows the user to connect the PLC to

high-speed local networks that may be different from
the network communications provided with the
PLC.
 I/O Specifications
▪ Nominal Input Voltage
This AC or DC value specifies
the magnitude and type of voltage
signal that will be accepted.

▪ On State Input Voltage Range

This value specifies the voltage
at which the input signal is
recognized as being absolutely on.
▪ Nominal Current Per Input
This value specifies the minimum input current that
the input devices must be capable of driving to operate
the input circuit.
 I/O Specifications
▪ Number of Inputs and Outputs per Card
This value indicates the number
of inputs or outputs that can be
connected to the module.
▪ Off-State Leakage Current per
Output
This value specifies the maximum value
of leakage current that flows through
the output in the OFF state.
▪ Maximum Output Current Rating Per Output
and Module
These values specify the maximum current of a single
output and the module as a whole.
 CPU - Processor
The term CPU is often used interchangeably with the
term processor. However, by strict definition, the CPU
term encompasses all the necessary elements that form
the intelligence of the system.
 Processor Unit

Status indicators provide system diagnostic information.

Keyswitch allows you select different modes of operation.

The processor executes the operating system, manages

memory, monitors inputs, evaluates the user logic, and
turns on the appropriate outputs.
Typical Processor Modes Of Operation

RUN Position
⮚ Places the processor in the Run
mode

⮚ Executes the ladder program and

energizes output devices

⮚ Prevents you from performing

online
program editing in this position

⮚ Prevents you from using a

program/operator interface device
Typical Processor Modes Of Operation

PROG Position
⮚ Places the processor in the program
mode
⮚ Prevents the processor from
scanning
or executing the ladder program, and
controller outputs are de-energized
⮚ Allows you to perform program
entry and editing
⮚ Prevents you from using a
program/operator interface device
to change the processor mode
Typical Processor Modes Of Operation
REM Position
⮚ Places the processor in the Remote
mode: either REMote Run, REMote
Program, or REMote Test mode

⮚ Allows you to change the processor

mode from a programmer/operator
interface device

⮚ Allows you to perform program

online program editing
 Electrostatic Discharge Protection
Many electronic components
found in modules are
sensitive to electrostatic
voltages.

Ground yourself before handling static-sensitive parts.

Wear a wrist strap that provides a path to bleed
off charges.
Be careful not to touch the backplane connector or
connector pins of the PLC system.
Create a static free work area by covering your
work bench and floor area with a conductive surface
that is grounded.
 Memory Design

Memory is a physical space

inside the CPU where the
program files and data files
are stored and manipulated.

Program File Data File

 Memory Requirements
The complexity of the program determines the amount of
memory required.

The program is stored in memory as 1’s and 0’s,

which are typically assembled in the form
of 16-bit words.
 Memory Requirements
Memory sizes are commonly expressed in thousands
of words (K) that can be stored in the system.

1K
64 K 2M

MicroLogic 1000 SLC 500

Controller Controller
1 K memory ControlLogix
Up to 64 K memory Controller
Up to 20 inputs Up to 4096 Inputs
Up to 14 outputs Up to 160 K through
and Outputs 2 M memory
Up to 128,000 Inputs
and Outputs
 Memory Design
Memory location refers to an address in the CPU’s
memory where a binary word can be stored. A word
usually consists of 16 bits.

Memory utilization refers to the number of memory

locations required to store each type of instruction.

A rule of thumb for memory

location is one location per Contact
coil or contact. One K of
memory would then allow
a program containing 1,000
Coil
coils and contacts to be
stored in memory.
 Input Status Table Or File
The memory of a PLC is divided into sections that have
specific functions.
If the input is closed the bit is
Each input has set to 1
one corresponding
bit in memory

If the input is
open the bit is set to 0
 Output Status Table Or File

Each output has one corresponding If the bit is a 0, the output

bit in memory will be OFF

If the bit is a 1, the output will be ON

 Status Table

Status Table Files: Stores information such as scan time, fault

codes, and watch dog timer; and some have
precision timing bits for use in the control
program.
 Timer Table

Timer Files: Are usually three words long. One word contains
timer status information; another contains the
preset value or set-point; the last contains the
accumulated value.
 Counter Table

Counter Files: Also three words long, having the same

configuration as the timer.
 Volatile Memory

Volatile memory will lose its stored information if

all operating power is lost or removed.

Volatile memory is easily

altered and quite suitable
for most applications when
supported by battery
backup.
 Nonvolatile Memory
Nonvolatile memory has the ability to retain stored
information when power is removed accidentally
or intentionally.

Although nonvolatile memory

generally is unalterable, there
are special types used in which
the stored information can be
changed.
 Memory Types

As the name implies, programmable logic controllers

have programmable memory that allows users to develop
and modify control programs. Data are stored in memory
locations by a process called writing. Data are retrieved
from memory by what is referred to as reading.

Read-Only Memory (ROM):

Is designed so that information
stored in memory can only be
read, and under ordinary
circumstances cannot be changed. ROM memory is used
by the PLC for permanent storage of the operating system.
Memory Types

Random Access Memory (RAM):

Is designed so that information
can be written into or read from the
memory. Commonly used for the
user’s program, timer/counter values,
and input/output status.
 Memory Types
Electrically Erasable Programmable Read-Only Memory
(EEPROM):
Is a nonvolatile memory
that offers the same flexibility
as does RAM. The EEPROM
can be electrically overwritten
with new data. Since it is
nonvolatile, it does not require
battery backup.
Typically, an EEPROM
memory module is used
to store, back up, or transfer
PLC programs.
 User Communications With PLC Circuits

Input Central Output

Module Processing Module
Unit

Programming Operator
Device Interface

The programming device

The operator interface device
is used to input the desired
allows the process information
instructions. These
to be displayed and new control
instructions determine what
parameters to be entered.
the PLC will do for a specific
input.
 Hand-Held Programming Device

Has a connecting cable so

that it can be plugged into
a PLC programming port.

Are compact, inexpensive,

and easy to use.

Contains keys for instruction entering and editing,

and navigation keys for moving around the program.

Have limited display capabilities!

 Personal Computer Programming Device

⮚Manufacturers
⮚ The computer can software required.
be used to document the
⮚Large
program.amounts of logic can be displayed.
⮚Circuit
⮚ PC software
elements can becut-and-paste
provides highlighted infeatures
color for
to indicatedeveloping
program status. and editing.
⮚More
⮚ Allows than
easyone program of
monitoring candata
be stored
tables.on
⮚ Easy to make copies of the program on floppy disk,
the
computers hard drive.
 Changing Programs

The PLC can have only

one program in memory
at a time!

To change the program in the PLC, it is necessary to

enter a new program directly from the keyboard or to
download one from the computer hard drive.
PLC Workstation

⮚ Shows operation in graphic format for easier viewing

⮚Allows operator to change settings using a touch
screen
⮚Shows alarms, complete with time of occurrence and
location
21. An analog signal:
(a) can only be in one of two possible states
(b) varies with time but not magnitude
(c) is repetitive over a given period of time
(d) can have an infinite number of states
22. Both analog inputs and outputs are
bit-oriented. (True/False)

23. Which module enables a PLC to operate a

seven-segment display?
(a) High-Speed Counter
(b) BCD-Output Module
(c) Stepper-Motor Module
(d) Encoder-Counter Module

24. Intelligent I/O modules have their own

microprocessor on board. (True/False)
25. A closed contact would have a binary 1 stored
in its respective location in the input status file.
(True/False)

26. An output that is “on” would have a binary 1

stored in its respective location in the output
status file. (True/False)

27. Hand-held programming devices have limited

display capabilities. (True/False)
 28. PLC memory is:
(a) a physical space inside the CPU
(b) where data files are stored
(c) where program files are stored
(d) all of these

29. The PLC can have more than one program

in memory at a time. (True/False)
30. Which type of memory is commonly used to
back up a program?

(a) ROM
(b) RAM
(c) PROM
(d) EEPROM