University of Baghdad
Al-Khwarizmi College of Engineering
Mechatronics Engineering Department
Programmable Logic Controllers
Asst. Lecturer: Oger Z. Amanuel
References:
• Book of programmable logic controllers by Frank Petruzella
• Book of programmable logic controllers by W. Bolton
Hardware Components
Contents
• List and describe the function of the hardware components used in
PLC systems
• Describe the basic circuitry and applications for discrete and analog
I/O modules, and interpret typical I/O and CPU specifications
• Explain I/O addressing
• Describe the general classes and types of PLC memory devices
• List and describe the different types of PLC peripheral support devices
available
1. The I/O Section
The input/output (I/O) section of a PLC is the section to which all field devices are connected and
provides the interface between them and the CPU. Input interface modules accept signals from the
machine or process devices and convert them into signals that can be used by the controller. Output
interface modules convert controller signals into external signals used to control the machine or process.
A typical PLC has room for several I/O modules, allowing it to be customized for a particular application
by selecting the appropriate modules. Each slot in the rack is capable of accommodating any type of I/O
module. The I/O system provides an interface between the hardwired components in the field and the
CPU.
The input interface allows status information regarding processes to be communicated to the CPU, and
thus allows the CPU to communicate operating signals through the output interface to the process devices
under its control.
The processor receives signals from the remote input modules and sends signals back to their output modules via
the communication module.
A rack is referred to as a remote rack when it is located away from the processor module. To communicate with
the processor, the remote rack uses a special communications network. Each remote rack requires a unique
station number to distinguish one from another. The remote racks are linked to the local rack through a
communications module. Cables connect the modules with each other. If fiber optic cable is used between the
CPU and I/O rack, it is possible to operate I/O points from distances greater than 20 miles with no voltage drop.
Coaxial cable will allow remote I/O to be installed at distances greater than two miles. Fiber optic cable will not
pick up noise caused by adjacent high power lines or equipment normally found in an industrial environment.
Coaxial cable is more susceptible to this type of noise.
Allen-Bradley controllers make a distinction between a PLC chassis and rack. The hardware
assembly that houses I/O modules, processor modules, and power supplies is referred to as the
chassis. Chassis come in different sizes according to the number of slots they contain. In general,
they can have 4, 8, 12, or 16 slots. A logical rack is an addressable unit consisting of 128 input points
and 128 output points. A rack uses 8 words in the input image table file and 8 words in the output
image table file. A word in the output image table file and its corresponding word in the input image
table file are called an I/O group. A rack can contain a maximum of 8 I/O groups (numbered from 0
through 7) for up to 128 discrete I/O.
The PLC’s memory system stores information about the status of all the inputs and outputs. To keep
track of all this information, it uses a system called addressing. An address is a label or number that
indicates where a certain piece of information is located in a PLC’s memory.
In general, rack/slot-based 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. This may be a combination of the
rack number and the slot number when using expansion racks.
Word and Bit: the word and bit are used to identify the actual terminal connection in a particular I/O
module.
The following are typical examples of input and output addresses:
Discrete I/O Modules
The most common type of I/O interface
module is the discrete type. This type of
interface connects field input devices of the
ON/OFF nature such as selector switches,
pushbuttons, and limit switches. Likewise,
output control is limited to devices such as
lights, relays, solenoids, and motor starters
that require simple ON/OFF switching. Each
discrete I/O module is powered by some field
supplied voltage source. Since these voltages
can be of different magnitude or type, I/O
modules are available at various AC and DC
voltage ratings.
The operation of the circuit can be summarized as follows:
• The input noise filter consisting of the capacitor and resistors R1 and R2 removes false signals that
are due to contact bounce or electrical interference.
• When the pushbutton is closed, 120 VAC is applied to the bridge rectifier input.
• This results in a low-level DC output voltage that is applied across the LED of the optical isolator.
• The zener diode (Zd) voltage rating sets the minimum threshold level of voltage that can be
detected.
• When light from the LED strikes the phototransistor, it switches into conduction and the status of the
pushbutton is communicated in logic to the processor.
• The optical isolator not only separates the higher AC input voltage from the logic circuits but also
prevents damage to the processor due to line voltage transients. In addition, this isolation also helps
reduce the effects of electrical noise, common in the industrial environment, which can cause erratic
operation of the processor.
Discrete output modules are used to turn field output devices either on or off. These modules
can be used to control any two-state device, and they are available in AC and DC versions and
in various voltage ranges and current ratings. Output modules can be purchased with transistor,
triac, or relay output.
Triac outputs can be used only for control of AC devices.
Transistor outputs can be used only for control of DC devices.
The discrete relay contact output module uses electromechanical as the switching element.
These relay outputs can be used with AC or DC devices.
Analog I/O Modules
Earlier PLCs were limited to discrete or digital I/O interfaces, which allowed only on/off-type devices to
be connected. This limitation meant that the PLC could have only partial control of many process
applications. Today, however, a complete range of both discrete and analog interfaces are available that
will allow controllers to be applied to practically any type of control process.
Discrete devices are inputs and outputs that have only two states: on and off. In comparison, analog
devices represent physical quantities that can have an infinite number of values.
Common physical quantities measured by a PLC analog module include temperature, speed, level, flow,
weight, pressure, and position.
The transition of an analog signal to digital values is accomplished by an analog-to digital (A/D)
converter, the main element of the analog input module. Analog voltage input modules are available in
two types: unipolar and bipolar. Unipolar modules can accept an input signal that varies in the positive
direction only. For example, if the field device outputs 0 V to 110 V, then the unipolar modules would be
used. Bipolar signals swing between a maximum negative value and a maximum positive value.
For example, if the field device outputs 210 V to 110 V a bipolar module would be used.
The resolution of an analog input channel refers to the smallest change in input signal value that can be
sensed and is based on the number of bits used in the digital representation. Analog input modules must
produce a range of digital values between a maximum and minimum value to represent the analog signal
over its entire span. Typical specifications are as follows:
Special I/O Modules
Many different types of I/O modules have been developed to meet special needs. These include:
HIGH-SPEED COUNTER MODULE
The high-speed counter module is used to provide an interface for applications requiring counter speeds
that surpass the capability of the PLC ladder program. High-speed counter modules are used to count
pulses ( Figure 2-26 ) from sensors, encoders, and switches that operate at very high speeds. They have the
electronics needed to count independently of the processor. A typical count rate available is 0 to 100 kHz,
which means the module would be able to count 100,000 pulses per second.
THUMBWHEEL MODULE
The thumbwheel module allows the use of thumbwheel switches for feeding information to the PLC
to be used in the control program. Thumbwheel uses BCD system.
TTL MODULE
The TTL module allows the transmitting and receiving of TTL (Transistor-Transistor-Logic)
signals.
This module allows devices that produce TTL-level signals to communicate with the PLC’s
processor.
ENCODER-COUNTER MODULE
An encoder-counter module allows the user to read the signal from an encoder on a real-time basis and
stores this information so it can be read later by the processor.
BASIC OR ASCII MODULE
The BASIC or ASCII module runs user-written BASIC and C programs. These programs are
independent of the PLC processor and provide an easy, fast interface between remote foreign devices
and the PLC processor. Typical applications include interfaces to bar code readers, robots, printers, and
displays.
BCD-OUTPUT MODULE
The BCD-output module enables a PLC to operate devices that require BCD-coded signals such as
seven- segment displays.
STEPPER-MOTOR MODULE
The stepper-motor module provides pulse trains to a stepper motor translator, which enables
control of a stepper motor. The commands for the module are determined by the control program
in the PLC.
Some special modules are referred to as intelligent I/O because they have their own
microprocessors on board that can function in parallel with the PLC. These include:
PID MODULE
The proportional-integral-derivative (PID) module is used in process control applications that
incorporate PID algorithms. An algorithm is a complex program based on mathematical
calculations. A PID module allows process control to take place outside the CPU. This
arrangement prevents the CPU from being burdened with complex calculations. The basic
function of this module is to provide the control action required to maintain a process variable
such as temperature, flow, level, or speed within set limits of a specified set point.
MOTION AND POSITION CONTROL MODULE
Motion and position control modules are used in applications involving accurate high speed
machining and packaging operations. Intelligent position and motion control modules permit PLCs
to control stepper and servo motors. These systems require a drive, which contains the power
electronics that translate the signals from the PLC module into signals required by the motor.
COMMUNICATION MODULES
Serial communications modules are used to establish point-to point connections with other
intelligent devices for the exchange of data. Such connections are normally established with
computers, operator stations, process control systems, and other PLCs. Communication modules
allow the user to connect the PLC to high-speed local networks that may be different from the
network communication provided with the PLC.
