What is a PLC System – Different Types of PLCs with Applications

Programmable Logic Controller (PLC) also known as Industrial Computer is the major component in the industrial automation sector. Due to its robust construction, exceptional functional features like PID controllers, sequential control, timers and counters, ease of programming, reliable controlling capabilities and ease of hardware usage – this PLC is more than a special-purpose digital computer in industries as well as in other control-system areas. Different types of PLCs from vast number of manufacturers are available in today’s market. Therefore, in the subsequent paragraphs, let us study about PLCs and their types.

Programmable Logic Controller (PLC)
Programmable Logic Controller (PLC)

What is a PLC System?

PLC is invented to replace traditional control panels whose operations depend on the electromagnetic logic relays that are based on timers in industrial control systems. PLCs are capable of monitoring the inputs continuously from sensors and producing the output decisions to operate the actuators based on the program. Every PLC system needs at least these three modules:

  • CPU Module
  • Power Supply Module
  • One or more I/O Module
PLC Architecture

CPU Module

CPU Module of PLC
CPU Module of PLC

CPU module consists of a central processor and its memory. The Processor is responsible for doing all the necessary computations and data processing by accepting the inputs and producing appropriate outputs. Memory includes both ROM and RAM memories. The ROM memory contains the operating system, driver and application programs, whereas the RAM stores user-written programs and working data. These PLCs use retentive memory to save user programs and data when the power supply breaks or fails and to resume the execution of a user program ones the power is restored. Thus, these PLCs do not need any use of a keyboard or monitor for reprograming the processor each time. The retentive memory can be implemented with the use of long-life batteries, EEPROM modules and flash memory methods.

BUS or Rack

PLC BUS or Rack
PLC BUS or Rack

In some modular PLCs bus or rack is provided in the backplane of the circuit into which all the modules like CPU and other I/O modules are plugged to the corresponding slots. This bus enables the communication between CPU and I/O modules to send or receive the data. This communication is established by addressing the I/O modules according to the location from CPU module along the bus. Suppose, if the input module is located in the second slot, then the address must be I2:1.0 (second slot first channel only as an example). Some buses provide necessary power to I/O module circuitry, but they do not provide any power to sensors and actuators connected to I/O modules.

ABB PLC Power Supply
ABB PLC Power Supply

Power Supply Module

These modules supply the necessary power required for the whole system by converting the available AC power to DC power required for CPU and I/O modules. The output 5V DC drives the computer circuitry, and in some PLCs 24DC on the bus rack drives few sensors and actuators.

I/O Modules

PLC I/O Modules
PLC I/O Modules

Input and output modules of the PLC allow to connect the sensors and actuators to the system to sense or control the real-time variables such as temperature, pressure flow, etc. These I/O modules vary in type, range, and capabilities and some of these include the following:


Digital I/O module: These are used to connect the sensors and actuator that are of digital in nature, i.e., only for switch ON and OFF purpose. These modules are available on both AC and DC voltages and currents with variable number of digital inputs and outputs.

Analog I/O modules: These are used to connect the sensors and actuators that provide the analog electric signals. Inside these modules, analog to digital converter is used to convert the analog to processor understandable data, i.e., digital data. This module’s number of channel’s availability is also can be varied depending on the application,

Communication Interface Modules: These are intelligent I/O modules that exchange the information between a CPU and communication network. These are used for communicating with other PLCs and computers that are placed at a remote or far away distance.

Types of PLCs

Programmable Logic Controllers (PLCs) are integrated as either single or modular units.

An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.

Integrated or Compact PLCs
Integrated or Compact PLCs

A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.

A modular Types of PLC
A modular Types of PLC

Modular PLCs are further divided into small, medium and large PLCs based on the program memory size and the number of I/O features.

Small, Medium and Large sized Types of PLCs
Small, Medium and Large sized Types of PLCs

Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.

Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.

Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.

Some of the manufacturers or types of PLCs are given below:

Manufacturers or Types of PLCs
Manufacturers or Types of PLCs
  • Allen Bradley PLCs (AB)
  • ABB PLCs (Asea Brown Boveri)
  • Siemens PLCs
  • Omron PLCs
  • Mitsubishi PLCs
  • Hitachi PLCs
  • Delta PLCs
  • General Electric (GE) PLCs
  • Honeywell PLCs
  • Modicon PLCs
  • Schneider Electric PLCs
  • Bosch PLCs

Applications of PLC

The below figure shows the operation of a PLC for a simple process control application wherein the conveyor belt operation, the number of boxes’ measurement and other control operations are performed by the PLC. Here, the position sensor and other sensor outputs are connected to the input module of the PLC, and from the output modules – a motor is controlled. When the sensors are activated, then the CPU of the PLC reads the inputs, and correspondingly processes them according to the program and produces the outputs to operate the motor so that the conveyor is controlled.

Applications of PLC
Applications of PLC

PLC and SCADA combination of control structure is mostly used in industrial automation sector and also in electrical utility systems like power transmission and distribution systems. Programmable sequential switching operation is another major application area of the PLC.

Therefore, the selection of PLCs for certain application needs several considerations over different types of PLCs. Thus we believe that the information here about the topic gives you a better understanding well supported by some appropriate and effective images. Do write to us if you have any technical doubts on this topic, and also on PLC based projects for students as well as for industries.

Photo Credits:

  • Programmable Logic Controller (PLC) by blogspot
  • CPU Module of PLC by aotewell
  • ABB PLC Power Supply by tlauk
  • PLC I/O Modules by thomasnet
  • Integrated or Compact PLCs by bse-tech
  • A modular Types of PLC by delta
  • Small, Medium and Large sized Types of PLCs by ecogate
  • Manufacturers or Types of PLCs by amci
  • Applications of PLC by ytimg

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