PLC - different types of PLC

We can choose different types of PLCs based on their function, I/O module and the structure of the PLC. There are two categories in classification:

  • Classification by I/O modules.
  • Classification by application ranges.

Classification by I/O module:

Discrete I/O:

It is the most common type of I / O device. This type of interface connects field input devices of the ON / OFF nature, such as selector switches, push buttons and limit switches. Each bit of discrete input and output represents a complete information element in itself and provides the status of some external contact or advises of the presence or absence of power in a processing circuit.

Analog I/O:

Typical analog inputs and outputs vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. Analog inputs are continuous such as the signal from pressure and temperature transmitter. Analog outputs are control signals used for equipment such as electrical motor drives.

Classification by application ranges:

Integrated or Compact PLC:


Here the I/O capability of the PLC is determined by the manufacturer. A single case contains the several modules which are built-in. The integral PLC has the integral power supply, CPU, I / O interfaces and other components are concentrated housed in a chassis with a compact, small size. Some of the integrated PLCs allow connecting additional I/Os.

Modular PLC:


Modular PLC is made of many several modules such as CPU, I/O modules, Power modules that are plugged into a common rack. It can extend the I/O capabilities. The modular PLC is flexible and can design for the purpose. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.

Small size:

The small size PLC is compact, lightweight, installed next to the equipment to be controlled. This type of PLC is used to replace logics of wired relays, counters, timers, etc.

It is monolithic in construction, Fixed casing, Mono processor and no processor computer capability.

Medium size:

It is the modular design, which allows many plug-in modules. It has up to 2048 I/O’s and memories up to 32 KB. Communication module facilities, input/outputs are provided by adding additional I/O cards.

Large size:

Modular PLC with the network of remote I/Os. It is most sophisticated and used for complex processes. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules.

These PLCs are used in large plants with SCADA and DCS.