The Types of PLCs

By Benjamin Aries

A programmable logic controller, or PLC, is a special type of computer that is commonly used in commercial and industrial settings. PLCs are designed to run automated systems, such as robotics or other machines, and use inputs and outputs to read and send data. Unlike consumer computers, PLCs are designed to be very rugged and can be operated nearly nonstop.


A unitary PLC is the more simple type of controller, and contains all of the basic system components within a single housing, or box. These components typically include the processor, which runs the software program, in addition to ports for input and output connections. Unitary PLCs are typically attached directly to the device or application that is being controlled.A commonly used example of a unitary PLC type is the Micrologix 1000, built by Allen Bradley. The Micrologix 1000 includes on-board memory for storing programs, 32 digital input and output ports, and a communications port used to program the unit. This setup is typical of many unitary systems.


A modular PLC contains several different modules that can be coupled together to build a customized controller. Typically, a base module contains core functions such as electrical power regulation, the computer processor, and input connections. Additional modules, including analog to digital signal converters or additional outputs, can be added to this core unit as needed. This modular design allows a PLC to be customized and changed easily.The Allen Bradley Micrologix 1200 is a commonly used example of the modular PLC type. This unit is able to handle between 23 and 40 inputs and outputs. The actual number of connections can be expanded easily by adding modules. This provides a wide range of flexibility and is typical of a modular PLC.

Rack Mounting

The rack mounting type of PLC is similar to the modular concept, but is implemented differently. Whereas each module in a modular PLC connects to the base unit directly, a rack mounting PLC keeps each module separate. All extra modules are connected through a network, and modules are held in organized racks. This approach allows for larger systems to be built without becoming overly cluttered and complicated. Modules are well organized on the rack and can be removed and reinserted as needed.The commercial unit SLC 500 is an industry-standard example of the rack mounting PLC type. There are essentially no limits on the number of modules that can be added to this system, each mounted on a standard rack chassis. This setup allows large, scalable automation solutions to be built and is common in factory settings.