PLC is the acronym for a programmable logic controller. It is an industrial computer that is one of the most important constituents of the industrial process. The computer regularly overseas the condition of input devices in a company and it is able to achieve this by making decisions that are in line with a custom program.

Benefits of using the PLC

Programmable logic controllers such as the Unitrionics Unistream immediately improve all kinds of production lines and machine functions. However, the main use of the programmable language control is that it has the ability to either alter or copy any operation process while at the same time collating and transmitting important details.


Steve, expert at says “being modular is the second benefit of the PLC system”. “By being modular, it allows the user to combine output and input device to fit the application in use”. He concludes.

History of the programmable logic controller

The PLC was invented by Dick Morley in 1969. This first design was known as model 084. It was, however, not until 1973 that the PLC became a commercial success. The first commercial design of the PLC was the work of Michael Greenberg with the PLC model 184.

With the controllers, the need for rewiring or bringing in additional hardware for every new configuration of logic was eliminated. With the new system, the functionality of the controls was drastically increased while the cabinet space that housed the logic was reduced.

A look into the interior of the PLC

The PLC is made up of a Central Processing unit. It houses an internal program that notifies the PLC of how to perform specific functions. Some of the functions the CPU helps the PLC accomplish include the following:

  • Performance of housekeeping activities such as communications and internal diagnostics
  • Communication with a variety of devices such as I/O devices, programming devices, networks as well as other PLCs.
  • Execution of control instructions that are found in the User’s Programs. The program is stored in non-volatile memory which means that the program will not be lost even when power is disconnected.

How PLCs function

The PLC function is made up of four main operations that continuously run in a repeating loop. The four basic steps in this operation includes: Input Scan, Output Scan, Program Scan and Housekeeping.

The input scan determines the state of all input devices that are connected to the PLC, the program scan executes the user created program logic, the output scan energises and de-energises the output devices connected to the PLC while the housekeeping step involves communications with internal diagnostics, programming terminals etc.

Programming Language Used In Programming A PLC

The Ladder Logic is the major programming language used in PLCs but there are other languages. They include the Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL) Sequential Function Chart (SFC) and the Ladder Diagram (LD).