Programmable Logic Controllers Basics

Programmable Logic Controllers

A PLC is a microprocessor based controller designed for industrial and commercial applications, they were originally developed to replace relay logic applications. However, because of their flexibility and sophistication they have now developed far beyond there original application. PLC’s are available form very small sizes e.g. 6 to 10 inputs up to controllers capable of handling thousands of inputs. There constituent parts are as follows.

Input modules. The input module receives the signal. I.e. a voltage from the sensor out in the plant. It isolates them from the delicate electronics of the processor and reduces the voltage level to a very low value comparable with the processor. Every sensor or manual control switch in the scheme will have a separate conductor to the terminal of the input module. Naturally the number of input modules required will reflect the complexity of the process to be controlled.

CPU Central Processing Unit. The CPU contains the logic processor, which makes the logical decisions based upon the comparison of the input signals and the stored program. This CPU is protected from external plant voltages by optocupler in both the input and output sides.


Memory. The Memory of the PLC contains the user instructions. I.e the stored programme. This is installed in the memory through a programming device, naturally it must be written in a specific language which is recognised by the memory and the CPU. This can generally take two forms, ladder language or a statement list. These two forms are interchangeable.

Output Module. This is the interface between the CPU and the actuators, (relays, contactors, sounders, valves, indicators). These will normally operate at 24V, 110V, 220V, and 380V. The actuators perform the work.

Programming Unit. This enables the user to input the programme which has been designed to deliver a control solution to the CPU. It can take on of two forms. I.e a dedicated unit specific to the controller or dedicated software operating through a computer.

Simple Programme. Any programme will have inputs and outputs and will operate on the basis of the presents or absents of a signal (voltage). The position and the status of the input is important where the inputs can only have one of two states. Active or inactive. Inputs are designated X1 while outputs are Y1.