PLC controllers are low cost, compact, versatile units based on the standard microprocessor architecture used in the control of machines or processes. They are designed for ease of programming and maintenance. The plc systems replace the old relay logic control systems in automated manufacturing and are designed to provide an easy and efficient replacements for the bulky relay logic controllers. The PLC, also known as programmable controller (PC) is defined by the National Electrical Manufacturers Association (NEMA) in 1978 as:

"a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process".

They essentially operate by detecting the on/off (logic) or analog inputs and depending on the control programs - the ladder diagrams - outputs of the same type (usually logic) are produced.

In PLC implementation, field wiring between the logic elements remains unaltered, but there are no more hard wired connections between the devices. Instead, the connections are stored in computer memory. This allows the programming of these connections, which is in turn made easier as they are entered in ladder logic. Ladder logic is a simple programming technique that requires minimal programming training.

PLC systems have considerable advantages over the old relay logic systems. They include:

all the capabilities of the earlier systems,

dramatic performance increase over the relay logic systems

greater reliability

little maintenance due to no moving parts

no special programming skills required by maintenance personnel

physical size of the PLC system is much smaller than the conventional relay based logic

and most importantly much lower cost

Although the PLC systems have many advantages, there are also disadvantages. These include fault finding, as PLC systems are often much more complex than the hard-wired relay systems. Secondly, the failure of the PLC may completely stop the controlled process, whereas a fault in a conventional control system would only disrupt the process. And thirdly, external electrical interference may disrupt the PLC memory.