Programmable Logic Controllers (PLCs) have basic architecture compared to normal general purpose computers. A normal computer can be converted to PLCs by providing a way that the computer can able to receive information or signal from the field devices such as push buttons, switches and valve positions. The computer requires some software to process the information obtained from the input to generate an output which decides whether to close or open the valve position on the process side.
Some of the important features and characteristics that distinguish between the general purpose computers and Programmable Logic Controllers (PLCs) are given below:
- PLCs are designed to operate under industrial environments (PLCs have to operate under a wide range of temperature conditions, humidity and other environmental conditions). They are least affected by the electrical noise and are inherent to electrical noise.
- Programming in PLCs is through Relay Ladder Logic or other easily learned languages. PLCs comes with program language built in its memory. PLCs do not contain input and output devices such as keyboards, mouse, monitor, CD drives and other hard disks. It is simply a self-contained box with communication ports and set of terminals for input and output devices.
- Unlike computers which perform numerous tasks simultaneously, PLCs execute a single program in an orderly and sequential manner from first instruction to the last instruction.
- PLCs have been designed for installation and maintenance by plant electricians. Programming in PLCs is simple (Relay Ladder Programming), it does not include any advanced code. Troubleshooting is simpler and many PLCs are designed to include fault details and written fault details on display screen.