PLC as a Computer

A PLC is a computer, but a different type form the one we are probably used to seeing and working with. Most of people are familiar with data-processing computers, especially microcomputers such as those from Apple and IBM.

These machines sit on your desk, or even on your lap, and have powerful systems and applications software that let you play games, do word processing, create computer-aided design (CAD) drawings, and layout spread sheets.

Such computers process reams of data, which is why they are called data-processing machines. Their input peripherals are the keyboard and mouse; their output peripherals, the video display terminal (VDT), Printer, and plotter.

There is another type of computer, however, known as a process-control computer.  Although it, of course, processes data, its main function is to control manufacturing and industrial processes (machinery, robots, assembly lines. etc).

Such computers are said to be event driven. Although they may have a keyboard input peripheral, their control inputs are switches and sensors, and although output peripherals such as VDTs and printers may be attached, the process-control computer primarily controls such devices as motors, solenoids, lights, and heaters. Such process-control computers, which number in the millions, are the control element in virtually all modern factory operations.

PLCs are a type of process-control computer: small relatively inexpensive environmentally hardened, and easy to program, operate, maintain, and repair.  They are often installed close to the machinery or process they control and are thus seen as an extension of industrial equipment.

Although PLCs are similar to ‘conventional’ computers in terms of hardware technology, they have specific features suited to industrial control:

·         Rugged, noise immune equipment

·         Modular plug-in construction, allowing easy replacement/ addition of units (e.g. input/output);

·         Standard input/output connections and signal levels;

·         Easily understood programming language (e.g. ladder diagram or function chart)

·         Ease of programming and reprogramming in-plant

These features make programmable controllers highly desirable in a wide variety of industrial-plant and process-control situations.