Introduction to PLC and SCADA Systems - EIT

1 Introduction to PLC and SCADA Systems Introduction Programmable logic controller Necessity is the mother of invention, states a well-known quotation. In the past years, the manufacturing process was cumbersome and often the desired quantity and quality were not achieved. Since the relays and timers such as cam timers are considered mechanical devices, the lifetime of their functionality is limited and quite often they become cumbersome, especially in large applications where thousands of them are required. As the electrical relays have to be hard-wired, and as many relays have to be worked with, the trouble-shooting can become complicated. The first programmable logic controller (PLC) project was created in 1968, by Bedford Associates, MA, USA, who had initiated the development of an electronic device called the Modular Digital Controller for controlling the operations in an automobile assembly line in General Motors (GM).

The processor of a PLC (Courtesy: Allen-Bradley) The main processor (or CPU) is a microprocessor-based system that executes the control program, after reading the status of the field inputs. It, in turn, sends commands to the field outputs. Depending on the manufacturer, it is possible to find bit or word processors. If a bit processor is

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