Programmable Logic Controller
As mentioned previously, the programmable logic controller (PLC or PC) is by far the most important commercially available logic control device. Its development began in 1968 when General Motors Corp. solicited the electronics industry for an alternative to the eletrome-chanical relay logic systems used to control machining, materials handling, and assembly in automobile manufacture. Their relay systems had become very large and complex and required a great amount of rewiring and even outright scrapping when changes in manufacturing processes were needed. This became particularly critical during the years model changeovers typical of the auto industry. A number of manufacturers responded and the software-based programmable controller was born. It could duplicate all the logic control functions of relay networks, but implemented them in software (a program), just like a computer. This meant that process revisions or complete model changeovers did not now require scrapping or tedious rewiring of relay systems, but instead needed only relatively quick and easy changes in the PLC’ s program.
