ABSTRACT
Sequencing has traditionally been realized with relay techniques. Until the beginning of the 1970s, electromechanical relays and pneumatic couplings dominated the market. During the 1970s programmable logical controllers (PLC) became more and more common, and sequencing is normally implemented in software. PLC are microcomputers developed to handle Boolean operations. A PLC produces on/off voltage outputs and can actuate elements such as electric motors, solenoids, fans, heaters, and light switches. The PLC was initially developed by a group of engineers from General Motors in 1968, where the initial specification was formulated: it had to be easily programmed and reprogrammed, preferably in-plant, easily maintained and repaired, smaller than its relay equivalent, and cost-competitive with the solid-state and relay panels then in use. Graphical tools for programming and operator communication are also becoming a standard of many commercial systems. To make programming of PLDs less complex, several software packages are available for personal computers.
