In the late 1970s distributed control systems (DCSs) began replacing individual loop controllers and centralizing the process analog control environment. The first PLCs were large (though much smaller than the walls of relays), and programming was done with dedicated terminals and a very limited instruction set. In the early 1970s the PLC was created and began to be widely used in industrial applications replacing relay systems. This posed many challenges including the consumption of very large spaces for the relays, expensive and time consuming changes, and often monumental troubleshooting efforts. Up until the late 1960s control systems consisted of relays controlling discrete functions and independent loop controllers controlling analog functions.
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