Prior to PLCs, many of these control tasks were solved with contactor or relay controls. This is often referred to as hard-wired control. Circuit diagrams had to be designed, electrical components specified and installed, and wiring lists created. Electricians would then wire the components necessary to perform a specific task. If an error was made the wires had to be reconnected correctly. A change in function or system expansion required extensive component changes and rewiring.
DRAWBACKS:
Bulky and complex wiring.
Difficult to change the logic.
Unreliable.
RELAY :
RELAY LOGIC – AND GATE:
ELECTRONICS CONTROL
Logic gates and Electronic Circuits are used
Ease of programming
Ease of maintenance
Drawbacks:
Difficult to Troubleshoot while Change the Logic of the Process
Difficult to expand
Not suitable for industrial conditions
