Comparison between PLC and Relay logic



Relay logic uses relay control in electronic circuits. It is sometimes called a HARD-WIRED control system. When the control logic is installed it can only be changed manually.

The relay system is still preferred in some safety systems as it is very difficult to check the software operation of a PLC for faults

Here the relays are replaced by a microprocessor which is programmed to provide the switching logic.

Online editing of the programs is possible in PLCs. A PLC collects inputs and distributes outputs in the same way as a relay logic circuit.

RAM can be reprogrammed at any time.