Difference between hard wired control and PLC control?

Hard-wired control systems

In hard wired control systems, relays are used. For example: In Electrical-control, the wiring of control elements such as sensors, solenoids, counters etc. are through relays control. Such relay controlled systems are also called as hard-wired control system because any modification in control program involves rewiring of the circuit.

Therefore, hardwired controls are cumbersome and difficult to modify when production requirement changes regularly. Hard-wired control systems are difficult to maintain because any small problem in design could be a major problem in terms of tracing and rewiring.

Hard wired control systems consists of three division

1. Input section – Consists of push –buttons, switches and sensors. They transfer signals to the
processing section

2. Processing section – Consists of relay coils and contacts. They determined the relationship
between the inputs received and outputs required

3. Output section – Consists of solenoids, lamps, and contactor coils etc. The processed signals
are transferred to this section.

PLC Systems.

PLC systems offer number of advantages over hard wired electromechanical relay control
systems. Unlike the electromechanical relays, PLCs are not hard-wired to perform specific
functions. Thus, when system operation requirement change, a software program is readily
changed instead of having to physically rewire relays.

In addition, PLCs are more reliable, faster in operation, smaller in size, and can be readily expanded.

PLC systems consists of three division

1. Input section – Consists of push –buttons, switches and sensors which are connected to
specific input addresses in the program. They transfer address information to the processing

2. Processing section – The microprocessor receives the input signals from input sections and
executes the information (called instructions) in the software program and sends the processed
signals to output section

3. Output section – Takes the signal from processing section and modify the signal from the
processor to operate output devices connected to specific output addresses.


Excellent explanation. Could you please provide source also?