PLC inputs and outputs

A PLC has two kinds of connection. Inputs tell it what is happening. Outputs are the things it switches on and off. That is all a PLC ever deals with.

Inputs: they tell the PLC something

An input carries a signal into the PLC. Is the Start button pressed? Is the tank full? Is the door shut? How hot is the water?

Buttons, switches and sensors are all inputs.

Outputs: the PLC tells them what to do

An output carries a signal out of the PLC, to something that acts. Run the pump. Open the valve. Turn on the warning light. Sound the horn.

Motors, pumps, valves, lights and alarms are all outputs.

A light switch, done the PLC way

Think of the light in your kitchen. The switch is the input. The bulb is the output. At home, a wire joins them directly.

In a factory, the switch is wired into the PLC, and the PLC decides whether to light the bulb. That means you can change the rules without rewiring anything. For example: only light the bulb if the door is shut.

Always look from the PLC's side

This is the one thing that trips people up. A warning light gives people news, but for the PLC it is an output: the PLC tells it to light up. A Start button gives an order, but for the PLC it is an input: it tells the PLC someone pressed it.

So ask one question: does the signal go into the PLC, or come out of it?

Try it

Sort eight parts into inputs and outputs. Each answer is explained before the next part.

Part 1 of 8

Start button

Look from the PLC's side. Which way does the signal go?

Inputs They tell the PLC something

Sorted parts appear here.

Outputs The PLC tells them what to do