Lessons · Industrial control · an input: what the PLC can see
Inputs: how the PLC senses
A button, a limit switch, a sensor wires to an input. Each scan the PLC reads every input into memory, and the program reads that copy.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
The program never touches the wire; it reads the snapshot taken at the top of the scan. That is why an input cannot change halfway through the program, and why very short pulses can be missed.
How to think about it
Name inputs for what they sense (DoorClosed, PartPresent), not for the terminal number, and use them as contacts on the left side of rungs.
Worked example
[PartPresent] (Clamp)PartPresent is a sensor wired to an input; the rung reads its snapshot.
[DoorClosed] [KeyOn] (Ready)Two inputs in series: both must be on.
Your turn
Read the stop button as a condition.
[] (Motor)
Run a rung and watch the scan
The trap
A pulse shorter than one scan can arrive and vanish between two reads. Fast events need an input filter, a latch, or a faster scan.