Lessons · Industrial control · an output: what the PLC drives
Outputs: how the PLC acts
A motor starter, a valve, a lamp is driven by an output. The program sets output bits; at the end of the scan the PLC writes them to the terminals all at once.
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
Outputs are the whole reason the PLC exists. Understanding that they are written once per scan, after every rung has been solved, explains why two rungs writing the same coil fight.
How to think about it
One coil per output, written from exactly one rung. Put every condition for that output on that rung, in series and in branches, rather than writing the coil in several places.
Worked example
[Ready] [Start] (Motor)Motor is an output: set by this rung, written to the terminal at the end of the scan.
[Motor] (RunLamp)An output can be read as a contact: the lamp mirrors the motor.
Your turn
Drive the valve when the tank is low.
[TankLow] ()
Run a rung and watch the scan
The trap
Two rungs writing the same coil: the lower one wins every scan, and the upper one might as well not exist.