Lessons · Industrial control · an output that will not drop out
An output that will not drop out
Two causes, and they are the first two things to check: a path around the stop, or a second rung writing the coil.
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
An output that ignores its stop is the fault that hurts somebody, so it is worth having both causes at your fingertips.
How to think about it
First trace every path through the rung for one that misses the stop. If they all cross it, cross-reference the coil: a lower rung writing it on beats this rung writing it off, every scan.
Worked example
{ [Start] | [Motor] [/Stop] } (Motor)The first cause. Start on scan 2, Stop held from scan 3, Start held again on scan 4. Motor: 0 1 1 1Your turn
If every path crosses the stop and it still will not drop out, look for
a second rung writing the same
Run a rung and watch the scan
The trap
A genuine emergency stop is not left to the program at all. It removes power in hardware, and the PLC is told about it afterwards.