Lessons · Industrial control · a healthy stop button reads 1
A healthy stop button reads 1
Wired normally closed, the circuit is made at rest. Current flows, and the PLC input is on until somebody presses the button.
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
It reads backwards the first time, and getting it the wrong way round produces a machine that is dangerous rather than broken.
How to think about it
Arrange every input so that a failure produces the harmless result. A stop is normally closed, so losing it stops the machine. A start is normally open, so losing it cannot start anything.
Worked example
{ [Start] | [Motor] } [Stop] (Motor)Stop healthy (reading 1) throughout, Start pressed on scan 2 and released, then the wire breaks on scan 4 and stays broken. Motor: 0 1 1 0 0Your turn
A normally closed stop button, at rest, reads
Run a rung and watch the scan
The trap
The machine going down on a broken wire is not a shortcoming of this design. It is the design.