Lessons · Industrial control · closed in the field, open in the rung
Closed in the field, open in the rung
The input is already 1 when all is well, so the rung wants an ordinary contact: [Stop], not [/Stop].
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 is the most confusing sentence on the road, and the reason is worth saying slowly: normally closed in the FIELD is examined with a normally OPEN contact in the PROGRAM.
How to think about it
Do not reason about the button. Reason about the value the PLC sees. It is 1 while all is well, so an ordinary contact passes power while all is well.
Worked example
{ [Start] | [Motor] } [Stop] (Motor)The right spelling for a normally closed button. Started on scan 2, then the button is pressed (or the wire breaks) on scan 4. Motor: 0 1 1 0 0Your turn
A normally closed button in the field is examined with
[]
Run a rung and watch the scan
The trap
Writing [/Stop] against a normally closed button gives a machine that will not start until the stop is held. That is the merciful case: it announces itself.