Lessons · Industrial control · what changed
What changed
Working logic does not rot. If a machine ran on Friday and not on Monday, something moved.
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 cheapest question you will ask all day, and it beats reading the program often enough to be worth asking first, every time.
How to think about it
Ask three people and believe none of them entirely: what was swapped, what was edited, what was downloaded. Then ask what the machine was DOING when it stopped, because that names the rungs that were live.
Worked example
[Start] [/Stop] [PartPresent] (Motor)Nothing here is wrong, and the timeline shows it: Start held throughout, and PartPresent lost on the second scan. Motor: 1 0 -- the rung did exactly what it says. A sensor knocked out of line will produce this, and no amount of reading the rung will show it.
Your turn
Name the question worth asking before opening the program.
What ?
Run a rung and watch the scan
The trap
'Nothing changed' is what everybody says, and it is usually sincere. Somebody swapped a sensor for one with a different output type and did not think of it as a change.