Lessons · Industrial control · the double coil trap
One coil, two rungs, one winner
If two rungs write the same coil, the last one each scan wins; the first is overwritten before outputs update.
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
The classic novice fault: an alarm that only sometimes fires.
How to think about it
Does any coil have two rungs? Every coil gets exactly one rung. Combine conditions with a branch (OR) or series (AND) instead.
Worked example
{ [HighTemp] | [LowPressure] } (Alarm)One rung, both conditions.Your turn
Merge two writers into one rung.
{ [A] [B] } (Out)Run a rung and watch the scan
The trap
It 'works' in testing when only the last condition is exercised.
Practise the double coil trap on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.