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.
Now you try
Next in Industrial control
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.