Lessons · Industrial control · the watchdog
Sequences and the timer that watches
Step sequences are a chain of latches and timers; a watchdog timer trips if a step takes too long.
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
Batch processes, wash cycles, packaging: most of what PLCs do is steps.
How to think about it
What ends each step, and what if it never comes? Latch Running. Each step: a timer or sensor that ends it and starts the next. Each output = Running AND its step. Unlatch Running at the end.
Worked example
[Go] (L Running)Go latches the sequence on.
[Running] [TON S1 3s]Step 1 timer.
[Running] [/S1.DN] (Fill)Fill during step 1.
[S1.DN] (U Running)End.
Your turn
Gate the output by the step.
[Running] [S1.DN] (Fill)
Run a rung and watch the scan
The trap
No exit from a step when a sensor never arrives. Add a watchdog timer that faults.
Practise the watchdog on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.