Lessons · Industrial control · the reset that can never run
The reset that can never run
A latch with no unlatch is set for good. A latch whose unlatch cannot be reached is exactly as stuck, and looks better.
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
This is the alarm that will not clear, the fault lamp that stays on after the fault has gone, and the machine that 'just needs a power cycle'.
How to think about it
Two checks on every latch, and it is the second one people skip. Is there an unlatch? And can its conditions be true from the state the latch leaves the machine in?
Worked example
[A] (L Lamp) [B] (U Lamp)A on scan 2, B on scan 4. Lamp: 0 1 1 0 -- this one has a reset and it is reachable.
Your turn
The second question to ask about every unlatch.
can its conditions ever be ?
Run a rung and watch the scan
The trap
An unlatch gated on the machine running, for a fault that stops the machine, is a deadlock: the reset needs the state the fault prevents.