Write the normal exchange in plain language with one writer per signal. Request held until an agreed response, acceptance separate from completion, and explicit clearing conditions. The notes should describe both sides of the same conversation.
I'd bring whoever supports each side into that discussion. We can make a tidy table in one room and still leave the relief technician with two incompatible drawings when the application changes.
Yes. I said normal exchange as a starting point, not the whole specification. Startup, timeout and retained states need their own decisions, including when work is uncertain and must not be automatically repeated.
Then those are the two questions for the meeting, not just approve the interface. A named review is useful, but your operators still need the agreed waiting and escalation states before this is handed over.