our request disappears before the app sees it

PLC author means 'request offered'. App author means 'request accepted'. Same bit name, different contract.

There's a useful starting point. Define the meaning of each transition as well as its owner, including what remains asserted until the other side confirms it. Names alone won't resolve that disagreement.

Does the handover documentation include what should happen when the PC restarts halfway through, because we inherited a cell where both programs seemed reasonable individually and neither expected to meet the other's old signals on startup?

Include that in the contract. Normal completion, timeout, reconnect and restart need defined outcomes, not an instruction for the operator to keep clearing bits until the screen looks ready again.

No restart definition exists. We're reviewing the job state diagram before changing either program.

Use the missed-request trace as one example for that review. It should explain the observed loss, and the revised design should specify how receipt is confirmed despite different polling schedules.

Our training bench had two writers clearing one status bit. Could you check the actual writers against the proposed ownership table, not only the comments?

Found an old cleanup routine clearing request too. Comments never mentioned it. Added to the review.

That sounds familiar; who will own the final interface document when both programmers leave, so maintenance has one place to look instead of choosing whichever comment seems newest?

Assign a joint acceptance test too. Each side passing its own test isn't enough if the tests assume different clearing rules. Exercise the complete job interface, including interrupted exchanges, in a controlled environment before deployment.

And make the operator recovery procedure match those tested states. Otherwise the software contract will be precise and the shift instructions will still say 'restart and try again', which reintroduces the uncertainty.

Controls lead owns the draft contract. Joint tests and recovery instructions pending; no live interface change yet.