Why a Sequence of Operation Comes Before the Code

What it actually contains

Each step of the cycle in order, written in sentences rather than in code or ladder logic.

The conditions that must be satisfied before each step starts, which is where most of the useful argument happens.

What the machine does when something is wrong: a part missing, a sensor not made, a station faulted mid cycle. This is the part people skip and the part that matters most.

What the operator is expected to do, and what happens if they do something else.

Why it is written before the software

Software written without an agreed sequence encodes the programmer's assumptions. Those assumptions are discovered during commissioning, by which point changing them is expensive.

A written sequence lets both sides disagree on paper, where disagreement costs an afternoon rather than a week of machine time.

It is the reference for everything afterwards

Acceptance testing is run against it, step by step, rather than against a general impression that the machine works.

Operator training uses it, because it is already in plain language.

Troubleshooting uses it, because it says what should have happened.

Any future change is assessed against it, and the document is updated when the change is approved.

For regulated equipment it is not optional

Where the equipment has to be validated, the sequence of operation is what validation references. A machine whose behaviour is not written down cannot be shown to behave as intended.

What to ask for as a buyer

Ask to see the sequence of operation before software work starts, and read the failure cases rather than the happy path.

Ask that acceptance testing be run against it explicitly.

Ask that the as built version be part of the handover package, not an internal document.

If a supplier cannot produce one, that tells you something useful about how the project will run.