Datum

What changed? It is the first useful question after any fault, and on most plant machines nobody can answer it. The person who set the machine up has moved on. The change log records intentions, not facts. Four hours in, three parts have been replaced and none of them was faulty.

Datum is a book about building the tool that answers the question. Not a survey of products, and not a tour of a finished program: a build, from an empty file to an installer, in the order it actually happened, with the mistakes left in. You will write a collector that reads a machine and records only the facts that are supposed to hold still, a comparison engine that returns its verdict as an exit code, and a frozen executable that runs from a USB stick on a machine you do not own and changes nothing on it.

Every stage ends with something you can run: not a finished module and not a passing test, but a command you type that produces output on real hardware. And every bug is left in, fifteen of them in the order they were found, including three that turned up inside the tools written to catch bugs. For controls engineers, integrators, and maintenance technicians who have spent an afternoon asking a machine what it used to be.