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Functional Specs

Functional specifications define what a system must do — before defining how it does it. In manufacturing, they are the logic layer that sits between P&ID diagrams and control system implementation.

What a Functional Spec Covers​

A functional specification (FS) bridges the engineer's intent and the programmer's implementation. It describes:

  • Process conditions: What the system should do at each operating state (startup, normal, shutdown, emergency)
  • Interlocks: The safety conditions that override normal operation
  • Setpoints: The targets the control system maintains
  • Sequences: The ordered steps that transition between states
  • Alarms: What triggers operator attention and at what threshold

The relationship to P&ID: The Process and Instrument Diagram shows WHAT is connected. The Functional Specification shows WHAT it does. A P&ID without a functional spec is a map without a legend.

Agent-native manufacturing: As AI agents begin to manage control systems in real time, the functional spec becomes the intent document that the agent executes against. An agent that violates the spec is doing what was not intended. Verifiable intent systems encode the functional spec as the reference against which execution is verified.

Software equivalence: The functional specification in software is the product requirement document (PRD). Both define expected behavior without specifying implementation. Both are the authority when implementation and intent diverge.

Context​

Questions​

At what complexity level does a functional specification require formal methods rather than natural language — and who is qualified to verify the spec is correct?

  • How does the functional specification change when AI agents replace programmers as the primary implementers of control logic?
  • Which aspect of a functional spec — interlocks, sequences, or setpoints — is most commonly wrong in commissioned systems, and what does that imply about the review process?
  • If the functional spec is the verifiable intent document for a manufacturing system, what is the equivalent for an AI agent operating in an unstructured environment?