Predictable PLC State Machines: A Practical Pattern That Scales
Modern PLC projects suffer when control logic is scattered across rungs, hidden in timer branches, or tightly coupled to raw I/O. The result is brittle behavior, difficult commissioning, and painful offline testing. The approach below is a simple, repeatable pattern for building state-driven modules that stay deterministic, testable, and vendor‑friendly. It divides each routine into clear sections—Inputs → Commands → Interlocks → Permissives → State & Transitions → Status → Control Resources → Outputs—and enforces one rule above all: states govern state; device control happens only in Outputs. Whether you write in ST or Ladder, this structure keeps behavior predictable and easy to review. ...