Ladder Logic, Interlocks & State Sequences

Good PLC logic tells a state story. Separate commands, permissives, interlocks, proof, modes, alarms, and safety functions.

Start logic that is reviewable

A conveyor run request might require automatic mode, cell ready, downstream available, drive ready, guards/safety ready, no jam, and no active trip. The output command should be followed by running feedback within a timeout. If proof is absent, remove the command when appropriate and raise a specific fault.

Avoid a single opaque Ready bit with no provenance. Build intermediate tags that answer an operator’s question: Downstream_NotReady, Drive_NotReady, Jam_Detected.

State model

For an index-and-clamp station:

1. Idle: wait for part and start conditions.
2. Indexing: command conveyor; require position proof before timeout.
3. Clamping: command clamp; verify closed and pressure.
4. Processing: command tool; monitor completion and process limits.
5. Unclamping: retract and verify.
6. Complete: transfer permission and cycle count.
7. Faulted: de-energize or hold per risk assessment; announce cause and recovery steps.

Each transition has trigger, guard, actions, timeout, and failure destination. Manual mode should not bypass safety; it changes operational authority and may use reduced, hold-to-run, or step behavior defined by the application.

Alarm quality

An alarm should name condition, consequence, likely checks, and reset criteria. Prevent floods by suppressing dependent symptoms when a root cause is known. Log first-out cause and timestamps. Reset should clear a latched indication only after the underlying condition is healthy; it should not automatically restart motion.

Safety separation

OSHA describes industrial robot safety through risk assessment, safeguards, modes, procedures, training, and validated functions ¹. Application PLC logic can request a stop but must not be mistaken for the safety-related implementation unless designed and validated for that function.

Sequence review

Write pseudocode, ladder, or SFC for the station above. Add sensor stuck-on, stuck-off, part removed midcycle, pressure loss, downstream blocked, power cycle, and manual recovery. Peer-review whether every waiting state ends in completion or a bounded fault.