Interlocks and permissives are conditions that control whether a machine action is allowed. They help prevent invalid sequences, expose why equipment is waiting, and provide a maintainable relationship between the process and PLC logic.
Permissives
A permissive is a condition that must be true before an action may start. A conveyor might require the guard circuit healthy, downstream ready, drive ready, air pressure available, and no active stop request.
Interlocks
An interlock prevents an action when a conflicting or unsafe condition exists. Examples include preventing two valves from opening together, stopping a pump on a low-level condition, or blocking motion when another axis occupies the required space.
Make the reason visible
A single “not permitted” bit is hard to troubleshoot. Use individual named conditions and HMI diagnostics so maintenance can see which condition is false. Keep standard process interlocks separate from safety functions that require a safety-rated design.
Testing
- Test each permissive independently.
- Verify the expected output remains off when a condition is false.
- Test recovery after the condition returns.
- Confirm alarms identify the first meaningful cause.
Interlocks belong in a broader sequence structure with states, feedback, and timeouts. See What is ladder logic? and TechControl’s PLC programming service.
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