Automated machinery should not command an action merely because an operator or sequence requested it. The controller must also check whether current conditions make the action allowed.

A permissive is a condition that must be true before an action may start. An interlock prevents or removes a command when an incompatible process condition exists.

For example, a spindle command might be defined conceptually as

$$Run = StartRequest \land GuardClosed \land ClampConfirmed \land \neg Inhibit.$$

The request alone is insufficient. If the guard is open, the clamp is not confirmed, or an inhibit condition is active, the run command remains false.

Permissives are often associated with prerequisites for starting:

part present
AND clamp confirmed
AND tooling ready
        ↓
operation permitted

Interlocks also prevent incompatible outputs from being active together. If two actuators must never command opposite motions simultaneously, each command can inhibit the other according to the physical machine design.

Good control logic distinguishes the requested action from the conditions that authorize it. This makes machine behavior easier to review because process requirements are represented explicitly instead of being scattered through unrelated program branches.

Interlocks should be derived from the physical process and risk analysis. Ordinary control logic can coordinate operation, but protective functions whose failure could cause harm may require dedicated safety-rated architectures rather than ordinary PLC code.