Which PLC function makes the state of two mechanisms mutually dependent?

• A. A data logger
• B. A safety relay
• C. Interlock
• D. A timer function

Answer: (C)

Interlock is a control feature that binds the operation of two mechanisms so that one cannot proceed unless the other is in a safe or specific state. This mutual dependency ensures coordinated, safe operation between parts of a system. For example, in a machine with a guarding door and a press, the interlock prevents the press from starting while the door is open; only when the door is closed and the interlock confirms that state can the machine run. If the door state changes to unsafe during operation, the interlock can stop the machine to keep the two parts in compatible states. This mutual dependency is what differentiates an interlock from other PLC functions: a data logger merely records data, a safety relay provides protective action without tying two devices’ states together, and a timer handles delays or sequencing without enforcing dependence between two mechanisms.