Explain the function of a 'watchdog timer' in a PLC system.

The function of a watchdog timer in a PLC system is to provide a safety mechanism that detects and recovers from program malfunctions or hardware failures that could cause the PLC to stop executing its program correctly. The watchdog timer is a timer circuit that must be periodically reset by the PLC program. If the PLC program fails to reset the watchdog timer within a specified time interval, the timer will expire and trigger a predefined action, typically a system reset or a shutdown of the controlled process. This ensures that the PLC does not remain in an undefined or potentially dangerous state due to a program crash, infinite loop, or hardware fault. The watchdog timer prevents the system from becoming unresponsive or operating in an uncontrolled manner. The programmer configures the watchdog timer with a specific timeout value, which represents the maximum time that the PLC program is expected to take to complete its normal execution cycle. The PLC program then includes a routine that resets the watchdog timer at the end of each cycle. If the program execution is interrupted or stalled, the reset routine will not be executed, and the watchdog timer will expire, triggering the fault response. This fault response often includes shutting down the controlled equipment in a safe and orderly manner.