A Watchdog Timer is a timer that is used to make sure some unexpected problem does not keep the system processing as it should. There are several of them in the SLC and they are common to every PLC. In the most common sense (and probably what you are referring to), there is a Watchdog Timer that covers the processor scan. If the scan takes too long to complete due to a poorly written loop or jump routine, the Watchdog Timer will expire and fault out the PLC. This is a major fault and is only recoverable by clearing the fault, fixing the problem, and starting the PLC back up.
It is primary purpose is to prevent the PLC from going into LaLa land (technical term) due to a programming error. You generally will not have a problem with this error. You can change the watchdog timer period in the case that you have a massive program that needs a long time to process. Generally that means you have the wrong processor for the job - need more power. With the SLC there are also hardware watchdogs that look for unexpected hardware issues, STI watchdog, and the program scan watchdog.
The Watchdog Timer was conceived shortly after Digital computer began to be used in control application. According to System Level Integrity there are three basics types of Watchdog Timers which are used in control system supervision:
• Basic CPU Watchdog
• Control system Watchdog
• Communications Watchdog
The most basic is the Watchdog Timer local to the CPU board. This device or circuit monitors CPU activity local to the microprocessor and if it fails to detect CPU activity, it resets the microprocessor. In most control system applications, it is required to supervise the system’s ability to stay in control of a machine or process.