FAQ01120 of Programmable Controllers FAQ

A CPU error will occur when the microcomputer in the CPU Unit does not operate properly.
The watchdog timer can be used to check whether the CPU Unit is operating properly.
The watchdog timer is just one way to check whether the CPU Unit is operating properly.
The watchdog timer is a normal processing routine that runs a timer that will be reset if an instruction is executed within a set period of time. If the set period of time passes and the timer is not reset, it is determined that the program has entered an abnormal routine, and an interrupt is generated to detect the error.
This is the method generally used to detect hardware errors.
Take, for example, a program that executes an instruction, and 100 ms later executes the same instruction again. If 130 ms pass and the instruction has not been executed, a hardware error will be detected and program execution will be stopped.
The timer that monitors this time is known as the watchdog timer, and when a specified time has passed, it interrupts program execution and sends notification.