Siemens PLC monitoring timer reset command - Database & Sql Blog Articles

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The watchdog (watchdog), also known as the watchdog, has a timing of 300ms. It is automatically reset every time it is scanned. If the scan period is less than 300ms during normal operation, the watchdog timer does not work. If strong external interference causes the plc (programmable controller) to deviate from the normal program execution route, the monitoring timer is no longer periodically reset, and the PLC (programmable controller) will stop running when the timing time expires.
The scan cycle may be greater than 300ms in the following cases, and the watchdog timer will stop executing the user program:
(1) The user program is very long.
(2) When an interrupt event occurs, the execution of the interrupt program takes a long time.
(3) The loop instruction lengthens the scan time.
To prevent the watchdog timer from operating under normal conditions, the watchdog timer reset (WDR) instruction can be inserted into the appropriate place in the program to reset the watchdog timer. If the execution time of the FOR/NEXT loop program may exceed the timing of the watchdog timer, the WDR instruction can be inserted into the loop program. If the conditional jump instruction JMP is after its corresponding label (that is, the program jumps back), the program between them may be repeatedly executed due to continuous repeated skips, and the total execution time exceeds the timing of the monitoring timer. To avoid this, a WDR instruction can be inserted between the JMP instruction and the corresponding label.
After using the WDR instruction, the following operations will be disabled before terminating this scan:
(1) Communication (except for free port mode).
(2) I/O update (except immediate I/O).
(3) Forced update.
(4) SM bit update (SM0, SM5 to SM29 cannot be updated).
(5) Run time diagnosis.
(6) The STOP instruction in the interrupt program.
(7) If the scan time exceeds 25s, the 10ms timer and the lOOms timer cannot accumulate the time correctly.
If the mode switch of the PLC CPU is switched to the STOP position, the CPU in the 1.4S goes to the stop mode.