Ch a pt er 6 Ap p l i ed I ns t ruc t i ons6 - 4 2 96.20.2 Explanation of Other InstructionsAPI Instruction code Operand Function1900 WDT P ─ Watchdog timerPulse instruction 16-bit instruction (1 step) 32-bit instruction─ AH500 ─Symbol:Explanation:1. In the AH500 series PLC, there is a watchdog timer which is used to monitor the operation ofthe system.2. The instruction WDT is used to reset the watchdog timer in the PLC. If the program scanningtime exceeds 200 milliseconds, the error LED indicator is ON, and the PLC stops running.3. The particular point when the watchdog timer acts: The system is abnormal. The execution of the program takes much time, and therefore the scam time is largerthan the setting value of the watchdog timer. There are two way users can use to improvethe situation.1. Using the instruction WDTt1 t2STEP0 ENDWDT2. Please refer to ISPSoft User Manual for more information about changing thesetting value of the watchdog timer.Example:Suppose the program scanning time is 300 milliseconds. After the program is divided into two parts,and the instruction WDT is inserted between these two parts, the time it takes to scan either the firstpart of the program or the second part of the program is less than 200 milliseconds.Additional remark:Please refer to ISPSoft User Manual for more information related to the setting of the watchdogtimer.
