Chapter 3 Devices Watchdog timer 1. The watchdog timer is used to monitor a scan cycle. If the scan cycle is greater than the watchdog timer value, the ERROR LED indicator on the module will be turned ON, and all the output devices will be turned OFF. 2. The watchdog timer is initailly set to 200. If the program is long, or the operation is complex, users can change the watchdog timer value by means of the instruction MOV. In the example below, the watchdog timer value is changed to 300. SR000 SM002 0 3. The maximum value which can be stored in the watchdog timer is 32,767. However, the larger the watchdog timer value is, the more time it takes to detect any operation error.
As a result, if there is no complex operation resulting in a scan cycle longer than 200 milliseconds, it is suggested that the watchdog timer value should be less than 200. 4. If an opration is complex, the scan cycle may be long. Users can check whether the scan cycle is greater than the value stored in SR000 by monitoring SR010~SR012. If the scan cycle is greater than the value stored in SR000, the users can change the value in SR000. Initial pulse 1. Users can set the time it takes for the input terminals to respond by setting SR020. The value in SR020 must be in the range of 0 to 20. (Unit: ms) 2.
If the module is turned form OFF to ON, the value in SR020 will automatically become 10, and the value in SR021 will automatically become 10. Input f ilter SR020 Terminal Response time 0 X0.0 0ms 1ms 1 SR020 is set. The value in Sr020 is 10. 10 Regenerating an input X0.7 10ms 15 3. If the program below is executed, the time is takes for the input terminal to respond will be 0 milliseconds. Owing to the fact that the input terminals are connected to resistor–capacitor circuits in series, the shortest time it takes for the input terminals to respond is 50 microseconds. 15ms The state is memorized. SM000 4.
If high-spedd counters and interrupts are used in a program, the value in SR020 does not have any effect. Normally-open contact 3-29 MOV K300 SR000 MOV K0 SR020