4 Special Functions
3-98 SmartAXIS Touch User's Manual
4.17 Watchdog Timer Setting
This section describes watchdog timer setting with monitoring the operation state of the Touch.
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Feature Description
A watchdog error occurs when the processing time for one scan exceeds the allowed time while the ladder program
or FBD program is running.
If a watchdog error occurs, the system is reset with the purpose of returning the system to normal operation.
If the watchdog error occurs frequently, you can assume that there is a hardware problem and that the Touch
requires replacement.
Some of the reasons why a watchdog timer will occur are problems with the Touch hardware and the length of the
processing time in the ladder program. If the processing time for one scan of the ladder program exceeds the time
configured for the watchdog timer on the ladder program, place a NOP (no operation) instruction in the ladder
program. The watchdog timer is reset when the NOP instruction is executed.
For FBD program, the watchdog timer cannot be reset in the program.
Watchdog timer setting time
The watchdog timer setting time can be changed in the function area settings. Decide on the watchdog timer setting
time in either of the following two ways.
• Decide by user system specification
Determine the maximum on time (milliseconds) so the output signals do not stay on if the Touch loses control and
set that value as the watchdog timer setting time. However, if the ladder program processing time takes longer
than the design specification, use an NOP (no operation) instruction in the ladder program. For example, when the
watchdog timer is set to 100 milliseconds and the maximum value for one scan is 120 milliseconds on the ladder
program, insert an NOP instruction in the ladder logic program to ensure that the error does not occur.
• Match the ladder program processing time
Check the maximum value for one scan (D8024) and set the watchdog timer value as a value with a safety margin
added to the maximum value.
Select the watch dog timer setting between 100 milliseconds and 5,000 milliseconds. The default time is 400
milliseconds.
When changing the watchdog timer setting value, select an appropriate value that fully takes into
consideration the safety of the system.
The ladder program or FBD program execution time can be checked with the value of LSD38.
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