GFK-0467K Chapter 4 Series 90-30/20/Micro Instructions Set 4-149
4
SVCREQ #8: Reset Watchdog Timer
Use SVCREQ function #8 to reset the watchdog timer during the sweep.
Note
Of the CPUs discussed in this manual, Service Request 8 is supported only
by 90-30 CPUs, beginning with Release 8.0.
When the watchdog timer expires, the PLC shuts down without warning. This function allows the
timer to keep going during a time-consuming task (for example, while waiting for a response from a
communications line).
Caution
Be sure that restarting the watchdog timer does not adversely affect the
controlled process.
This function has no associated parameter block; however, the programming software requires that
an entry be made for PARM. Enter any appropriate reference here; it will not be used.
Example:
In the following example, when enabling output %Q0127 or input %I1476 or internal coil
%M0010 is set, the watchdog timer is reset.
| _____
| %Q0127 | |
|——| |———+—————————| SVC_|—
| | | REQ |
| %I1476 | | |
|——| |———| CONST —|FNC |
| | 0008 | |
| %M0010 | | |
|——| |———+ %AI001—|PARM |
| |_____|
|