Hitachi HF-W6500 45/40 Feature Manual page 31

2. ITEMS MONITORED BY THE RAS FEATURE
In Figure 2-2, the monitored program is configured to periodically retrigger the watchdog
timer.
The monitoring program periodically checks the timeout counter of the watchdog timer
and if the value of the timeout counter (that is, the remaining time until timeout expires) is
0, it is determined that the timeout occurred.
The fact that timeout occurred means that the monitored program was not able to retrigger
the watchdog timer for at least the length of the timeout. In this example, another program
detects timeout of the watchdog timer, but if you use the RAS external contacts interface,
the WDTTO contact is closed when timeout occurs for the watchdog timer. In this case,
an external hardware connected to the RAS external contacts interface can monitor the
operational state of a user program.
A program can use a watchdog timer by calling the library function WdtControl. For
information about how to use the WdtControl function and how the WDTTO contact of
the RAS external contacts interface works, see "6.1.3 Watchdog timer control function
(WdtControl)".
NOTE
When the WdtControl function is used, you cannot use the Automatic
retriggering feature for a watchdog timer. In order to use the WdtControl
function, select Retriggered by application program under Watchdog timer
setting. For information about how to use the RAS Setup window, see "3.1.3
Using the RAS Setup window".
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