Advantech PCM-3864 Manual page 62

Jumpers J1 and J3 control the watchdog timer hardware settings.
To enable the watchdog timer, you must write a program which
reads I/O port address 443 (hex) at regular intervals. The first
time your program reads the port, it enables the watchdog timer.
After that, your program must read the port at time interval of less
than 1.6 seconds, otherwise the watchdog timer will activate and
reset the CPU. When you want to disable the watchdog timer,
your program should read I/O port 043 (hex).
If CPU processing comes to a standstill because of EMI or a
software bug, your program's signals to I/O port address 443 to
the timer will be interrupted. The timer will then automatically
reset the CPU, and data processing will continue normally.
You must write your program so that it reads I/O port address 443
at an interval shorter than the timer's preset interval. The timer's
intervals have a tolerance of ±30%, so you should program an
instruction that will refresh the timer about every second.
The following program shows how you might program the
watchdog timer in BASIC:
10 REM Watchdog timer example program
20 X=INP (&H443) REM
30 GOSUB 1000 REM
40 X=INP (&H443) REM
50 GOSUB 2000 REM
60 X=INP (&H043) REM
70 END
1000 REM Subroutine #1, takes 1 second to complete
.
.
.
1070 RETURN
2000 REM Subroutine #2, takes 1 second to complete
.
.
.
2090 RETURN
56 PCM-3864/3860 User's Manual
Enable and refresh the watchdog
Task #1, takes 1 second to complete
Refresh the watchdog
Task #2, takes 1 second to complete
Disable the watchdog