9
2 Principle of Operation
This chapter describes how the Serial Watchdog works. It does not
describe the usage and installation of the watchdog program which is
necessary to operate the watchdog. For questions concerning this
program, please refer to the corresponding user manual / online help.
2.1 General system behaviour
After setting up all parameters in the watchdog program, monitoring is
activated by pressing the START-button. This sends the START-
command to the Serial Watchdog which monitors the computer now.
The COMMUNICATION-LED turns on and the STATE-OF-HEALTH-
LED starts red flashing. Now the watchdog program sends the PING-
command with parameter 0x01 every 30 seconds until the program’s
timer reaches the preset watchdog time. Every time a file is created
within the monitored directory this timer will be cleared and the PING-
command with parameter 0x00 is sent to the watchdog. This changes
the STATE-OF-HEALTH-LED from red flashing to continously green. If
the timer reaches the preset watchdog time without having detected a
file-writing event, the watchdog program stops sending the PING-
command and tries to shut down and restart Windows. For safety
reasons a fixed 30s overtime period is always added to the preset value.
So, for instance, if desired measurement time is 60s, it is recommended
to set the watchdog time to the same value, i.e. 60s. Thus, the effective
time-out will be 60+30=90s.
Exactly 30 seconds after the last PING-command was received by the
watchdog, it changes the STATE-OF-HELTH-LED to continously red
and generates a periodical acoustic signal if audible alarm is enabled. If
no PING-command is received within the next 5 minutes, the watchdog
will open the power relay for 40 seconds which acts as a hard reset for
the computer. Next the watchdog runs its initialization procedure like
descibed in the chapter „System behaviour after a power failure“. If the
operating system isn’t in trouble, the computer will finish the reboot
within 5 minutes. After a successful reboot the watchdog program will be
restarted automatically. It sends immediately the PING-command (with
parameter 0x01) to the Serial Watchog to prevent a hard reset in this
moment. Now the next monitoring cycle starts like described above.