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The WatchDog timer is designed to overpass a computer hanging up. Usually computer almost never hangs up.
However, if it hangs up and gets stuck, usually there is nobody at the site to press the reset or nobody knows
where the stuck computer is because there were no problem with it for a long time. The WatchDog timer is used
just for such situations. It automatically restarts the computer that is hung up. The WatchDog timer is off after
starting of the controller. To switch it on, the command “Esc[WW” should be sent and timeout (number of ticks, one
tick is 0.3 seconds) should be set by indicating required W, which is a value between 0 and 255. To switch the
timer off, timeout should be set to 0 by "Esc[W0" command. The special driver is used to load and to control
timeout counter. To ensure non-stop running of the computer, the driver should set the timeout periodically to
avoid that the WatchDog hits with following “reset” contacts closing. If your computer ever locks up, the driver
no longer sets the timeout and the WatchDog hits – “reset” contacts are closed. The WatchDog is designed in such
way that it hits only once. This allows to avoid it hitting again during the file system check which will probably
follow after the reset (or after power on start). When the computer comes up again, the driver program needs to
re-enable it (to send the command Esc[WW with needed timeout). The WatchDog guarantees that the system is always
able to execute programs. It does not guarantee that application is still running and responding. To check such
things, you can use a crontab entry for Linux/UNIX or other programs. You can be confident that the crontab will
be working because the WatchDog ensures that software in general is still executing. For example, you can design
a script that is triggered by a cronjob and downloads a webpage from your webserver every 50 seconds. However, you
have to be careful with that: a webserver can get heavily loaded with a lot of requests and it is normal if it does
not answer all of them.
For our example we have created a script watchdog.cgi to learn the WatchDog timer facilities in interactive mode.
Operation of this script is tested in OS SuSE Linux, however they
should operate in any other Unix like OS. Interface with the controller is allocated in special module TeleServ.pm.
You should copy this file in any directory and you should define environment variable PERLLIB. The easiest way is
to add to the file /etc/profile the following string:
PERLLIB=”the name of your directory”
export PERLLIB
You can indicate directory where modules would be searched in the script adding at the beginning of the script the
following directive:
use libs “the name of your directory”
This way is more preferable because the script is run as a CGI program under web server Apache.
To get it, both web server and script watchdog.cgi that provides such
service should be installed and run. This script is tested with installed web server Apache on controlled (remote)
host. Execution of CGI programs should be enabled in Apachе settings (refer to documentation of web server for
instructions). Script watchdog.cgi should be copied to controlled host in directory where CGI program are located.
To test it you can run web browser and enter local URL of script in the address string, for example:
http://127.0.0.1/cgi-bin/watchdog
The following page with table should appear:
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Download watchdog