Most software doesn't run very well if the sytem board hangs, or the power
supply dies. To be effective a watchdog need to be outside of the box
being monitored, and looking for a periodic signal that the box is still
working. A typical, far from perfect, but nevertheless easy method is to
have a box with a bell, light, or whatever that is turned on by a relay
dropping out. The relay is held picked by a re-settable one shot, e.g.,
555. If the one shot is not reset within a pre-set interval, say 1 second,
it drops the relay and the bell goes off. What's the reset? Well it might
be a program, running on the system that brings one pin on a parallel port
high for a millisecond once every 1/2 second. As long as the program is
running the bell is quiet.
Alan Mead <[EMAIL PROTECTED]> on 07/14/2000 09:39:52 AM
Please respond to [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
cc:
Subject: Re: Hardware Watchdogs / Alternatives
At 03:55 AM 7/14/00 , you wrote:
>All,
>
>I'm looking for a good watchdog card for Linux and am coming-up short.
>Any pointers?
I don't know what a 'watchdog card' is. There are software packages like
'heartbeat' that monitor something and raise an alarm if that something
stops.
>Another thing I'd like to try out is some sort of device which will
>reboot a Linux server remotely if need-be. Sometimes you get into a
>situation where the kernel hasn't crashed, and still answers pings, etc.
>and would probably look to a hardware watchdog like everything's okay,
>but the machine is still unreponsive and you can't telnet to it.
I don't know if it's still being made but we have a set of devices called,
I think, X10 or X11 that are designed for this. There are client devices
that mediate between the computer and the wall socket and another device on
the controlling computer's serial port (and also plugged into the wall
socket). You have to write serial port software to control the client
computers. I've seen similar units sold for home automation. IIRC, they
don't work across circuit boundaries and they require that the controlled
devices actually get plugged into the wall (as opposed to, say, a UPS).
All that said, the only times I've ever had to reset a Linux box was when I
was screwing around with the hardware. (We bought these X10 units for
controlling a multi-computer DOS-based homegrown C process for processing
faxed test scores into psychological reports.) So I don't know whether I
would bother.
-Alan
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