On Mon, Dec 31, 2007 at 03:37:46PM +0000, Stuart Henderson wrote:
quoted text
> On 2007/12/31 15:07, chefren wrote:
> > And look at the workings of your heartbeat monitor: I bet it needs a loop in
> > the software that "pings" it. With software failures: Big chance that loop
> > still works and thus the heartbeat monitor isn't triggered while the system
> > as a whole can be considered broken.

Put it somewhere in the main software loop.  Hook up a second trigger
off the money-detector.  The computer decides what tune to play and
plays it.  The heartbeat monitor can detect the money and listen for any
sound out of the speaker.

quoted text
> 
> Even so, it still allows recovery from some serious problems without
> touching the machine. There are quite a few situations where this could
> be very useful, though it might not be worth the extra expense and
> complexity of adding an external device, watchdog timers aren't too
> uncommon in PC hardware these days.
> 
> > Your heartbeat monitor also needs a way
> > to power-cycle the whole system. Relays? How is/are these powered?  Don't
> > forget for all the cables and connectors needed.
> 

Last time I needed such a timer was about 20 years ago.  Used a mil-spec
556 TTL dual-timer IC and appropriate caps, resistor, and relay.  Fit it
inside a small box with a 110V recepticle and a power cord, and
connected with a serial cord.  Powered it right off the serial port.


Doug.