As long as one of your systems is up you could use it to trigger the contacts for another one.
The simplest way would be to wire a small solid state relay to the parallel port header (on board) on one and then the contacts to the power pins on the other system. If one system goes down you can trigger a power up from the other. There are lots of utils around for setting pins on parallel ports so you would just script together calling such a util from a web page trigger.
Another way to handle this would be to have the systems watch each other. So if you use a parallel port to control a relay then the same port could use an input to monitor the 5V line on the other system. If you poll that parallel port every minute then when it sees the 5V line low it could fire the relay for 1 second to restart the system. Repeating that on each system in a daisy chain (loop) would ensure that as long as one system is up then all the others would soon start up too.
The only thing you need for this is some wires and solid state relay with input suitable for parallel port outputs. I could draw up a simple wiring diagram for N number of systems if that would help.
An opto-isolator like a 4N25
would be a better way than a relay and easier to wire up too... about 0.25$ each.
R1 1 ----------- 5
D(x) ----1k------| Opto- |-----+ to power switch + pin
| Isolator |
GND -------------| |-+
2 ----------- 4|
CNY 17 or | R2
4N25 | 1K
+--\/\/\/\/--- - to power switch GND pin
This is a very clever idea. I like the fact that I could control the power via a script. The limitation seems to be that it would be from one computer to another, or several computers via daisy chain. I think I would like to keep this idea in my back pocket for special situations. Regardless, I'll send a small donation you way!
The solution that I am most keen on right now is to disable ATX soft power. Soft power is described here: http://www.jeae.dk/computere/atx_-_psu.htm
. Basically, when an ATX PSU is connected to a motherboard it is always providing a small amount of power to be able to sense the power on signal. If you ground the POWER ON (green) wire you will be telling the PSU to give the motherboard power without using the switch. People often short POWER ON and a ground wire to run an ATX power supply without a motherboard.
What I am thinking that I will do is to quick splice the POWER ON wire with a ground wire as shown below.
Accomplished by using one of these, which run about $0.50:
By doing this, whenever the PSU has power (and PSU's power switch is in the on position) the motherboard will turn on/stay on. To control the on/off state of the computer, I will use my switched power distribution unit. With the PDU, via a web interface, I can turn an outlet on or off, which will turn the computer on or off since soft power will be disabled.UPDATE
Several of my boards did not like this configuration. GPU, CPU, etc. would power on, but it would not wake the motherboard itself. Thus, YMMV