pete%RAND-UNIX@vortex.UUCP (12/15/83)
I have not actually done this yet, but it should work. On page 12 of 12
of the System board schematic in the OLD PC Tech Reference, you will find
a wire called POWER GOOD. The page number may be different for the PC2 or
XT, but IBM traditionally puts connectors on the last page of each schematic.
If this wire is grounded, you will reset the system (truly reset it, not just
interrupt). There are two possible problems with this:
1. The memory refresh will be stopped while the line is held low, and for a
short time afterward. This should not cause loss of information, but may.
2. The device(s) supplying this line in the power supply are not
mentioned anywhere, and might not tolerate even momentary shorts to
ground. If (as the spec implies) the device is LSTTL, it would
tolerate SHORT shorts to ground with no damage. A solution to this,
if desired, is to re-route the signal through an AND gate, and to
connect the button to the other input of this new gate. The gate
could be made up of diodes and a resistor, or a "flying chip", or a
search for a free gate on the system board could be done. This last
would almost certainly be different for the two board styles of the
PC and the third of the XT.
My own inclination has been simply to ground the line in parallel with the
power supply, but I have never gotten around to trying it.
-- Pete