Sporadic Model M - PSU?

[Phaedrus2129]

So I'm this old Athlon XP rig that I've brought back from the dead and is currently a work in progress. These old computers use power differently than modern ones; modern PSUs draw mainly from the +12V rail, and thus modern "ATX12V v2.x" PSUs are designed to deliver most of their power on the +12V rail. Older systems draw mainly from the +5V and +3.3V rails, and so "ATX 1.x" PSUs are designed to give at least half of their power on the +5V and +3.3V.

Well this old system needs an ATX v1.3 PSU, but I don't have one on hand so I'm using an ATX12V Antec Earthwatts EA430 until I can pick one up. It's a group regulated unit, so when under a heavy +5V/+3.3V load the voltages go a little wonky. The +5V runs at ~4.80-4.85V and the +12V runs at 12.43-12.55V. Not dangerous, but close to the edge of what the ATX specs allow.



ANYWAY, I decided that an old computer should have an old keyboard, so I've hooked up my Model M to it. And it's funny; one time in three on starting the computer, it will halt on POST and say "Keyboard error; no keyboard detected".

Now I know some motherboards don't follow the specs as closely as they should. But this is an Asus A7N8X Deluxe, and Asus, in my experience, has been pretty good about that, even on modern mainboards. I've tested my Model M on three different Asus mobos, and this is the only one with issues.

My theory is that since the PS/2 port is powered off the +5V rail, the low voltage is causing the Model M to not start up properly. It's getting power; the LEDs light up. But I'm guessing that maybe the voltage isn't rising quick enough to make the keyboard work in time to send its ok signal to the motherboard.

Sound plausible?


In any case, I'm picking up a Zippy 300W ATX v1.3 PSU soon, as well as 2x512MB DDR to replace the current 512+128+128.

[InSanCen]

I'm working on a simple mod that may solve your problem. 12v from the PSU, through a wire, dropped down via a small resistor inline, and connected straight to the PS/2 port (Underside of Motherboard seems logical).

The alternative  for those of us with stable 5v rails, is to take a wire from the PSU atraight to the appropriate pin on the ps/2 port. Take your pick as to where to run the wire from, HD, Floppy and 20/24pin connectors all carry 5v and 12v, so no resistor needed. Your board layout and PSU cabling will determine which is best for you. I Plan on putting in a floppy or similar connector somewhere to make it detachable (Chances of me soldering in a 1.2KW PSU are precisely Nil).

[Phaedrus2129]

Hm, I expect the Zippy should clear up the problem (ordered it last night, since the +12V rail was hitting 12.63V; out of spec), but if not it shouldn't hurt to try your mod. Unless it fries something. That wouldn't be good. *thinking*

[InSanCen]

If you want to make *really* sure, cut the 5v trace to the PS/2 port.

[Phaedrus2129]

It works perfectly on other computers. I used this Model M for nearly three months on my main PC before getting my MX-11800; in fact, I prefer the Model M for typing, but the noise and huge size make it, ah... Not ideal for gaming.

[Phaedrus2129]

If it is indeed the PSU at fault then using USB won't matter, because it's still powered off the same +5V source. And keep in mind it's a sporadic problem, and doesn't happen all the time. But the PSU's out-of-whack voltage seems like a likely cause since the +5V is running at 4.78-4.83V during POST; and it takes a few miliseconds for the voltage to rise to its desired level.