Fun with computer modifications
A year or so ago, I traded a bunch of computer hardware that I don't use anymore for parts of a disassembled iMac which the original owner had stripped because he wanted to make it into a rackmount box. After I got the parts, I filed them away for later assembly (using one part -- a Morex D2D -- as the power supply on the oggOmatic) and only got down to that layer of computer junk this weekend.
One of the annoying things about Apple is that they want to thimk different, even with the hardware they use. For example, they used SCSI drives until the point where IDE drives cost about 1/10th that of the SCSI parts (yes, there are some really high performance SCSI parts out there, but you can throw a lot of IDE hardware at the problem for less money, and if you're looking at small file servers (<1/4th terabyte) it's really cheap to just bung in 5 80GB IDE drives and a hardware disk array controller, plus a couple of 250gb drives on the backup machine.) On the iMac, they thumk differently by using a power supply that was almost, but not quite, an ATX supply (they didn't use the -5V line, and they changed the signalling on the PS_ON line.) This, by itself, isn't a problem, but to make things wierder they put a power filter card into their machine that takes input power on 24 pins and puts out power on 26 pins.
No new voltages. No new signal lines, just an additional 2 pins.
But this isn't the problem either.
There's a fairly large community of people who modify their iMacs into other cases. Some of them do it for fun (if I'd had a functional iMac instead of a carcass, I would have ended up ripping it apart because I don't like having big CRTs on my desk) but, at least from reading the web, most of them do it because the part of the power supply that drives the tube has the annoying habit of self-destructing, leaving them with a perfectly functional PowerPC box that has (a) no head and (b) an OS that isn't usable without a head. So those people rip the boxes apart and mod them so they won't be out an iMac. But they way they do it is to pull out the system board (not a motherboard, because the iMac is a single board computer), the power filter board, and glue the power filter board to an ATX power supply.
All of the webpages I've found for modding iMacs give quite detailed descriptions of how to glue the power filter board to an ATX power supply. Many of those pages then give a quick description of the pinout of the power attachment to the system board (26 pins for +3.3v, +5v, +12v, -12v, PS_ON#, +5v standby, and ground; nothing like redundancy) and, as a quick aside, say "we decided not to connect to the powerfilter, but instead to the system board."
No description of how they did this, of course.
I can sort of see why you wouldn't do this; after all, you're ripping apart a PC and reassembling it, so it should be fairly trivial to just tie across the lines and solder together the magic transister hack to make PS_ON work, but this is Apple and you can never know whether one of the line has the invisible note (connect input pin 1 to output pin 7 or your IDE subsystem will melt) attached to it.
My great plan (assuming I can get home early enough to solder everything together tonight and I don't get distracted assembling the worktable I'm building for the bears, is to build an adaptor that connects the pins together like:
| ATX | signal | iMac filter | iMac mboard |
|---|
| 1 | 3.3 | 18-19 | 17-18 |
| 2 | 3.3 | 20 | 19-20 |
| 3 | G | 1 | 4 |
| 4 | 5 | 10 | 5+1 |
| 5 | G | 3 | 3 |
| 6 | 5 | 11 | 6 |
| 7 | G | 1 | 12 |
| 8 | PWR_OK |
| 9 | +5VSB | 24 | 26 |
| 10 | 12 | 15+16 | 2+10 |
| 11 | 3.3 | 21 | 15 |
| 12 | -12 | 17 | 16 |
| 13 | G | 2 | 11 |
| 14 | PS_ON# |
| 15 | G | 5 | 14 |
| 16 | G | 6 | 13 |
| 17 | G | 7 | 21+22 |
| 18 | -5 |
| 19 | 5 | 12 | 8 |
| 20 | 5 | 13 | 7 |
Hopefully, after doing this the little iMac will give the ping of wanting a boot disk, and then I can wedge it into a new tiny case and make it look pretty.