Not putting two iMacDV motherboards in the same case

April 16th, 2007 — Wordman

For even longer than this blog has existed, I have had a plan to take two previously mentioned iMacDV computers apart and place their innards into a single ATX tower case, essentially jettisoning their bulky and power-hungry CRTs and running them from a single power supply.

The threat of high voltage and lack of motivation caused this plan to languish for years. Now, having given one of these machines away, I’m forced to abandon the idea altogether. I did, however, spend quite a bit of time looking into the concept, so thought I’d share what I discovered so someone else with a stack of iMacDV’s can make it happen.

There are a number of issues that prevent such a project from being an easy task. They are:

  1. Discharging the high voltage in the iMac’s CRTs without killing yourself
  2. Connecting a motherboard built expecting a non-standard power supply to use an ATX power supply
  3. Converting the video signal on the motherboard into a VGA signal
  4. Converting the motherboard’s non-standard drive cabling into more typical IDE cabling
  5. Powering both motherboards at once with a single power supply
  6. Wiring up the power switch to correctly power down the two motherboards

All but one of these issues also occur with a project that would put just a single motherboard in the case. For all but the last two problems, a solution that works in a single board machine could be applied to both boards. The last two issues, however, are interrelated and the addition of multiple boards makes the problems more complicated, particularly the final one.

When I started looking into this project, there were a one or two documented attempts on the net to move an old tray-loading iMac into an ATX case, but the slot-loading iMacDV turned out to be a different beast in many ways, so these were not as helpful. There was also a company that sold kits to convert tray-loading iMacs into 1U rack mounted machines. They later started selling kits for the slot-loading iMacDV. These kits were expensive, however (over $200) and the company looks like they no longer sell them.

After waiting a while, however, a site materialized detailing the iMaxATX, a conversion of a single iMacDV into an ATX case. This site looked to have solved most of the tough bits, with good diagrams for the power supply, video and hard-drive wiring problems. Prior to this page, I hadn’t even known that the video was an issue. After all, the iMacDV has a perfectly functioning VGA connection on the back of it, so it wasn’t (and still isn’t) clear to me why this needed to be converted. Still the pictures on the page are fairly convincing that the video needs some kind of tinkering.

I also am not sure if the hard-drive connection tinkering is really an issue, particularly if you are willing to sacrifice the DVD drives, which appear to be the reason for the non-standard cabling. It occurred to me that you may be able to forgo IDE cables all together and just use Firewire. This would require either drives with native firewire interfaces (which don’t appear to actually exist) or IDE/Firewire bridges. If possible, this would be advantageous because it would circumvent a limit of these motherboards that prevents the onboard IDE from supporting drives larger than 128GB. This would look a bit odd in the case, as the Firewire port that is external when in the iMacDV case would essentially be mounted internally to an ATX case, with Firewire cables running inside. This would, however, eliminate the big gray ribbon cables, so might even look more slick. (I should mention that I was intending to mount all this in a clear acrylic ATX case.)

This leaves the power issue. Even though the iMacATX page mentioned above does provide good wiring information, it doesn’t address some problems, including those unique to a two-motherboards-in-one-box solution. In the first place, can a single power supply support two iMacDV motherboards? I think the answer to this is that the PowerPC chip draws so little power that it is possible with a 500w or greater power supply, but I’ve never tested this. If not, using twin power supplies would work, and would make certain things much easier, but wouldn’t be as cool.

One problem with supporting two boards from one power supply is that there is only one main feed from an ATX power supply; however, since you already have to do a bunch of custom wiring to hook this up to one iMacDV motherboard, splicing an extra feed in parallel into this should not be much more difficult, at least in principle. It’s all just voltage, after all. A socketed power supply might also make this process easier. So would using heat shrink tubing.

There is one big wrinkle here, however, having to do with rebooting. The way machines reboot is by, essentially, telling the power supply to turn on and off. With ATX power supplies, the signal to do this is sent by grounding a specific wire from the supply. The iMac motherboards, however, indicate they want a reboot by setting a wire from ground to +5v. So, even in the case of a single motherboard, there is an issue with rebooting (one not covered by the iMaxATX page, as far as I can tell). There are various posts for solving this problem, some using resistors and other parts, others using simple integrated circuits.

It gets even more tricky when running two motherboards from a single power supply, though. You don’t want to have a reboot or power down signal from one of the boards shutting off everything, because that would turn the other board off as well. You really only want the power to cycle if both boards want to power down, and this requires a bit of brains (or, at least, switches) in-between the power supply and the boards. I worked out a method of doing this in my head, but it makes the wiring much more complicated (you can’t just splice the power wires together anymore) and is probably wrong. I’m sure someone with more EE experience could find a better way.

I may never know, however, since I don’t have one of the machines any more (and the other may be dead). If you get it to work, please let me know.