I know it could run dual 604e and that the second card was a special one.
Well? HOW special? What was done? How was it done? Was it speed? Voltage? Certain pins shorted out in a proprietary way? Could someone with little common sense and a lot of money and time actually build a functional G4 based secondary CPU card for this beastie?
Imagine a Dual G4 CPU S900 at around 800mhz. You could give them new fangled mirrored door pretty Macs a run fer their money ya could! :)
XLR8 tried to make this work and I think the effort broke them. The guy working on it was pretty bright, so I don't think it was a trivial problem.
The pinout in the second CPU slot is very similar to the pin out in the primary slot, except (I think) that the CPU Bus Arbitration signals are different. However, that's not the tricky part. The tricky part is getting two CPUs hooked up so that they can coexist on the bus. I don't know how that is done.
Also, any card in the secondary slot needs to be facing the other direction from the card in the primary slot. That is, a standard primary slot card just won't fit physically, because the card sticks out in the wrong direction adn the heat sink faces the wrong way.
It was easier for the 604e than it would be for the G4, because the S900 was built to handle it with 604es. The G4 uses a slightly different arbitration scheme which can cause problems in our old machines and probably does if you try to implement two G4s. This is most apparent in other (non-S900) six slot machines where G4s cause problems interacting with the two PCI busses and freeze up the machine.
Anyway, if you could get a reversed G4 card built, and if you knew what to connect between the two cards it could probably be done, but no-one is going to bother, because the cost would be too high for the volume you could ever sell.
Jeff Walther
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