MDD CPU in a Quicksilver: why won't it work?

[ArmorAlley]

What's the reason that a CPU from an MDD won't work in a Quicksilver or other G4?

• Bus speed: 133MHz vs 167MHz?
• Difference/incompatibility in the ROM?
• Physical size: MDD card & heatsink won't fit?
• Power draw: the power draw of dual 7455s is too much for the QS PSU?

Or are there other reasons?

I have 2 MDDs with broken PSUs in my cupboard and a recently acquired 733MHz QS.
733MHz is a grand speed but if I can simply transplant one of the CPUs, I would have a faster QS.

 

[Phipli]

@GRudolf94?

One for you?

 

[GRudolf94]

Mechanically incompatible, and I wouldn't be surprised if electrically too. Apple has reused the same connector in a few places, but it doesn't mean they kept a pinout or signaling levels. You can, with some work, interchange Sawtooth/QS cards (and their GbE/DA variants), but not MDDs.

While the MPX bus is technically the same (which is why we can have 1GHz upgrade CPUs on a Sawtooth), Saw/QS are Uni-N machines, and MDDs are...Intrepid? I think so - and as such, much more architecturally distant.

 

[ArmorAlley]

Mechanically incompatible, and I wouldn't be surprised if electrically too. Apple has reused the same connector in a few places, but it doesn't mean they kept a pinout or signaling levels. You can, with some work, interchange Sawtooth/QS cards (and their GbE/DA variants), but not MDDs.

While the MPX bus is technically the same (which is why we can have 1GHz upgrade CPUs on a Sawtooth), Saw/QS are Uni-N machines, and MDDs are...Intrepid? I think so - and as such, much more architecturally distant.

I had long known that it wasn't possible but was not actually sure why.
And it's nothing so simple as replacing a crystal for the bus-speed or getting a beefier PSU.
Thanks for this.