I previously acquired a PowerMac G3 Blue & White system that had a dead mainboard, only gives 4 beeps on startup indicating something's wrong with the boot rom. I did manage to find a really good deal on a replacement mainboard with only one issue. The replacement board is a REV. A with the original bugged PCI646U2 IDE controller while my dead board is a REV B. with the patched 646U2-402 controller.

Looking at the boards themselves though they seem to be identical in layout, which makes me wonder if a straight swap of the IDE controller would be possible to have bug-free IDE on my working REV. A board. The only difference I've noticed is on the back, directly underneath the IDE controller. The REV. A board includes a PALCE chip (U48) with a few added capacitors and resistors while the REV. B board omits these.


REV. A board with PALCE 16V8H-103C/4 in U48 (Left/Top) & REV. B board with empty pads at the same location (Right/Bottom)

I'm not really sure what the functionality of this PAL chip is on the REV. A board that was no longer needed on the REV. B and if it would interfere with the updated IDE controller.

If anyone has anymore knowledge on this I would love to know before I take a soldering iron to anything.

Thanks.

@XodiumLabs has done this, the CMD 642 is a direct pin compatible replacement.

>> chasnah said:

REV. A board with PALCE 16V8H-103C/4 in U48 (Left/Top) & REV. B board with empty pads at the same location (Right/Bottom) I'm not really sure what the functionality of this PAL chip is on the REV. A board that was no longer needed on the REV. B and if it would interfere with the updated IDE controller. Click to expand...

B&W G3 Yosemite REV C schematics at:
https://www.macdat.net/repair/apple_schematics.html
The pages are not in order and it's not OCR'd and Apple's OCR has a tough time with 80% of the text.
There's a revision history with many items but they are not detailed.

For U48, it says "PROG, CMD HACK, YOSEMITE" in the BOM's "NO STUFF" list.
The schematic is at sheet 29-D3. It says "CMD HACK PLD, PRGMD"
It's job appears to be to affect the ULTRAREQ signal based on some PCI signals to the CMD646.