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Macintosh IIfx motherboard revision A vs B differences.

PowerPC

Member
Does one revision have an advantage over the other?
There are some noticeable differences between the two like all the names printed on revision A and some of the chips are different. Black slot vs white slot differences besides color?

820-0282-A

820-0282-B

92534667-B54E-43AD-B338-D3BD7F006A57.jpeg

C2977F68-D5D6-428F-8317-40377DC3D63B.jpeg

 

Arisotura

Active member
Only real difference I can see are that revision A has some jumpers that are missing in revision B. Rev B also doesn't have the list of names at the top.

The chips under the RAM slots are slightly different (yellow instead of black on rev B), but I saw some pics of rev B motherboards with black chips there, so that probably depended on what was available during assembly. Same for the NuBus/PDS connectors, I guess.

Seems there are other revisions of this motherboard as well, the one I got is a revision 03. It is identical to revision A, except the list of names is different, and it doesn't say 'Macintosh IIfx'.

IMAG1687.jpg

 

Arisotura

Active member
According to a quick check in the schematics:

* J20 connects the CPU's /CDIS pin to ground, that pin is otherwise pulled high.

* J103 connects /CIOUT (wherever that comes from) to pin 64 of the memory controller, that pin is otherwise pulled to ground.

* J106 connects pin 11 of UK6 to ground, that pin is otherwise pulled high.

Rev A motherboards seem to have J103 closed and the others open, and mine did too. The rev B motherboard shown above doesn't have the jumper headers at all, but that W1 butterfly pad thing that connects to where J103 used to be, presumably that makes it closed by default and you can open it by cutting through the butterfly pad.

Checking the 68030 datasheet, I get the following info:

* /CDIS disables the CPU cache.

* /CIOUT is the 'cache inhibit output', which "reflects the state of the CI bit in the address translation cacheentry for the referenced logical address, indicating that an external cache should ignore thebus transfer.". The signal goes from the CPU to UP8, and presumably from there to the rest of the system.

* J106 might be affecting address selection logic? Not sure there.

Either way, it seems these jumpers would be used to configure the system for an external cache card or something in that vein. No idea if they ever made cache cards for the IIfx.

 

Arisotura

Active member
They're probably the same thing, couldn't find much about that though. Couldn't find a revA picture with enough detail to identify these components. Ssearching the part number I have on mine gives no results whatsoever. In any case, the writing on them says: TOKIN / 551-0007-F / 93Y9.

Shrug.

Also, the revA motherboard has a spot for installing a Molex connector, next to the HDD power connector. On the revB board, it's mostly gone, the holes are still there but they're tiny, and no silkscreen markings (other than the original 1/4 pin numbers).

I did some googling, but the rev03 thing is still mysterious. Everybody seems to have either revA or revB, with revB seeming to be more common. Googling the rev03 part number gives absolutely no results.

 

Arisotura

Active member
Another lil' difference: on revA boards, within the spot for Y1 (RTC crystal), there are alternate pads for a SMD crystal; on revB boards these pads were moved to an empty spot below the RTC chip. Speaking of, were there any boards fitted with SMD crystals? All the ones I see around have through-hole crystals.

This feels like one of these 'spot the differences' games, heh.

Dunno if there are any good pics of the back of the boards available too? The Bomarc schematics are for the revA board, and while repairing my board I ran into some different component numbers on the back. Whether these are just different numbering or actually different circuitry (or errors in the schematics), dunno...

 
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