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Mystery VRAM SIMMs?
Posted by: zigzagjoe on 2024-09-11 11:52:40
Got a few of these at VCF. They result in display corruption on a Q650 and some oddly partially functional effects on a Q605. Interestingly, VRAM tests pass. Seems like they don't agree with the actual display output. Anybody have a clue what these go to? They're 512K, 80NS.

20240910_212615.jpg
Posted by: Fizzbinn on 2024-09-11 19:26:11
Wild guess, "GWorld" SIMMs?

64-Pin 4MB SIMMs for Apple Display Card 8*24 GC

The best deal on eBay were these two, but those are listed for an Amiga. Does anyone know if these would work? One is parity the other is non-parity. The other question is could they be used together on the video card? https://www.ebay.com/itm/334471305723 https://www.ebay.com/itm/334471303537
68kmla.org 68kmla.org
Posted by: zigzagjoe on 2024-09-11 19:58:22
Wild guess, "GWorld" SIMMs?

64-Pin 4MB SIMMs for Apple Display Card 8*24 GC

The best deal on eBay were these two, but those are listed for an Amiga. Does anyone know if these would work? One is parity the other is non-parity. The other question is could they be used together on the video card? https://www.ebay.com/itm/334471305723 https://www.ebay.com/itm/334471303537
68kmla.org 68kmla.org
Hmm, I don't think so... Those look to be 64 pin and use DRAM, these are 68 pin and have VRAM chips.
Posted by: Fizzbinn on 2024-09-11 20:12:50
64... 68 pins, whatever it takes...


yeah, I didn't look close enough
Posted by: Mk.558 on 2024-09-11 20:34:11
Might be for an LC. I have a 68 pin SIMM out of an LC II, compared it to a 68 pin SIMM out of a Q610 and the pins ... don't exactly line up with each other.
Posted by: zigzagjoe on 2024-09-24 16:43:31
Went on a bit of a quixotic quest and made some headway. TLDR: It's LC2 VRAM. Presumably third party.

---------------------------------

How these simms behave in 16 and 8 bit color respectively. Seems to occur on a 1024 byte boundary. Even mixing a single apple vram and one of these produces similar display.

1727217633568.jpeg1727217649927.jpeg

The VRAM chips used on these (OKI MS514252A) don't support the JEDEC special functions DSF/QSF, which Quadra 605/650 seem to need. This would align with the chips behaving as expected (VRAM testing OK) when accessed via the independent parallel bus interface.

From the Micron MT42C4256 datasheet, used on the working SIMMs:

1727217127101.png

DSF (pin 22 in SOJ28 form factor) is wired to both the internal VRAM (if applicable) as well as pin 2 on the 68 pin slot. Interestingly, the mystery SIMMs are also wired correctly for DSF, just the chip doesn't support it.

The question I guess becomes, what would be the missing link machines which use 512K VRAM SIMMs yet have an "older" video subsystem? Some research showed the LC2 supports 512K vram simms yet DSF is grounded, so that's our winner.

LC: 256K. 100ns also. DSF (pin 2) is grounded on the connector.
LC2: 256K or 512K: DSF (pin 2) is grounded on the connector.
LC3, Q700, Q900: 256K. 80ns. DSF is implemented on the VRAM connector
Q475, Q605: 512K. 80ns. DSF is implemented on the VRAM connector

LC3:

1727221306031.png

Q650 devnote:

1727220324060.png

Q605 devnote:
1727219194454.png

I find it interesting nothing explicitly called the requirement for the more advanced VRAM, but I suspect there weren't too many third party VRAM simms floating around....?

Anyways, quite enough silliness.
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