Upgrading video RAM for an on-board GD5428 in an IBM PS/1? \ VOGONS

Upgrading video RAM for an on-board GD5428 in an IBM PS/1?

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First post, by AtomicPlayboy

Posted on 2022-01-17, 17:50

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My IBM PS/1 Expert 2168-33T model comes with 512Kb of video ram accompanying the on-board CL GD 5428 chip. I'd like to upgrade this to 2Mb so that I can take advantage of some higher resolutions/color settings under Win 3.11 and OS/2 Warp. Looking at the hardware manual, this does not seem to be possible, _however_ when I popped out the riser card I found an _undocumented_ expansion slot U53 that looks identical to the one used by other PS/1 systems for video memory upgrades. It's a ZIP/zigzag-style slot which should accept something like the 1Mb chip I've attached a photo of. See attachments for the official manual diagram for my model from the IBM hardware manual, which does not include this slot, and a photo of the motherboard showing it.

My question - what chip should I buy in order to upgrade this to 2Mb? Another thread here mentions a similar scenario, but does not provide details about the purchased hardware. And then there's this link, which states that 1Mb may be the max upgrade, but I'm not sure I trust it given that this upgrade path isn't even documented by IBM, and this information is for the Aptiva line, not my PS/1.

So, it's unclear to me whether (1) this will work at all, (2) I need to get a full 2MB to replace the VRAM, or just 1.5MB to supplement it, (3) what combination of 40-pin slot interface and actual RAM chips I would purchase. Any help here would be greatly appreciated.

Reply 1 of 8, by Plasma

Posted on 2022-01-17, 18:57

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I suspect you can only upgrade to 1MB total. The current chip you have is 256K x 16 (512KB). The zip chip pictured is 128K x 8 (128KB = 1M bits) so you don't want that one.

I would try a 256K x 16 ZIP-40 chip such as MT4C16256Z or UPD424170V.

Reply 2 of 8, by AtomicPlayboy

Posted on 2022-01-18, 00:50

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Plasma wrote on 2022-01-17, 18:57:

I suspect you can only upgrade to 1MB total. The current chip you have is 256K x 16 (512KB). The zip chip pictured is 128K x 8 (128KB = 1M bits) so you don't want that one.

I would try a 256K x 16 ZIP-40 chip such as MT4C16256Z or UPD424170V.

Excellent, I will give that a try. Thank you.

Reply 3 of 8, by RichB93

Posted on 2022-01-18, 13:49

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Looking at the board, it appears to have a 512K chip with a space next to it which is unpopulated - I guess you could potentially solder another 512K chip there for 1MB onboard, and then add a 1MB module. The CL GD 5428 can take up t0 2MB total so theoretically this would be doable.

Reply 4 of 8, by AtomicPlayboy

Posted on 2022-02-05, 19:19

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Following up on this - I gave it a shot with a 512Kb chip, and wow did it not work. I got some garbled text and then an odd screen pattern upon boot and then ... the strong scent of ozone as the chip started overheating! Luckily no harm done except for a small blister from trying to pull the chip by hand, not knowing is was ultra-hot. So I guess this port is undocumented with reason, as it is not useable. Oh, well, was worth a shot.

Reply 5 of 8, by Plasma

Posted on 2022-02-05, 19:29

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Maybe it was installed backwards?

Reply 6 of 8, by Sphere478

Posted on 2022-02-06, 06:15

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Yikes!!!

That is no good..

Yes, was it installed backwards?

My approach would be to buy a pci card with that chipset and the desired amount of ram and just steal the ram from it.

I assume that the pci card won’t have that ram slot though. Just surface mount chips. Which should be easy to remove and solder on.

Sphere's PCB projects.
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Sphere’s socket 5/7 cpu collection.
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SUCCESSFUL K6-2+ to K6-3+ Full Cache Enable Mod
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Tyan S1564S to S1564D single to dual processor conversion (also s1563 and s1562)

Reply 7 of 8, by dionb

Posted on 2022-02-06, 23:50

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Another thing to bear in mind: 2MB is nearly completely pointless on CL GD-542x chips. Up to 1MB you get the expected benefits (1024x768@8b, 800x600@16b, 640x480@24b), above that 2MB just gives access to a few nasty interlaced modes, nothing actually usable.

So a second 512kB is only realistic option.

As for your blister - exactly which chip were you using? Wrong way round and/or short-circuits due to soldering are likely if it's the correct chip, if not a different pinout can give this sort of effect...

Reply 8 of 8, by AtomicPlayboy

Posted on 2022-02-18, 21:11

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Answering the questions above:
- The spec on the chip was "UPD424260V-70 V20 256KX16 FAST PAGE DRAM 40 PIN ZIP NEC", with the eBay listing here: https://www.ebay.com/itm/153203289210.
- I was completely unaware that a zip-40 form factor chip could be installed backwards. It fit each time that I seated it in the slot, and maybe I just happened to insert it the right way the first few times and another the final time. I got different results over a few tries: nothing at all at first (caused by a bent pin), then garbled text/video but a regular boot cycle (after I straightened the pin), then the final meltdown.
- I did ultimately settle on a 512K upgrade, for a total of 1M, for the reason that you've described @dionb. No real return after 1M.

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