VOGONS


First post, by dionb

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One of the more obscure VGA chipsets - UMC's UM85C418F. Surprisingly it has extremely good raw DOS performance, coming very close to Ark 1000VL. I'm working on an all UMC build, and have two VLB cards with this chipset.

The oddest thing about the chipset is that it also supports I/O functions. Cards with it invariably have an IDE interface and sometimes full I/O (IDE, floppy, game, serial, parallell) as well as VGA. Gerwin posted a topic about a system with one of those cards a few years back:
486 VLB UMC-Chipset, what is it?

I have the same card and a different revision of the same motherboard - but I also have a second card, a Puretek PT-2037. Information about this card seems non-existent apart from an FCC registration (ID: H52PT-2037). It "only" has VGA and IDE, and 1MB of RAM in two SMD SOJ chips. However it has space for 8x 44256 (1Mb) chips, which would add up to 1MB. When I see empty PCB like that I start itching to grab my soldering iron. But... 2MB RAM on old VLB chipsets doesn't always make sense (CL GD542x is a case in point), so before I start I'd like a second opinion on what if anything this might do.

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Now, in his 2014 topic gerwin already mentioned he couldn't find any datasheets for the 85C418F. I've given it another try but not found anything either. The only hint comes from the FCC registration of the PT-2037. Its title (really...) states: Puretek Industrial Co Ltd VL Bus VGA Interface Card, Maximum Resolution 1280 X 1024 (interlaced). PT-2037. That suggests that it's just as DAC-limited as the CL chips. However unlike them it doesn't have an integrated DAC. Instead it has a UMC UM70C188. Now, finding info on that is almost as hard as on the 85C418F, but a single line here gives a lead: UM70C188 (Color palette with triple DAC, same as UM70C171). And datasheets for the UM70C171 are available: pdf.datasheetcatalog.com/datasheet/UMC/mXyztwts.pdf

Unfortunately I'm not able to figure out resolution or refresh rate limits from that datasheet, all I can interpret is that it's limited to 256 concurrent colours from a 256k palette, courtesy of 6bit operation.

So, anyone want to hazard a guess what if anything might happen if I were to solder in the extra RAM (and filter caps)?

Reply 1 of 5, by mpe

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I have no idea if the upgrade can make this more useful. I'd say no, even if actually unlocking more non-interlaced or true-color modes. There are better cards for Windows and for DOS probably doesn't matter.

But given the PCB layout, It looks like it is something that's not too hard. I'd do it just for the science. Makes a nice weekend project and self-satisfaction if nothing else.

Definitely report back if you end up doing it.

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Reply 2 of 5, by dionb

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Next time I'm ordering components I'll get the necessary sockets and caps, and see about the chips. I'd love high colour at 1024x768 non-interlaced, but that's probably out of the question with this RAMDAC anyway. Dead right that this is so easy there's no real reason not to do it.

Bigger challenge: finding the missing JP7 on the card. According to silkscreen on the back that would let me set the IDE to a different I/O address so that this could work as secondary interface. Unlike the RAM that would actually be useful...

Reply 3 of 5, by gerwin

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Nice card there. So the IDE functionality is actually in that UM85C418 chip.
Just a crazy thought: Can it be solder spots for memory to use instead of the two SMD SOJ chips? My card also has solder spots for two types of memory, but they overlap, so that only one type can actually be used.

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Reply 4 of 5, by dionb

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gerwin wrote on 2020-03-04, 22:06:

Nice card there. So the IDE functionality is actually in that UM85C418 chip.

It certainly looks that way. Can't figure out what the UM9502 is doing, but given most of the traces go directly into the UM85C418 it must be the active component.

Just a crazy thought: Can it be solder spots for memory to use instead of the two SMD SOJ chips? My card also has solder spots for two types of memory, but they overlap, so that only one type can actually be used.

Not crazy at all, that was one of my concerns. I can see some of the traces overlap - but not all. However on a multilayer board that doesn't tell the whole picture anyway. Any idea how I could determine that?

Reply 5 of 5, by mpe

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I assume this is a chip with 32bit interface without bank interleaving. So most of the bus will be shared. But there should be at least one connection ( OE or CAS line) that will be wired to the main chip differently when comparing 2x 256k×16 and 8x 256k×4 banks if this is really 2M upgrade and not just an alternate memory configuration

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