First post, by dionb
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.
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)?