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First post, by quicknick

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Hi folks,

I recently acquired some XT-class parts, and since I haven't touched this kind of stuff for more than 20 years... Well, I have some questions.

There is this DTK PIM-640K motherboard, couldn't find much about it but it seems it is quite similar to other DTK models, so jumper/switch settings are not a problem. It had 640KB of RAM, but one 4164 chip was missing so I removed the whole 3rd bank until I can find a spare.
But I'm left wondering... Why so many (empty) ROM sockets? What could go in those? ROM BASIC? Is XT-IDE a possibility? Are there more "modern" BIOSes that add features that were lacking in the original XT?
The board works, started at first attempt with one of my 16-bit VGA cards that also work on 8-bit slots, but hangs with a "Disk not setting ok" error. Haven't tried to go further, because I don't have any XT keyboard and I don't know yet if any of my multi-IO cards will work on 8-bit slots (only have 16-bit cards). To solve the keyboard part I guess that I'll have to build the AT2XT keyboard converter described on Vintage Computer Forum, but if anyone has other ideas... please shout.

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The second board is a Samsung Electronics XT board, also with 640KB RAM - it has onboard serial, parallel ports, floppy controller, real-time clock and video (Paradise PVC4, CGA-compatible from what I've read), all of which can be individually enabled/disabled with jumpers. Things are quite straightforward with this one: also POSTed at first attempt, but after a few test it halts with a "Keyboard Error, press F1 to continue". 😁 I had to use the same VGA card because I don't have any CGA monitor around (and cannot get one because of space)... And this brings another question - what can I use to convert CGA/EGA to VGA? Ebay seems flooded with converters, do I choose one at random or does anyone use a specific model and can recommend it to me?

Regarding both boards, do you think they will support 1.44MB floppies?

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Last, there is this 8-bit video card. Couldn't find much info about it. The main chip is Compaq branded, 104495-001. The ROM is an OKI M3864, which has some peculiarities (two Chip Select lines) so my TL866 can't read it. There are four 4416 RAM chips. There is a D-Sub 9 pin connector, a RCA one (composite?) and a 3-pin header (light pen?). Couldn't test it, as I said there's no monitor, and I'm yet to source a cable to connect what I suspect is Composite out to my TV.
The text on board:
ASSEMBLY NO. 000525
DIAGRAM NO. 000526
VDU CONTROLLER, GA3
BOARD NO. 00527-001

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Reply 1 of 6, by dionb

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quicknick wrote:
[...] […]
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[...]

Last, there is this 8-bit video card. Couldn't find much info about it. The main chip is Compaq branded, 104495-001. The ROM is an OKI M3864, which has some peculiarities (two Chip Select lines) so my TL866 can't read it. There are four 4416 RAM chips. There is a D-Sub 9 pin connector, a RCA one (composite?) and a 3-pin header (light pen?). Couldn't test it, as I said there's no monitor, and I'm yet to source a cable to connect what I suspect is Composite out to my TV.
The text on board:
ASSEMBLY NO. 000525
DIAGRAM NO. 000526
VDU CONTROLLER, GA3
BOARD NO. 00527-001

Given that CGA doesn't need/have its own BIOS (those functions handled by motherboard BIOS), this is almost certainly an EGA card - quite a late one too given the level of integration - despite beging a 3/4-length board, it's almost empty with that Compaq-branded controller doing the heavy lifting. The RCA is indeed composite, for backward compatibility with CGA/MDA. Only question is whether it does PAL or NTSC.

The whole thing looks strikingly similar to my ATi Small Wonder - although that does squeeze everything onto a 1/4-length board.

Reply 2 of 6, by quicknick

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Good point about the BIOS. But the Samsung XT board also has a 27C64 ROM near the Paradise PVC4 chip, would that mean it is an EGA (and not CGA as found on the net)?
If the composite is NTSC won't I get anything on screen, or will it be black&white only?

Reply 3 of 6, by Plasma

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quicknick wrote:

ROM BASIC? Is XT-IDE a possibility? Are there more "modern" BIOSes that add features that were lacking in the original XT?

Yes, yes and yes.

quicknick wrote:

And this brings another question - what can I use to convert CGA/EGA to VGA? Ebay seems flooded with converters, do I choose one at random or does anyone use a specific model and can recommend it to me?

The cheap gonbes converters on ebay won't work directly since they are intended for arcade machines. I recommend the "MCE2VGA" project.

quicknick wrote:

Regarding both boards, do you think they will support 1.44MB floppies?

The DTK won't but the Samsung might. You can add HD floppy support by using a 16-bit multi I/O card and either adding an HD floppy BIOS or booting DR-DOS.

Reply 4 of 6, by bakemono

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Given that CGA doesn't need/have its own BIOS (those functions handled by motherboard BIOS), this is almost certainly an EGA card

Doesn't seem like much RAM for an EGA. But the oscillator might be the big clue, is 18.9MHz common on EGA cards? I think mono cards used 16.2MHz.

Reply 5 of 6, by quicknick

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@Plasma Thanks! Your project is gold! I had just a quick read, so I might be wrong, but with all those extensions the DTK board should be able to cover everything (BASIC, boot from hdd, HD floppy), I just have to order enough 2764 EPROMs, thus surpassing the Samsung board which uses a 27128 EPROM and I guess isn't really moddable.

@bakemono After the quickest search, most EGA cards seem to use a 16.257MHz crystal...

Reply 6 of 6, by Plasma

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Yes, but you will need an XT-IDE card to use a modern HDD (the IDE on a 16-bit multi I/O won't work). You can also use an 8-bit SCSI card with a boot ROM like the TMC-850. Or go old school with an MFM controller.