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Which kind of bus is this MDA card intented for?

Rauli

Experienced Member
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Sep 16, 2013
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I post a photo of the latest card I got: MDA.jpg

It's just a regular MDA/parallel clone... except for the bus connector. It should be 8-bit ISA, but it has the 16-bit ISA extension and a 2nd very strange (and unknown for me) extension.

More details about the card:
- External connectors are DB-9 and DB-25 (both female).
- Looks like MDA/Parallel or Hercules/Parallel
- Newest chip is dated 8731
- Chips include a 6845 (CTRC), two TMM2016 (RAM), and a stickered chip with UV window (EPROM or micro-controller).
- Smaller ones are just 74LS and PALs (the stickered ones).
- According to datasheet each RAM chip is 2 Kbytes, so not Hercules/Parallel, but MDA/Parallel.

What do you think about it? Did you know about the existance of such connector?

Note: I got replies in other forums claiming it's a VESA Local Bus connector. IT IS NOT. The VLB extension has narrow fingers, the same width than PCI connector fingers. This card 2nd extension fingers are the same width than the 8-bit and 16-bit ISA connector fingers.

Note 2: The big stickered chip turned out to be an 4 Kbyte EPROM with characters definitions (nothing else, no BIOS or such).
 
Last edited:
I post a photo of the latest card I got: View attachment 38751

It's just a regular MDA/parallel clone... except for the bus connector. It should be 8-bit ISA, but it has the 16-bit ISA extension and a 2nd very strange (and unknown for me) extension.

More details about the card:
- External connectors are DB-9 and DB-25 (both female).
- Looks like MDA/Parallel or Hercules/Parallel
- Newest chip is dated 8731
- Chips include a 6845 (CTRC), two TMM2016 (RAM), and a stickered chip with UV window (EPROM or micro-controller).
- Smaller ones are just 74LS and PALs (the stickered ones).
- According to datasheet each RAM chip is 2 Kbytes, so not Hercules/Parallel, but MDA/Parallel.

What do you think about it? Did you know about the existance of such connector?

Note: I got replies in other forums claiming it's a VESA Local Bus connector. IT IS NOT. The VLB extension has narrow fingers, the same width than PCI connector fingers. This card 2nd extension fingers are the same width than the 8-bit and 16-bit ISA connector fingers.

Note 2: The big stickered chip turned out to be an 4 Kbyte EPROM with characters definitions (nothing else, no BIOS or such).

The J3 connector looks a little odd also.

Possibly an industrial board. Is J3 connected directly to the extra card edge connector?
 
Is J3 connected directly to the extra card edge connector?

Yes! The 3 P3 fingers seen in the photo, go to one of the PALs and also go to J3.

The 2 P3 fingers on the other side of the card: One goes to the jumper block U16 (*) and I can't trace the other one.

(*) I don't know what this jumpers do. I expected they change the LPT port address or IRQ, but they don't. And I don't know which other thing can be configured on an MDA/parallel card.
 
My guess is that this board can be operated as a 16-bit MDA card, making screen writing much faster. The extra traces on J3 are a mystery.

Have you tried this card in an 8-bit slot to see if it works as a normal 8-bit MDA card? Clearly, there's not enough memory here to do graphics.
 
My guess is that this board can be operated as a 16-bit MDA card, making screen writing much faster. The extra traces on J3 are a mystery.

Yes, but I don't know if both 2 Kbyte RAM chips are arranged as 4 Kbytes, or as 2 Kwords (storing separately even and odd addresses). Only the last case could benefit from 16-bit access mode. Is there any tool to measure video memory speed in both 8-bit and 16-bit access? Like VGASPEED but for MDA, and not requiring a 386.

Have you tried this card in an 8-bit slot to see if it works as a normal 8-bit MDA card? Clearly, there's not enough memory here to do graphics.

Well, my monochrome screen is not at home, so I tried the card blindly (no screen) alone and also as a secondary video adapter (together with a VGA), but on an 16-bit ISA slot. All I can say is that all sysinfo tools detect the card as an MDA. Next test will be on an 8-bit ISA slot, and later with the monochrome screen.
 
If I were the designer, I'd use the RAMs as 16-bit wide; even addresses: character, odd addresses: attribute. I doubt that either organization would be sub-ISA bus speed, so 16-bit access should be roughly twice the speed of an 8 bit only card.
 
What's the actual speed of 8-bit ISA? Something like 1 MB/s, isn't it?
For a 4 KB frame buffer, it gives 250 fps.
Who needs more, especially considering MDA's refresh rate is only 50 Hz?
 
Was there potentially a graphical overlay from another proprietary display adapter, fed in on the J3 connector?

From an old high-end CAD system, or something...

That's a pretty unique card. I hope you figure it out, and once you do I hope you let us know!
 
What's the actual speed of 8-bit ISA? Something like 1 MB/s, isn't it?
For a 4 KB frame buffer, it gives 250 fps.
Who needs more, especially considering MDA's refresh rate is only 50 Hz?

Scrolling is a big problem on PC text displays. Look at the BIOS code. Using a REP MOVSB is pretty slow. A REP MOVSW on a 16-bit CPU would be considerably faster. If you've got an MDA, try echoing the data from a 9600 bps feed. The move-memory scrolling can't keep up.

Some comms package got smart and changed the "start address" for the 6845 and allowed the buffer to "wrap around" the display memory, but that wasn't what was used in the IBM BIOS to scroll. In particular, the "Print Screen" function didn't work right. Others just buffered up data until they caught a break, then updated the screen.
 
Was there potentially a graphical overlay from another proprietary display adapter, fed in on the J3 connector?

From an old high-end CAD system, or something...

I don't think the card has the required circuitry to treat overlays. Not sure, but I think it would need more than 74LSs and old PALs.

That's a pretty unique card. I hope you figure it out, and once you do I hope you let us know!

I also hope it, thanks, but I'm more interested in the kind of bus, than in the card itself :p
 
What's the actual speed of 8-bit ISA? Something like 1 MB/s, isn't it?
For a 4 KB frame buffer, it gives 250 fps.

I think it's 8 Mbytes/s. But a 0-wait-state memory access takes 4 cycles to a 8088/8086 (at 4.77 or 8 MHz).
 
Granted I've never seen any card like this before, but perhaps it was meant to go in an Amiga 2000? The additional card edge would certainly line up in the overlapped Zorro/ISA bus slots.
 
It almost looks like the additional card edge connector was only meant for testing a prototype, and was intended to be snapped off on the production version.
 
I've seen a little card edge connector behind an ISA slot in...I want to say a Compaq Deskpro system? Might've even been 6-pin. I never figured out what it was for, the ISA slot was otherwise normal.
 
Granted I've never seen any card like this before, but perhaps it was meant to go in an Amiga 2000? The additional card edge would certainly line up in the overlapped Zorro/ISA bus slots.

The card seems to be manufactured in 1987. The card works in a PC using just the ISA-8 and ISA-16 part, leaving the strange extension unconnected. I'm not in the Amiga world, but with this information, could it still be an Amiga Zorro card?
 
Any other markings on the back of the card? Otherwise, I'd say dump the ROM and maybe there's a clue in there.

Nothing but a sequence of letter+numbers (I will look again and take note).

I dumped the ROM, it contains character definitions, no more. That's the only information MDA and Hercules cards need to know. The PC BIOS knows how to handle these video cards (and CGA, too).
 
I've seen a little card edge connector behind an ISA slot in...I want to say a Compaq Deskpro system? Might've even been 6-pin. I never figured out what it was for, the ISA slot was otherwise normal.

Do you mean you've seen a similar bus connector (like P1+P2+P3 connector parts) or a similar pin connector (like J3)? That angle-pin connectors are not unusual where there's not much space and straight pins can't be used. I've seen the same, for example, inside keyboards, where the cable is connected.
 
Granted I've never seen any card like this before, but perhaps it was meant to go in an Amiga 2000? The additional card edge would certainly line up in the overlapped Zorro/ISA bus slots.

Apart of mechanically compatible, definitely not a Zorro card. I got the Zorro bus specification and the address and data lines are in very different position than ISA bus. If it were a Zorro card, it wouldn't have worked in my PC.

So the title question is still unanswered.
 
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