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TTL Monitor: HGC does not equal CGA/EGA?

kyodai

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Jan 26, 2011
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Kerpen, Germany
I was gifted a nice IBM Portable clone (probably a Taiwanese product) today by a workmate.

It has an internal CRT that is powered by a cute Hercules card via DE9 (No surprise).



Well now we were talking about monochrome monitors and none of us really knows too much about them. We understand that Hercules, CGA and EGA all used TTL CRT displays and DE9 connectors. So the idea that the displays are compatible arose and we were curious if the HGC card could be replaced by a CGA or EGA card.

From my research on wikipedia i learned that HGC is compatible with MDA - no surprise since they use the same resolution and hercules was meant to be a replacement for MDA to display graphics. So I am 99.9% sure MDA and Hercules cards could be swapped.

One sentence from Wikipedia kept me from trying anything with this wonderful computer:

Even though these all use the same DE9 connector, the displays cannot all be interchanged and monitors or video interfaces may be damaged if connected to an incompatible device using the same connector.


So obviously i don't wanna destroy the display.


Does anyone know more about the compability of these old TTL monochrome displays?

There's some adaptors to connect a modern display to an old HGC/CGA/EGA card - but is there also something to do it the other way round? WOuld be cool to have some VGA card in that machine and see the output on the monochrome display. But i guess that isn't possible or doesn't exist?
 

PeterNC

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reenigne

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The critical issue preventing compatibility between MDA/Hercules and CGA (or 200-line EGA) isn't text vs graphics or composite vs TTL, it's the horizontal and vertical timings. MDA/Hercules is ~18.4kHz horizontal and ~50Hz vertical while the CGA standard is ~15.7kHz horizontal and ~60Hz vertical. If you try to drive a MDA monitor with a CGA card the picture would be scrambled (and the monitor could be damaged if the sync pulses drive the ramp generators directly instead of going through PLLs).

It is possible to reprogram a CGA card to generate MDA-compatible timings, but what would be the point? The resolution would be incompatible with existing software and if any such software tried to set up its own timings then you get into dangerous territory again. Plus, of course, the raison d'etre of a CGA card is to display colour but there's no way to do that with an MDA monitor. So might as well save your CGA cards to use with your CGA monitors and keep your MDA monitors connected to MDA/Hercules cards.

It would be possible to have a device that converts CGA or VGA signals to MDA, but I've never heard of anybody making such a thing (I guess because old monitors are scarcer than their corresponding display adapters, so the opposite problem to the one you have is more common). It would probably have be an FPGA-based device, with enough RAM to store a frame or two's worth of image data. I'm not sure, but it might be possible to coax something like a Gonbes GBS-8200 into doing the conversion - if it is possible it would probably be a fairly involved piece of hacking, though.
 

creepingnet

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I have one of those NEC monitors - how they work is there is a switching module board in the back - that's the board I had to replace the 9-pin connector on and fix the cracked traces. That entire board is dedicated to switching between Analog and then Digital, and then changing the "Sync" settings for each using Resistor Packs and other IC's or discreet components. I found the digital section because that is the section that I had to patch a few more traces together on.
 
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Plasma

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ATI EGA Wonder can display CGA/EGA graphics on a monochrome TTL (MDA/Herc) monitor.

Arkanoid2_IBM_Portable_PC.JPG
 

rpiguy2

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Plasma, that is likely 15khz 640x200 monochrome screen almost definitely not a herc/MDA display, which is 720x350 and requires 18khz.
 

Plasma

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Yes, that screenshot is not a good example. It is an IBM 5155 with a 15khz monitor. But the EGA Wonder also works with 18khz monitors.

8N1I39W.jpg


 
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