VOGONS

Hercules graphics on VGA monitors (mod)

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First post, by Jo22

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Hello everyone,

This is a little thread dedicated to a modification or "hack" for a Hercules Monochrome Card.

I got inspired by oldCat's "Childhood Embers" - 286 AT with Hercules graphics build thread.
I'm also making this thread as a means of indemnification for all those readers of my other project, the 128KB card in this thread over there.

Now, how can we make a VGA screen display Hercules graqphics ?
The most reasonable answer would be that we can't, because they differ in so many ways:

Hercules: 720x350, 2-Bit data, progressive, TTL-level signaling, 50Hz, 18,42KHz horizontal sync.
VGA/SVGA: various resolutions, RGB, prorgressive/interlaced, analogue level signaling, 50 to ~100Hz, 31.47kHz+ horizontal sync.

So under normal circumstances. a mechanical adapter simply won't do. 🙁

(Except we're cheating, of course, and use an LCD TV set with VGA-Input. 😉
They can do as a low as 15Khz, so maybe they support refresh rates all the way up to 31.47kHz.)

Next, we could try to develop a device with a framebuffer. It would catch the Hercules (MDA) picture,
stores it in its SRAM. The controller of that device would then grab the image and generate an 800x600 picture
with the correct timings required for the (S)VGA monitor..

All these ideas are nice, but still a bit out of reach for the poor amateur.

Another approach is what Peter described on his website.
In essence, it is a hack for Hercules clone cards (MGA or MDPA are more generic terms for the Hercules compatible card).

This modification or "hack" involves changing the onboard crystal by one of higher frequency.
The effect is similar to an overclock, so it has to be taken with care. The older chipsets might not support this at all.
If it works, however, both the v-sync/h-sync frequencies will rise. Eventually, reaching the ~31kHz range required for VGA's H-Syc.

Will it really work ?

Let's see! I'll do these modifications step by step for you.

Just relax. 😀

Best regards,
Jo22

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 1 of 42, by Jo22

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Update. I finished making the mechanical adapter. 😀
It looks like this:

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 2 of 42, by Jo22

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Update. The adapter fits. The installed card at the moment is a "Video&Printer-Karte" by Geosoft. I'll have to replace it later.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 3 of 42, by Jo22

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Update: Good news!
The mechnaical adapter seems to work. I get an "out of range" message instead of "no signal".
Video available here: https://youtu.be/H5IhQLKz1j0

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 4 of 42, by Jo22

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Update: I've got found a spare Hercules clone card for testing/modifying.. 😀
Edit: The card is still in unaltered condition.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 5 of 42, by Jo22

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Update. Here's a close up view of the 16MHz crystal, next to the TD3088A.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 7 of 42, by Jo22

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Me, too. If it works, it can be used with an VGA to Composite box maybe.
The monitor at the end of the box would get the proper timings then (I hope so).

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 8 of 42, by Jo22

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Update: I changed the 16MHz crystal by a ~27Mhz one from a CB radio.
The pins of the new one are slightly thicker, so I had to create an adaptor.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 9 of 42, by Jo22

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Update. I tested MGA graphics with TD3088A @27MHz and three different monitors.
Results were.. negative. I expected this, however.
The author of that tutorial himself noted that it doesn't work with all clones.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 11 of 42, by Jo22

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Or real mono TTL CRTs.. 😁

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 12 of 42, by Jo22

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Update. I'll try to swap the 27MHz crystal over to the Winbond card..
Edit: This card is equipped with a 16MHz oscillator crystal and a W86855AF chip.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 13 of 42, by Jo22

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Update. I changed the crystal on the Winbond card by the 27MHz crystal.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 14 of 42, by Jo22

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Update. Unfortunately, it didn't work the second time. 😕
Perhaps the crystal is bad or the frequency to high ? Who knows ?

Interestingly, however, the CRT reports 10.2KHz/250.0Hz.
- The same frequencies it did with with the first card before..

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 15 of 42, by Jo22

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Update. I checked the crystal (quartz). It's perfectly in resonance at 27,055MHz, as far as I can tell.
I also checked 9,183MHz, as well as 13,527MHz.. No signals there (good).

So I guess I'm lucky and this one likely has its basic resonance at 27.055MHz. :D
There's just one question left - Why doesn't it work then ? Is 27MHz simply too much ?

Questions over questions.. I'm done for now, though. And I think I'll go to bed soon. (ー。ー)

Last, but not least, here's the video link: https://youtu.be/KcbKjAlmw84

This little weekend experiment was fun nevertheless. - I hope you enjoyed it, too! :D

Maybe I'll continue it (as soon as I got new ideas).

If you've got any ideas or comments, please let me know.
- Feel free to post anything straight from the heart! :)

Best regards,
Jo22

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 17 of 42, by Jo22

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keropi wrote:

I just think that these cheap clones don't like anything else other than a 16mhz xtal....

Thanks, I'll keep that in mind during trouble shooting.
Luckily, I've got a card with the same chip as the author, at least. ^^

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 18 of 42, by Jo22

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Update. Just had a close look at the adapter and noticed that the author uses pin 2 for the video.
According to the German Wikipedia that pin is used for Green or a monochrome signal.
Other sources also say that this pin is can be used for Sync on Green, too.
Now I wonder, do all VGA monitors support monochrome mode ?
Or was this only possible on early models made between the mid-80s to early 90s ?
The other pins (for RED and Blue) are set to ground.
Edit. ID bits not set to ground.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 19 of 42, by Jo22

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Update. I ordered 25MHz crystals, just to be on the safe side..

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

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