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Monochrome display card with MDA/CGA/EGA to VGA converter problems

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rlauzon

Experienced Member
Joined
Feb 18, 2018
Messages
239
Location
Grand Rapids, MI
Back story:
I'm restoring an old IBM-XT clone and I need to hook it up to a more modern display that only has VGA.
So I picked up a MDA/CGA/EGA to VGA converter (https://sites.google.com/site/tandycocoloco/mda-cga-ega-to-vga).
It works pretty good. The computer came with a display card that I had assumed was a monochrome card, but later I found that it is a CGA card.
For this computer I want the old monochrome experience, so I got a monochrome video card.
That card works, but....

The problem:
When I use the monochrome card, the last 3 lines of the display are chopped off.
I already tried to adjust the display, but it's not a monitor problem.

I originally got a real IBM monochrome display adapter, and later a real Hercules monochrome display adapter. Both exhibit the same problem.

Do I just have a "bad" MDA/CGA/EGA to VGA converter? Or is there something else I might be missing?

The display with the monochrome adapters is way better than the CGA adapter, with the exception of the missing bottom 3 lines. So I'd really like to use those if I can.
 
3 rows of text is pretty extreme, I have not seen that. Make sure you have the DIP switches set correctly. They are different for mono (on off on), herc (on on on), and CGA (off off off).
 
I brought out my:
* IBM PC fitted with IBM monochrome card ("IBM Monochrome Display and Printer Adapter", A.K.A. IBM MDA)
* My MCE2VGA (of version 2)
* An LCD VGA monitor

Of note, my MCE2VGA has the switch block soldered on 180 degrees out, so switch 1 in the switch block is the documented switch 4, switch 2 is the documented switch 3, and so on. That mistake is shown at [here]. The 'DIP-Switches' section there, includes "The DIP-switches controls some features, from the 1 to 4 (1 closest to the DB-9 Connector)", but in the photos, the soldered on switch block has switch 4 closest to the DB-9 Connector, like on my MCE2VGA.

Also, catering for the aforementioned soldering mistake, I discovered that on my MCE2VGA, the operation of switches 3 and 4 (DIP 3 and DIP 4) is different to that documented at [here]. My switches 3 and 4 are per:
Switch 3: ON = Amber if 'monochrome emulation' mode is enabled, OFF = Green if 'monochrome emulation' mode is enabled
Switch 4: ON = RGB, OFF = monochrome emulation

So, on my MCE2VGA's switch block, catering for the aforementioned differences, I set switch 1 (i.e. closest to the DB-9 Connector) to ON (closed), switch 2 to OFF (open), switch 3 to ON (closed), switch 4 to ON (closed). I then powered on the IBM PC. The PC booted to Cassette BASIC, with 25 text lines showing on the VGA monitor.

I discovered that I did not need to power off/on the MCE2VGA if I wanted to change the setting of any of the 4 switches in the switch block - the effect was immediate.

But in 'playing' with all four switch block switches, I did not see any of the 25 text lines disappear.

I experimented with the 'Down' pushbutton, and one line was the most that I could get to drop off the screen, and whatever setting of that I made, disappeared on MCE2VGA power off/on.
 
Let's see if this works.
Here's what it looks like in "color" mode with the CGA card.
And here's what it looks like in monochrome mode with the Hercules Monochrome card.

I have played with setting the DIP switches on the MCE2VGA. I also adjusted the settings (vertical center and phase mostly) but the most it did was move everything up, but not display the missing lines.
 
I'm getting a sinking feeling that this is a problem with the PC itself.

There is a DIP switch on the motherboard that I have to change when I switch out the CGA/Hercules card.
 
Setting the motherboard DIP switch for the video config is normal for the XT and clones. This is necessary because mono/herc/CGA video cards have no onboard BIOS, so the system BIOS needs to know what video card is installed. For the XT, switch 5 and 6 should both be off for mono/herc. Some clones were different.

It looks like there is some garbage on the left side of the screen in your hercules photo. Do you see the same garbage with the IBM mono card?
 
rlauzon said:
..., but it's not a monitor problem.
Click to expand...
Why do you say that it is not a monitor problem? Is that because you tried a different VGA monitor?

Note that per the 'Output Resolutions' section at [here], the MCE2VGA outputs a different VGA signal for MDA and Hercules compared to CGA. Could it be that your particular VGA monitor has a problem with what the MCE2VGA outputs for MDA and Hercules?
 
rlauzon said:
texelec
Click to expand...

I also have texelec and it works fine with both original Hercules GB102 and with two 3rd party clones. I use an old 17" LG Flatron 1280x1024 LCD monitor. I tried it with a more recent 24" Samsung 1920x1200 LED monitor, but it was "eating" first column on the left.

As the person above said, try a few different monitors, see how it works. Alternatively, may be use mce2vga with CGA and get RGBtoHDMI for Hercules. RGBtoHDMI is much more tolerant to slight deviations in vsync/hsync, so if your card is a bit off the spec, it will work better then mce2vga.

BTW, I noticed some visual garbage on your hercules display photo. I also get that in 350 line modes (EGA 640x350, Hercules 720x348). I wonder if it's because of lack of some filtering caps or may be because PCB is basically bare, no RF shielding.
 
Plasma said:
Setting the motherboard DIP switch for the video config is normal for the XT and clones. This is necessary because mono/herc/CGA video cards have no onboard BIOS, so the system BIOS needs to know what video card is installed. For the XT, switch 5 and 6 should both be off for mono/herc. Some clones were different.

It looks like there is some garbage on the left side of the screen in your hercules photo. Do you see the same garbage with the IBM mono card?
Click to expand...

Ya, for me it's switches 3 and 4 on the block depending on whether I want 40 or 80 column color, or monochrome.

Yes, the garbage is there for all of the video cards. I think it's an artifact of the MDA/CGA to VGA converter.
 
modem7 said:
Why do you say that it is not a monitor problem? Is that because you tried a different VGA monitor?

Note that per the 'Output Resolutions' section at [here], the MCE2VGA outputs a different VGA signal for MDA and Hercules compared to CGA. Could it be that your particular VGA monitor has a problem with what the MCE2VGA outputs for MDA and Hercules?
Click to expand...

I have tried 2 monitors with the same results. But both monitors were 4:3 aspect ratio.

I think I can locate a VGA monitor that's 16:9 to see if there's a difference.
 
dhau said:
BTW, I noticed some visual garbage on your hercules display photo. I also get that in 350 line modes (EGA 640x350, Hercules 720x348 ). I wonder if it's because of lack of some filtering caps or may be because PCB is basically bare, no RF shielding.
Click to expand...

The MDA/CGA to VGA board is sitting on the wire mesh where the old tube monitor and controller board used to be. But it's pretty close to the power supply, so RF interference is certainly possible.
 
rlauzon said:
Ya, for me it's switches 3 and 4 on the block depending on whether I want 40 or 80 column color, or monochrome.

Yes, the garbage is there for all of the video cards. I think it's an artifact of the MDA/CGA to VGA converter.
Click to expand...

I think some part of your MCE2VGA is defective (SRAM?). Sometimes the image can be shifted very slightly but you should always be able to correct it with the adjustments. And there should not be garbage pixels.

If you bought it recently, maybe Texelec will exchange it for an RGBtoHDMI.
 
Plasma said:
I think some part of your MCE2VGA is defective (SRAM?). Sometimes the image can be shifted very slightly but you should always be able to correct it with the adjustments. And there should not be garbage pixels.

If you bought it recently, maybe Texelec will exchange it for an RGBtoHDMI.
Click to expand...

That's possible. It's been a while since I purchased the MCE2VGA. It's also possible that the device may have been damaged through my misuse. Anyway, I went and bought Texelec's RGBtoHDMI - 6-bit TTL Edition. If it doesn't fix my problem, I have other things I can use it for.
 
IIRC MDA runs at 50Hz scan rate rather than 60. The monitors you have tried may not try to sync to anything lower than 60? Although your problem seems pretty extreme; I think I'd double check jumper settings on the converter before doing anything too crazy.

But if all else fails, I think I'd try generating csync with an XOR gate and then mixing it with video and intensity using some resistors and a couple diodes, and feed it into the composite input of a monitor that can handle PAL signals.

Uhhhhhhhhh, lemme see..... Here's a circuit that does what I am talking about, but I've never tried this particular one. Option #2. http://boginjr.com/electronics/rad/rgbi/ .

(Please do your own research before taking any of my advice here; I'm going half from memory and half don't-know-what-I-am-talking-about.)
 
bladamson said:
IIRC MDA runs at 50Hz scan rate rather than 60. The monitors you have tried may not try to sync to anything lower than 60? Although your problem seems pretty extreme; I think I'd double check jumper settings on the converter before doing anything too crazy.
Click to expand...
Per the 'Output Resolutions' section at [here], the MCE2VGA converter outputs VGA at either 60Hz or 70Hz.
 
At this point, I've tried 2 different MDA and 2 different CGA cards.

Both MDA cards produce the same (bad) results: Text off the bottom of the screen.

Both CGA cards produce the same (not real good) results: Text is all on the screen, but the characters formed have some issues. Like some vertical lines are missing on the screen.

I've tried all 4 cards on 2 different monitors and the monitors produce the same results.

So that leaves the MCE2VGA converter board. I'm going to try something different in a week (whenever my RGBtoHDMI gets here) and see what happens.
 
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