Color MDA?

[vwestlife]

I heard a rumor years ago that IBM's MDA monochrome text adapter was originally planned to support color, and that the first production run even had circuit traces for the Red, Green, and Blue signals going from the video chip to the monitor output connector (although of course the color capability was ultimately unused and unsupported). Has anybody else heard anything about this?

[lynchaj]

Hi,

Yes, something similar to that. I seem to recall that the MDA started out life as an advanced video card for the IBM system 23 or some such. It "devolved" into the MDA and tossed most of its functionality out along the way. However many artifacts of its previous incarnations persisted.

http://oldcomputers.net/ibm5322.html

http://www.seasip.info/VintagePC/mda.html

http://www.seasip.info/VintagePC/mda.html#cmda

Apparently some MDAs can display color if attached to a proper display.

Not terribly surprising given the IBC PC 5150's rather "unique" heritage.

Thanks!

Andrew Lynch

[vwestlife]

Apparently some MDAs can display color if attached to a proper display.
I'd love to have a color-enabled MDA board, because my Mitsubishi Diamond Scan color monitor can display MDA/Hercules video just fine, as well as CGA, EGA, VGA, Super VGA up to 1024x768, as well as the video outputs of other computers such as Commodore 128, Amiga, Atari ST, etc., and it even has a composite input. Many older NEC MultiSyncs have similar capabilities (minus composite) and some even have a switch to let you simulate green or amber phosphor when displaying monochrome video.

It's hard to tell what was in the minds of the IBM engineers. Another well known "easter egg" is the "thin font" mode of the CGA board, which can be enabled by shorting the two-pin solder pad next to the 6845 chip. This gives a more "elegant" text display on a high-resolution RGB monitor, but allegedly it was unused because it's too thin to be displayed clearly on a composite or low-resolution RGB monitor. Indeed, IBM didn't even sell any color monitor for the PC until 1983, so clearly the intent was for early PC buyers to use a TV set or inexpensive composite monitor, just the same as with other popular home computers at the time.

[mbbrutman]

I'm confused about the supposed color capability of the MDA boards, and there wasn't enough in that web page to explain it.

Was it capable of sending different TTL outputs to a monitor similar to the CGA monitor? If so, how was color set on character? Is it just bits being set in the attribute bytes, similar to the way colors are used in text modes?

MDA and CGA both use the same CRT controller. I wasn't aware that IBM was confused on the design, but somehow it doesn't suprise me anymore.

(None of the PCjr announce letters or early marketing material show the PCjr color monitor - it's always used with the IBM 5153. They seem to be perpetually late with the color monitors.)

[lynchaj]

I have never seen a thorough explanation of the semi legendary color MDA modes. That webpage sort of substantiates it and I have seen other explanations on CCTALK or comp.os.cpm of the original MDA had lots of strange vestigal circuitry. Apparently some of it even appeared in early MDA clones until they figured out they could cut the cost but dropping the components. Given that, however, it is still just one tad better than rumor quality information. Maybe there is a retired IBM engineer who could shed some light on the subject.

Thanks!

Andrew Lynch

[vwestlife]

Another little-known peculiarity is the CGA's interlaced 50-line text mode. The display becomes very flickery because the refresh rate is cut from 60 to 30 Hz, but the text shrinks to half its normal height, leaving room for 50 lines on the screen. The pixel resolution becomes 640x400, but I don't think a standard CGA card has enough video RAM to support that full resolution in graphics mode.

I once tricked a Zenith 8088 clone into this mode just by dropping a book on the keyboard. :eek: Whatever was on the DOS 80x25 text mode screen at the time suddenly became very flickery on the CGA monitor and shrunk to the upper half of the display, with the lower half showing random garbage characters. This happened during programming class in high school, so I didn't have much chance to investigate how overflowing the keyboard buffer led to the 6845 flipping into interlaced mode!

Somewhere I have a small DOS program that lets you mess around with the 6845's registers, including the number of rows and columns of text displayed on the screen. You can pretty much fill up the entire overscan ("border") area with text before the display will lose sync and start to roll or tear.

Then of course there's color 6. The CGA board outputs it as dark yellow, and it is changed by circuitry in the monitor into brown! Aftermarket monitors which lack this extra circuitry display it as dark yellow.

[JohnElliott]

I have never seen a thorough explanation of the semi legendary color MDA modes. That webpage sort of substantiates it and I have seen other explanations on CCTALK or comp.os.cpm of the original MDA had lots of strange vestigal circuitry.

There certainly is some strange circuitry on the MDA, as I describe on the webpage above, but it mainly seems to relate to what looks like partial support for a CGA-like low resolution mode. The schematic does have a line on it marked "+B/W" (sheet 7 of the schematic, page D-42 in my copy of the tech ref). This is on pin 7 of U58, and on all four of my IBM MDAs nothing appears to be connected to this pin. There are also no traces that I can see to the "unused" pins on the monitor connector, which you'd need for colour (CGA has red/green/blue on pins 3,4,5; on the MDA these are not connected).

Perhaps everyone who's got an IBM MDA should take a look at it and see if there's anything connected to pins 3/4/5 of the video output?

[Anonymous Freak]

Another little-known peculiarity is the CGA's interlaced 50-line text mode. The display becomes very flickery because the refresh rate is cut from 60 to 30 Hz, but the text shrinks to half its normal height, leaving room for 50 lines on the screen. The pixel resolution becomes 640x400, but I don't think a standard CGA card has enough video RAM to support that full resolution in graphics mode.

I miss the undocumented pre-VESA modes that had really high text resolutions. I recall one video card that could do 132x50. It was very nice for programming pre-Windows.

[vwestlife]

I miss the undocumented pre-VESA modes that had really high text resolutions. I recall one video card that could do 132x50. It was very nice for programming pre-Windows.
132-column text mode was pretty common on older VGA cards. Even standard 256K VGA cards often supported it. I have a Western Digital 8-bit VGA card from 1988 and the owner's manual lists all the text and graphics modes it supports, including 132-column text mode. Some cards had a 100-column text mode instead.

[Trixter]

Another little-known peculiarity is the CGA's interlaced 50-line text mode. The display becomes very flickery because the refresh rate is cut from 60 to 30 Hz, but the text shrinks to half its normal height, leaving room for 50 lines on the screen. The pixel resolution becomes 640x400, but I don't think a standard CGA card has enough video RAM to support that full resolution in graphics mode.


This only works on clones. Andrew Jenner and I spent a significant amount of time trying to get this working on a true IBM CGA w/5153 and concluded that IBM's CGA card didn't implement the signals as advertised.


Somewhere I have a small DOS program that lets you mess around with the 6845's registers, including the number of rows and columns of text displayed on the screen. You can pretty much fill up the entire overscan ("border") area with text before the display will lose sync and start to roll or tear.


Yes, I've extended 80x25 to 90x30 without any ill effects this way.

Other fun tricks I've done include a 320x100 "letterbox" mode with 2 pages, and a 160x200 "column" mode with 2 pages. I plan on using these in a demo someday. Andrew and I have been trying our damnedest to trick the MC6845 into changing the display start address in the middle of the screen; hopefully someday we'll get it working.


Then of course there's color 6. The CGA board outputs it as dark yellow, and it is changed by circuitry in the monitor into brown! Aftermarket monitors which lack this extra circuitry display it as dark yellow.

You have no idea how difficult it was to convince emulator authors that was the case.