Tuesday, August 25, 2015

New IBM CGA to Old IBM CGA Mod

There has been a revised interest in the IBM CGA card over the last few years.  I attribute this in no small part to demos like 8088 Domination and 8088MPH as well as interest in vintage PC computing in general.  IBM CGA cards are not particularly uncommon to find, but the New cards are much more common than the Old cards.

Varieties of IBM CGA

There are four known varieties of IBM CGA boards.  Here is a table with their features :

Board Number
# of Resistors
Bracket Type
Composite Output Type
Schematic Reference
18504464
7
Black Oversized
Oldest
IBM Personal Computer Hardware Reference Library Technical Reference, First Edition August 1981, D25-30
18504472
8
Black Oversized
Old
IBM Personal Computer Hardware Reference Library Technical Reference, Revised Edition April 1983, D46-51
1501486
12
Silver
Old
IBM Personal Computer XT Hardware Reference Library Technical Reference, Revised Edition April 1983, D36-41
1501982
15
Silver
New
IBM Personal Computer Hardware Reference Library Technical Reference Options and Adapters, Revised Edition April 1984, Color/Graphics Monitor Adapter 28-33

The oldest IBM CGA board has a lower amount of resistance from the transistor to the composite video output.  What this means for the video output is unknown, but I would suspect that it would be somewhat brighter than later cards.  Also, the schematic does not give resistor values for the other four resistors connected to the composite video circuit and I cannot make out the values with the photo of the board I have.

The differences between 18504472 and 1501486 are the addition of 30 Ohm resistors on the bottom row for resistors.  These only affect the RGBI output signals, and were probably added to improve compatibility with digital RGBI monitors.

The oldest cards came with black brackets.  These brackets are sized for the five expansion slots of the IBM PC.  The IBM PC/XT used eight expansion slots which were set closer together, so this black bracket will block the slot above it.

Old IBM CGA cards almost invariably use a green PCB color and New IBM CGA cards usually use a brown PCB color.

Old IBM CGA cards almost invariably use MC6845 CRTC chips, New IBM CGA cards use either the MC6845 or HD4650/HD6845 CRTC.  I was lucky to have chosen the New IBM CGA card that had the MC6845.  This allowed me to play the party version of 8088MPH correctly, which only shows correct colors on an Old IBM CGA card.  The final version of the 8088MPH shows appropriate colors on either an Old or New IBM CGA card with either CRTC chip.

Modding a New IBM CGA card to an Old IBM CGA card

Here are the steps I took to mod my new IBM CGA card to an old IBM CGA card :

1. Lift pin 4 from U24 (the pin broke completely off for me so I had to replace the IC)
2. Solder a wire between pin 4 of 24 to pin 4 of U65 (pin 4 of U65 does not need to be lifted)
3. Remove all resistors in the first three rows.
4. Replace the resistors in the first three rows (IBM's designations are hard to follow) with the following, all values in Ohms:

100       51   Empty
3300   13000    5600
Empty   2200   Empty   Empty

5. Solder a 33 Ohm resistor to the bottom contact of R4 and the top contact of Q1. (Because of the tight space, you may want to do this from the solder side).

6.  If you have a New IBM CGA card with a yellow patch wire as shown below, make sure you reconnect it and do not make a solder bridge. If left unconnected, the light pen port will not work.

Note in my picture, I had to use a 1000 and a 12000 ohm resistor in series to make a 13000 ohm resistor.



Could the process be reversed?  Undoubtedly it could with the appropriate schematic, (see the chart above) but because Old IBM CGA cards are much less common, it would almost certainly be easier to just get a New IBM CGA card and mod it.

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