The MCA-Adapter is a device, which can convert MDA (and Hercules), EGA and CGA to analogue RGBS signal. It supports following features.
- Converts digital MDA, Hercules, CGA and EGA to analog RGBS or RGBHV/VGA signal
- Color DAC for 3, 4, 16 and 64 color modes
- Switchable composite sync from H- and V-Sync
- Loop through HV-Sync, colors conversion only mode
- Includes IBM's "brown-hack"for CGA and 16-colors EGA modes
- Hi/Low resolution EGA mode autodect for proper color representation
- Switchable green, white and amber monochrome screen emulation
Youtube Videos:
- Part 1: Why GBS-8200 is not a CGA/EGA converter https://youtu.be/7c3yLX52ZEs
- Part 2: Theory and implementation https://youtu.be/aUFKB4b5KpE
- Part 3: Showtime https://youtu.be/F1685u-QQ0k
Because why not? I wanted to get some practice with EDA and I needed such an adapter to test some of my old graphics cards.
If you want to connect your old PC with MDA, CGA or EGA graphics to a more modern VGA display, it's going to be difficult. First of all, this old graphics standards are digital, so RGB information is sent as a bunch of 1 and 0. VGA on the other hand understands only analogue signals, so f.e. the intensity of the red color is defined by some value between two voltages and not by a discrete value. So the first job of this adapter is to translates digital video signals into analogue video signals. However, this is may be not enough, since the horizontal sync signal of the old video standards is not supported by a usual VGA monitor. For this purpose, a very famous upscaler named GBS-8200 and compatible can be used to convert the sync signal to VGA compatible level. However, GBS-8200 expects such called composite sync signal, where horizontal and vertical sync signals are combined. This is, where this adapter comes into place, it doesn't only convert digital RGB signal to analogue, but also composite sync signal from incoming horizontal and vertical signals. Long story short, you will need this adapter to connect a MDA, CGA or EGA graphics card to an analogue RGB monitor, either by using GBS-8200 converter or the adapter standalone, in case your monitor supports horizontal frequencies down to 15kHz.
Well, first of all, I wanted to gain some experience in this. Second, I actually was heavily inspired by a YouTube video by TheRasteri, where he evaluates the output possibilities of an old PC, talks about the technical background and creates his own adapter. You can see the video here:
- Part 1: https://www.youtube.com/watch?v=kHhpTq-WSJk
- Part 2: https://www.youtube.com/watch?v=vwKA1z8tg1g
As I saw his solution with a ROM, I instantly thought, that the same thing can be done using a programmable logic IC, like GAL16V8. It would have some benefits in timing behaviour over a solution with ROM and be also very flexible. So, here it is.
The adapter can convert MDA, Hercules, CGA, 16 colors EGA and 64 colors EGA to standard VGA (RGBHV) or RGBS (combined HV-sync). The sync output frequency is always the same as the input frequency. The adapter can only compose HV-sync, but doesn't actively change it. The digital color signal is converted to analogue color signal, so any analogue RGB monitor should understand it.
ATTENTION: Jumper settings changed compared to previous revisions
Color mode selection J1:
| Mode | 1-2 | 3-4 |
|---|---|---|
| CGA/EGA (15kHz / 21kHz) | OFF | OFF |
| MDA/Hercules white (18kHz) | ON | ON |
| MDA/Hercules green (18kHz) | ON | OFF |
| MDA/Hercules amber (18kHz) | OFF | ON |
Sync selectiom J2:
| Mode | Setting |
|---|---|
| HV-Sync (VGA) | 1-2 |
| C-Sync (Composite) | 2-3 |
- Take the gerber files and make a PCB.
- Assemble the board
- Compile code/mceplug.eqn using EQN2JED tool
- Write the compiled jed file to a GAL16V8 IC
- Put it into the board and connect the cables f.e. CGA -> MCE Adapter -> GBS-8200 -> Analog RGB Monitor
| Reference | # | LCSC | Description |
|---|---|---|---|
| C1 C2 | 2 | C2839237 | capacitor 1µF |
| D1 | 1 | C402218 | diode 1N5819 |
| H1 | 1 | C492405 | pin header 1x6 |
| J1 | 1 | C492419 | pin header 2x2 |
| J2 | 1 | C429954 | pin header 1x3 |
| PWR1 | 1 | C46398 | USB-B mini port |
| PWR2 | 1 | C152154 | pin header 1x2 (+5V connector) |
| R1 R2 | 2 | C119317 | resistor 470 Ohm |
| R3 R5 R7 | 3 | C2848597 | resistor 1.3k Ohm |
| R4 R6 R8 | 3 | C119320 | resistor 680 Ohm |
| R9 | 1 | C173139 | resistor 10k Ohm |
| U1 | 1 | C6509 | GAL16V8B (or compatible) |
| VIDEO_IN1 | 1 | C141882 | DB9 female connector (MDA/EGA/CGA) |
| VIDEO_OUT1 | 1 | C75754 | DIB15 female connector (VGA) |
- GBS-8200 with original hardware (only 15kHz modes work)
- GBS-8200 with gbs-control firmaware (Hercules and hi-res EGA sync unstable)
- NEC Multisync LCD1970NX 19" TFT (everything works fine, Hercules is not in full screen, but works stable)
- EIZO L365 15" TFT (only Hi-Res EGA works, doesn't support 15kHz CGA nor 50Hz Hercules)
- Benq BL702A
