RGBS converter for MDA, Hercules, CGA and EGA
The MCA-Adapter is a device, which can convert MDA (and Hercules), EGA and CGA
to analogue RGBS signal. It supports following features.
Youtube Videos:
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:
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
| 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) |