ranger81/mce-adapter

RGBS converter for MDA, Hercules, CGA and EGA

MCE Adapter (Prototype!)

The MCA-Adapter is a device, which can convert MDA (and Hercules), EGA and CGA to analogue RGBS signal.

Why did I make this?

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.

Why might somebody need this?

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.

Why didn't I take an existing solution?

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.

Which modes does it support?

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.

Jumpers and switches

Basic jumper description:

• J1 mono (jumper off) or color (jumper on)
• J2 mode select (depends on the color or mono mode)
• J3 invert composite sync signal (see composite signal)
• J4 composite sync (1-2) or HV-sync (2-3)

Mode selection:

Mode	J1	J2	J3
MDA/Hercules white (18kHz)	OFF	OFF	ON
MDA/Hercules green (18kHz)	OFF	ON	ON
CGA/EGA 16 colors (15kHz)	ON	OFF	OFF
EGA 64 color (21kHz)	ON	ON	ON

Composite sync signal

Every end of line the graphics adapter sends a so called H-Sync or horizontal synchronization signal. Every end of the screen the graphics adapter sends so called V-Sync or vertical synchronization signal. The history of this principle goes many years back to cathode tubes, old TVs and monitors. This signals are used for the retraction of the ray in the monitor, so it knows when to restart to draw the next line or the whole screen. The history and the theory behind it is quite long and is not the point here. Important to understand is that different video modes, MDA, CGA, EGA are using different V- and H-Sync frequencies. It is important for the monitor to be able to distinguish them and it does by the frequency of the sync signals, but also by a polarity of the signals. Sometimes one of the sync signals is high positive and sometimes it is low positive. This also helps the monitor to detect it properly, but some monitors don't take it into account and in such cases the Jumper J3 can be helpful. If you have problems with the sync signal with one of the modes try to flip this jumper to get the sync signal inverted. This jumper has however no meaning, if you set the adapter to HV-sync mode (Jumper J4 to 2-3). Here is the overview of the different video modes, their sync frequencies and sync polarities:

Video Mode	H-Sync	V-Sync
MDA	+ 18kHz	- 50Hz
CGA	+ 15kHz	+ 60Hz
EGA low	+ 15kHz	+ 60Hz
EGA high	+ 21kHz	- 60Hz

Usually, you only need to invert the composite sync signal only if V-Sync signal is negative. In the columns H- and V-Sync + means high-positive and - means low-positive sync signal. EGA exists in two variants low resolution of up to 640x200 and high resolution with up to 720x350, hence the higher H-Sync frequency of 21kHz, since 15kHz are just not enough to draw 350 lines with 60Hz.

How to use this stuff?

1. Take the gerber files and make a PCB.
2. Assemble the board
3. Compile code/mceplug.eqn using EQN2JED tool
4. Write the compiled jed file to a GAL16V8 IC
5. Put it into the board and connect the cables f.e. CGA -> MCE Adapter -> GBS-8200 -> Analog RGB Monitor

Bill of materials

Reference	#	LCSC	Description
C1	1	C383039	electrolythic capacitor 220µF
D1	1	C402218	diode 1N5819
H1	1	C492405	pin header 1x6
J123	1	C65114	pin header 2x3
J4	1	C429954	pin header 1x3
PWR1	1	C46398	USB-B mini port
PWR2	1	C152154	pin header 1x2 (+5V connector)
R1 R2 R3	3	C58592	resistor 680 Ohm
R4 R5 R6	3	C58608	resistor 330 Ohm
R7	1	C57438	resistor 100 Ohm
R8	1	C57435	resistor 1k Ohm
U1	1	C6509	GAL16V8 (or compatible)
VIDEO_IN1	1	C141882	DB9 female connector (MDA/EGA/CGA)
VIDEO_OUT1	1	C75754	DIB15 female connector (VGA)