The 80 column output of the Commodore 128 is the same digital RGBI used by the original IBM CGA graphics adapter. Unfortunately nowadays is quite difficult to find a monitor with the suitable RGBI input. The CGA2RGBv2 adapter will convert the TTL RGBI to analog RGB suitable to be connected directly to a 15KHz capable RGB monitor or to the popular Gonbes GBS-8200 VGA converter. As an added bonus the CGA2RGBv2 can also convert MDA and EGA to an analog format for the enjoyment of all our retro PC friends.
Similarly to the older CGA2RGB-C02, the basic circuit is a triple 2-bit digital to analog converter. The main improvement is the addition of micro USB connector to power the circuit.
U1 decodes the RGBI input to 2-bit per color component according to the dip switch settings and generates the composite sync needed for the GBS-8200.
| Mode | DIP1 | DIP2 | DIP3 | DIP4 |
|---|---|---|---|---|
| CGA | OFF | OFF | OFF | OFF |
| EGA | ON | OFF | ON | ON |
| MDA White | ON | ON | OFF | OFF |
| MDA Green | ON | ON | ON | OFF |
For MDA the converter supports enhanced brightness and can be configured for white or green for a more retro look.
For EGA the conversion is direct as the digital signal is already 2-bit per component.
For CGA the decoding follows the standard color table which includes a special case for color 6.
| Commodore Color Number | CGA Color Number | RGBI | Color | R | G | B |
|---|---|---|---|---|---|---|
| 1 | 0 | 0000 | Black | 00 | 00 | 00 |
| 7 | 1 | 0010 | Blue | 00 | 00 | 10 |
| 6 | 2 | 0100 | Green | 00 | 10 | 00 |
| 12 | 3 | 0110 | Cyan | 00 | 10 | 10 |
| 3 | 4 | 1000 | Red | 10 | 00 | 00 |
| 9 | 5 | 1010 | Magenta | 10 | 00 | 10 |
| 10 | 6 | 1100 | Brown | 10 | 01 | 00 |
| 16 | 7 | 1110 | Light Grey | 10 | 10 | 10 |
| 13 | 8 | 0001 | Grey | 01 | 01 | 01 |
| 15 | 9 | 0011 | Light Blue | 01 | 01 | 11 |
| 14 | 10 | 0101 | Light Green | 01 | 11 | 01 |
| 4 | 11 | 0111 | Light Cyan | 01 | 11 | 11 |
| 11 | 12 | 1001 | Light Red | 11 | 01 | 01 |
| 5 | 13 | 1011 | Light Magenta | 11 | 01 | 11 |
| 8 | 14 | 1101 | Yellow | 11 | 11 | 01 |
| 2 | 15 | 1111 | White | 11 | 11 | 11 |
The resistor network R1,R2,R7 forms a simple R2R D/A converter. The output impedance of the converter is however too high to directly drive the 75ohm video cables. U2 buffers the signals and thanks to R12,R13,R15 provides a perfectly matched 75ohm impedance.
L1 an L2 and the related capacitors provide noise filtering some isolation between the analog and digital portion of the board.
The board requires a 5V power supply. It can be powered directly from the GBS-8200 or using an external USB power supply.
Jumper J4 allow to select between composite sync (pin 1-2) suitable for the GBS-8200 and separate sync (pin 2-3) suitable for 15KHz capable monitors.
Connector J5 can be used for internal connections.
| Pin | Signal |
|---|---|
| 1 | GND |
| 2 | B |
| 3 | G |
| 4 | R |
| 5 | GND |
| 6 | VSYNC |
| 7 | HSYNC/CSYNC |
| 8 | Video (if present, can be used as a CSYNC for SCART connections) |
Image Quality
The image quality of the GCA2RGB, like all analog video connections, heavily depends on the cable used. The standard multi colored cable included in the GBS-8200 is not an optimal choice. For sharp images a good quality VGA cable should be used.
Great care has been devoted to produce a sharp and noise free signal. The following pictures show the waveform for a single pixel line and a 1 to 7 pixel. Rise and fall times are consistently below 20ns with very limited overshoot.
Power filtering has been significantly improved as well to produce a noise free image even when using cheap 5V power supplies.
Accreditation?
While I'm appreciative that this is available for purchase, the GPL 3 license requires that you disclose this project is copied from necroware/mce-adapter (https://github.com/necroware/mce-adapter). Adding this information to your page and eBay listing will increase customer confidence and provide the open-source origin for this component so the community can "give back". This benefits everyone including gglabs as you will benefit from improvements made to the adapter over time.
CGA2RGB is not a derivative of mce-adapter
Thanks for the feedback. I never seen the mce-adapter before you sent the link.
In fact the first CGA2RGB design is from 2017 which is several years earlier than the mce-adapter publication on github. Therefore it's impossible for the CGA2RGB to be a derivative work of the mce-adapter.
Plastic Case
Hi
I've bought CGA2RGBv2, but I can't see a plastic case in order to mount this stuff inside it.
Do you know where I could find it ?
Thanks a lot.
Paez3D http://www.paez3d.com/
Paez3D http://www.paez3d.com/ used to sell cases for CGA2RGB and CGA2RGB+GBS-8200. Please contact them for availability.
Can be used from PC CGA?
I think since it says the Commodore uses the same CGA output as PC CGA, the implication is that I could use a CGA output from a PC and use this card? Also, I'm not familiar with this site, but I dont see where I can buy a board or finished product.. Is there more than just a schematic available? I am happy to order and build my own boards, but I would prefer not to re-enter the schematic and do the routing if I'm just missing something on this page?
PC CGA support
Yes, the converter will work for any CGA output including PC and Tandy 1000.
However, please keep in mind that it is not a scan converter and, in general, the output cannot be directly connected to a VGA monitor.
To purchase any pre-assembled items/PCBs, please check the "Shop" page that can be accesses in the navigation bar.