CGA2RGB - Digital RGBI to analog RGB for Commodore 128 and IBM PC CGA

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CGA2RGBThe 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 CGA2RGB 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.

Similarly to the older CGA2RGB-A00, the basic circuit is a triple 2-bit digital to analog converter. The main improvement is the addition of an analog video buffer to allow driving longer video cables with minimal signal degradation. A VGA connector has also been added to simplify the connection to the monitor of the scan converter.

U1 decodes the 4-bit RGBI input to the full 2-bit per color component and generates the composite sync needed for the GBS-8200.
The decoding follows the standard CGA 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.

The board requires a 5V power supply. It can be powered directly from the GBS-8200 or using an external 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.

CGA Colors

Performance Notes

The image quality of the GCA2RGB, like all analog video connections, heavily depends on the cable used. A high quality 75ohm coaxial cable is recommended for the video signals. The standard multi colored cable included in the GBS-8200 is not an optimal choice.
The following images show the difference between the standard cable and a good quality coaxial cable. In the first image is easy to note the large signal reflection due to impedance mismatch. The issue is compounded with additional discontinuities due to sub optimal grounding on the GBS-8200 PCB. The second picture shows a significant improvement.
For best quality we recommend to keep the wiring between the CGA2RGB and the GBS-8200 as short as possible.

Standard Cable75ohm Cable

Design Files
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