A new version of this project is available here
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-B01, the basic circuit is a triple 2-bit digital to analog converter. The main improvement is the addition of a dip switch to select the mode of operation and the improved power filtering to improve image quality with cheap power supplies.
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.
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|
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 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.
|8||Video (if present, can be used as a CSYNC for SCART connections)|
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.
The R and B label on the J5 silkscreen are swapped.