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

Project Status: 
released

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-B00, the basic circuit is a triple 2-bit digital to analog converter. The main improvement is the addition of J5 to improve the support for SCART and third party cases. Small capacitors have also been added to the output of the DAC to improve signal integrity.

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.

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

CGA Colors

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. The recommended configuration is to plug the CGA2RGB directly into the GBS HDB-15 input. If this is not an option a good quality VGA Male to Female 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 8 pixel. Rise and fall times are about 20ns with no visible overshoot.

B00 signal integrityB00 signal integrity

3D Printed CGA2RGB Cases

Our friends at Paez3D have developed a fantastic case for the CGA2RGB plus the Gonbes scaler. Please visit their website for information on how to purchase one.
Paez3D case

Design Files
Schematics: 
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Comments

Hi there,

I'd like one of these to attach to a IBM CGA 9pin video out to a generic VGA monitor.

Do you still have these and can you provide the cables as well?

kindest regards

Stephen Walters
London UK.

Hello,

CGA2RGB is available on eBay. The internal cables for connecting CGA2RGB to a scan doubler (like the GBS-8200) are *usually* provided with the scan doubler. Typically, you will receive a cable for video and another for power - both of these cables plug in to the GBS-8200 and the CGA2RGB (you will need to solder to the CGA2RGB if you use these cables). You will use both cables to drive the CGA2RGB. GBS-8200 scan doublers are available on eBay for a modest price. Be sure to confirm with your seller that the internal cables are provided, if you intend to use those cables to connect your CGA2RGB to the scan doubler.

For connecting the IBM's CGA port to the CGA2RGB: a 9-pin M-F cable (also available on eBay) is what you'll need. These can be serial cables but go with a shorter cable if possible.

As mentioned, enclosures are available as are complete solutions.

Hi, will this work with EGA 640x350 (21.8 kHz) and the full 64-color palette?

The CGA2RGB is specifically designed for CGA and will not work with EGA.
If there is enough demand we could consider a new design.

If I set the jumper to choose CSync output, will I get sync levels I can use with Scart?

the Csync voltage levels are optimized for a GBS style scaler and are not fully NTSC/PAL compliant.
Most TVs will be fine but Unfortunately not all of them.
If your CGA source has a b/w composite signal on CGA pin 7 (the C128 does) you can use that as a composite sync for the scart connector (the signal is also found on J5 pin 8.

I would love to buy one of these to use with my Apple IIe RGB card. Any for sale?

We are working on a new improved design that supports EGA and MDA in addition of CGA.
Units should be available around March/April 2019

I'm a little confused. To get 80 columns in color from my 128, what exactly do I need? I get that I need the CGA2RGB and a Gonbes scaler (which one--8200 or 8220?), but if I get the case from Paez3D, then I also need 3 different cables, right? One VGA cable, one RGBI cable (same connector on both ends?), and what to connect this unit to the Gonbes? And 40 columns would require a separate composite to VGA converter, correct?

as a minimum setup you need a CGA2RGB, a scaler with VGA connector input (either the gonbes 8200 or the 8220 will work), a DB9 straight cable (for the CGA signal) and a VGA cable (from the scaler to the monitor).
If you purchase the Paez 3D case you will need to use the cable that usually comes with the gonbes to connect the CGA2RGB to the gonbes internal connector (soldering might be required in this case).
If you don't want to solder, I believe Paez 3D will also sell complete units with all the required components assembled and tested.

Hi there,

I've purchased an CGA2RGB-B01 adapter some time ago, and I would like to use this adapter
with a Scart cable towards the TV, in order to maintain compatibility with the 15kHz scan rate
without further external devices involved.
I've made an 15pin d-sub to Scart cable to carry the RGB and the composite sync signals towards
the TV. Pins 1 and 2 of J4 jumper are shorted in order to pass the composite sync through to pin 13
of the VGA connector. After powering up everything (including the CGA2RGB board with external 5V)
I got only a blank screen. Sync is seems to reach the TV, because as I turn off the computer the
"no signal" message appears on the screen shortly.

The cable was wired the following way:
SVGA pin / Scart pin / Desc.
1 -> 15 Red
2 -> 11 Green
3 -> 7 Blue
6 -> 13 Red GND
7 -> 9 Green GND
8 -> 5 Blue GND
10 -> 18 Sync GND
13 -> 20 Sync

Is this OK, or maybe I'm missed something?

The csync output from the CGA2RGB is optimized for the GBS scan converters and it is not totally compliant to the TV specifications.
What is the source of the CGA signal? If your source outputs B/W video on CGA pin 7 you can use that for the SCART CSYNC (this signal is available on J5 pin 8)

Hello,
The source of the CGA signal is an RGBI output of a Commodore 128.
In the meantime I've managed to find the problem: it is necessary to provide
the "RGB select" signal for the TV. Pin#16 of the SCART connector must be pulled up 3 to 5 Volts
in order to switch into RGB mode. Below 3V the SCART input operates as composite,
and the display goes blank.

Providing this RGB signaling, my TV/monitor displays the picture correctly,
without any extra devices. So the cable was basically OK, only this signaling
was needed in order to work.