Commodore 128 80 Column RGBI/CGA to RGB Analog Adapter Part 2

This is a follow up to an older project I did. The RGBI (RGB Digital) to RGBA (RGB Analog) for the Commodore 128 80 Column Video. Commodore’s RGBI is essentially the same as CGA so the adapter works on either. I made my prototype adapter back in 2019. I wanted a project to work on and wanted to work with KiCad to see how that went.

The Original Project:

I had started making a PCB Design in Eagle back in 2019, but I never finished it. I didn’t want to keep using Eagle so I stopped working on it.

This project is the RGBI to RGBA project packaged into a custom PCB Design and with a few additions. Such as a dedicated 5V DC Power Jack for when using it with an IBM Based pc for CGA.

It still has all the Commodore functions and more passthrough functions now for the Commodore 128. The Commodore specific parts could be skipped though.

Below are some pictures of the PCB Design work in progress.

3/4/23 the finished v1.3. Is ready to download.

The board is sized to fit into the same case I used for the prototype. The problem with that is you can not get it into the case with the DB ports soldered on. I still laid it out for that size and the mounting holes to match that case. I have a couple of them yet. I think it would probably be better to design a 3d printed case. I can make it split at the place I need to get it in and out easily that way.

The board layout is basically a DB9 port on the left side for the RGBI/CGA input. Along the top the first port is for a single RCA Port for 80 Column Monochrome output from the Commodore 128’s RGBI Port. It is there, so I figured it is easy to include. You don’t need an “adapter” to use the Monochrome output, it is just Pin7 on the RGBI DB9 port on the back of the C128, I am passing it through the 74LS244 buffer though. The next footprint is a SVideo Port for from the Commodore 40 Column from the AV Port, including a 300 Ohm resistor on the Chroma line. The next port along the top is for another RCA Port and it is the Commodore 40 Column Composite Video from the AV Port. The last port along the top is a third RCA Port which is the Audio Output from the Commodore AV Port. There along the right side is the 15 Pin RGB Analog Output. The bottom left has the 5V DC Barrel Jack footprint (Center Positive). The Header pins are the C64 AV Port, it is oversized as I want to make it “keyed” with a missing pin and a plugged hole in the connector. The CSync/HSync is to set the Sync output for the normal HSync pin on the VGA port to either be Combined Sync (CSync) or to just pass through the HSync normally. The Audio header is a jumper to either output the Audio to the RCA Port above it when in the Left side or to output it on the VGA port on Pin10 for when I use it with the SCART to HDMI Converter box. InvertSync is another jumper to Invert the CSync/HSync line if needed. Finally the SCART Blanking is a jumper to enable sending VCC/5V to Pin9 on the VGA Port again this is for when I use the adapter with the SCART to HDMI Converter box, this enables the SCART RGB Detection on SCART Pin 16.

I think it is a reasonably sized board. If it was to be used for CGA with an IBM Computer then you can install the Power Jack, and omit the 3 RCA Ports, the Mini Din Svideo Port and the 10 Pin Header for the Commodore AV Port. If it is going to be used for a Commodore 128 RGBI, just build the whole thing minus the Power Jack. I think the only thing with the power Jack is that plugging it in while having the Commodore AV Port connected could end up very bad. I might think about that a bit more if the 3pin Barrel jack could be setup to bypass the AV Port power or something like that.

I did go with through hole for the ICs, headers and ports. I was thinking of going with dual footprints for the Resistors and Capacitors to allow either through hole or surface mount parts. The Surface Mount capacitors and Resistors are 0603 Imperial size parts. I have all the ICs already in DIP/DIL, and I want them in sockets for easy replacement if required.

I had to make the footprints for the RCA Jacks, and the SVideo Jack. The RCA Jacks should have square holes on the front corners. I couldn’t see how to make such cutouts. I figure that I may just have to shave the plastic a bit on the RCA Jacks.

The board as well as the original prototype build can do a combination of outputs depending on your needs.

The board in the fully configured mode outputs RGBS, Red, Green, Blue and Sync (CSync or Combined Sync) with custom Audio and SCART Blanking voltage on the HD15 port. This is what you use for a SCART Input. With the Audio to the RCA Port and the SCART Blanking disabled (may or may not be an issue if it is enabled) the GBS Control also takes RGBS input. The GBS Control might get confused due to VSync being included too on the HD15 all the time. With the SCART Cable the VSync pin is just not wired to anything.

If it is set to HSync with the jumper then it outputs RGBHV: Red, Green, Blue, HSync, and VSync. VSync is always on the HD15 port. The GBS Control takes RGBHV, and that is the setup I use with it.

It will also do RGBS with an Inverted Sync with the other jumper. I think did that variation because of something to do with the GBS boards at the time. I don’t remember what it was about though for sure.

The other two jumpers are specific for doing the RGBS with SCART, that puts Red, Green, Blue, Sync, Audio and 5V on the 15 Pin output. It is how I used this adapter until I build a GBS Control to go with it. The GBS boards weren’t as good for this use until the GBS Control project came out.

When using the adapter in RGBHV mode the 74LS86 is not required. The 74LS86 is just used to make the CSync by combining the HSync and VSync. It is also used to do the Inverted CSync if that is enabled (which it would also Invert the HSync technically if it was enabled while the other jumper was in the HSync position). If you look at the schematic you will see it is just bypassed. The 74LS244 is a buffer to protect the Computer video output. The 74LS138 handles the CGA Brown Fix.

I have received the parts I had on order and checked all the footprints. There was an issue with the DB9 port footprint that I had to fix. I also had a few adjustments to make to the Mini Din 4 for the SVideo port. I made changes for footprint corrections and some adjustments to labeling then put in an order for the v1.2 PCBs. I am going to build one to replace the one I made in 2019 for my Commodore 128 RGBI, as I want SVideo. I plan to scrap the prototype, reusing the short DB9 cable and the short Commodore AV Din cable I made for it. I also plan to build up a second one for IBM CGA without the Commodore AV Port related components. If the board tests out, I will post the KiCad project files and Gerber files on Github. Once I have the boards built up, I intend to design a case that can be 3d printed.

Update: 3/4/23 v1.3 I am correcting the schematic, the original one had the 80 Column Monochrome Video from the Commodore 128 RGBI port going through the 74LS244, and it shouldn’t be. I am also adding R11 as an option for a “Resistive Ground (RG)” on the Shell/Shield of the 15pin output port, it probably should have a 100 Ohm resistor between ground to help prevent potential Ground Loop issues. To prevent ground loops with the cable shield is generally recommended to only be “grounded” on one end, so you can put in the optional R11 or omit it. All of the other connectors have their shield to GND/Ground.

Parts List:

0603 – Resistors
1) 150
1) 300
2) 470
1) 560
3) 680
1) 1k
1) 2k

0603 – Capacitor
3) .1uF 100nF

RCA Jacks
1) Black RCA Jack
1) Red RCA Jack
1) Yellow

Svideo Jack
1) 4Pin Mini Din

1) DB9 Male
1) DB HD15

1) 74LS244
1) 74LS138
1) 74LS86

Optionally a surface mount barrel power jack. Various pin headers and a couple jumpers.

See the next part of this project. Building, testing and fixes below:

Finished project:


4 thoughts on “Commodore 128 80 Column RGBI/CGA to RGB Analog Adapter Part 2

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