The C128Neo is a project to create new mainboards for the Commodore 128 computer. The foundation is a reverse engineering of the revision 9 mainboard and then schematic and PCB layout capture using KiCad.
Latest blog entries
Issue on C128Neo boards
22 June 2021
Dot clock problems A customer reported an issue with the C128Neo boards causing certain cartridges to not work correctly, in his case a Swiftlink cartridge.
The email he sent me had full details on the issue which is always very appreciated. He also posted about it on lemon64.
This issue affects all current C128Neo boards.
The problem R47 on the original board had for some reason ended up where R101 should be located and there where no solder points for a resistor where R47 should be.
Open source version released
22 March 2021
Open128 released The open source version of the C128Neo called Open128 has now been released, you can find it on github. This is almost identical to the revision 3.3 of the C128Neo, with changed logos.
It is licensed under CC BY-SA 4.0 and can be used as you please, feel free to issue pull requests if you have improvements.
There are issues with the silk screen but it should be functional in any case.
06 July 2020
Backstory and first attempts As I mentioned in my last post I’ve been working on reverse engineering the PLA chip in the C128. I’m now mostly done with this process so I think it’s time to share some of the findings.
This has been a very interesting project as I did not really know much about semiconductor design and manufacturing. My existing knowledge extended to having seen some die shots and admiring the pretty looking pictures.
Now available for purchase
12 May 2020
At long last, the boards are now available for purchase on Tindie.
I’ve not been sat idle in the mean time as I have started working on reverse engineering the PLA chip of the C128.
More to come on this topic in a future post.
06 April 2020
Here’s another interesting discrepancy between the schematic in the service manual and the actual PCB.
Though neither the jumper of the pin on the ROM it’s connected to is labelled, it is documented as A12.
To understand what’s going on here a bit better we need to look into how the VIC display characters.
VIC-IIe memory banking Let’s first have a look at how the VIC is using the memory.
Revision 3.1 boards
22 March 2020
It’s been a while since I updated the blog now but I’ve not been inactive.
Base on feedback from some of the early testers, I’ve made some small updates to the Neo board and created revision 3.1. The changes are small, mostly adjusting the size of the mounting holes for some of the components for easier assembly.
Seeing as I’ve not heard of any major issues with the boards I went ahead and ordered a production run.
BOM and early boards
21 January 2020
Bill of materials Hi all. The first revision of the bill of materials have been uploaded. The link can also be found in the menu and on the assembly guide page. Some components can be a bit tricky to source unfortunately.
Early bird boards I also have three spare revision 3 boards still for sale. They are fully functional electrically and mechanically but do have some minor cosmetic issues like a misplaced silkscreen text and similar things.
Fixes: VDC R/W
13 January 2020
Continuing with the series on fixes compared to the original Commodore PCB, we now come to these two wires:
(Ignore the capacitor, that’s a different story)
If we look closely, we can see that one pin on the VDC has been isolated from the PCB traces by drilling them out in two places. Then new wires have been pulled to connect the now isolated pin on the VDC somewhere and another wire was used to bypass that segment for the original trace.
12 January 2020
This week I received the revision 3 boards I had ordered.
I promptly set about assembling one for testing. I’ve finally managed to sort out some of the mechanical sizing issues I’ve been having with some of the components, in this particular case the RF modulator and the RF cage around the video chips which now fit as they should.
All worked as it should after assembly and I only noted some minor cosmetic issues to be resolved.
05 January 2020
So, I figured I would make a small series of posts on the various little fixes that has been implemented on the C128 Neo boards. First up is this little fellow that is manually soldered to the underside of the PCB close to the 8502 CPU.
Since there are no markings I ended up following traces around to figure out what it is for. In the commodore schematic it is marked as R102 and sits between the 1MHZ and D1MHZ lines.