wolfgang ziegler

„make stuff and blog about it“

Fixing the Serial Port of my C64

April 11, 2023

A while ago, when listening to Scott Hanselman's podcast, I learned about C64 OS, a new (no kidding!) operating system for the C64.

I immediately knew that this was something I had to try out and I ordered an SD card with this brand-new OS for my C64.


If you are wondering now how to get the data from an SD-card on a C64, that's a solved problem fortunately. There are plenty of devices from SD2IEC device family to chose from. I went with a self-assembly kit, that I ordered on eBay.

The SD2IEC DIY assembly kit

After soldering the parts together and connecting the SD2IEC to my C64, I tried to access the SD card's contents.

LOAD "$", 8

Unfortunately, to no avail ...

LOADING ... forever

My C64 was stuck in the LOADING operation and would never finish.

LOADING hangs forever

I was quite confident that the SD2IEC was working correctly, but the C64's serial port ... not such much. So far I had never connected anything to this C64's serial port since I had bought it a few years ago.

Searching the web for this problem (LOADING forever, broken serial port, ...) first led me into the direction of a broken CIA chip. Apparently, this is a frequent cause of serial port problems and if you are lucky this chip is already socketed and can be replaced easily. In my case, the CIA was not socketed and also I would not even have a spare part.

So, I continued searching and found another promising hint: the serial port's protective diodes.

The serial port protective diodes

A damaged diode is also a common cause for the problem I was experiencing. So I gave this a chance, unsoldered the diodes, tested them and ... lo and behold ... one of those diodes (CR17) was damaged!

I replaced it with a spare diode, tried again and nearly could not believe what I saw!


My C64 was answering with READY. and let me LIST the SD card's contents without any further issues.

C64 loading successfully from the SD2IEC

I swear, this one of my proudest moments lately! I had done an actual hardware repair of a computer involving a soldering iron and spare parts.

I could physically feel my geek-cred levelling up at this moment 😆.