Commodore serial (IEC) bus interface to Raspberry Pi
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Commodore serial (IEC) bus interface for Raspberry Pi

raspiec is intended for connecting Raspberry Pi to the Commodore serial bus. It can take the role of the computer or the disk drive. It has been developed and tested with a Commodore 64 and a 1541-II.

This is the development version. What it can currently do:

  • Load and save single PRG files to the disk drive
  • Serve single PRG files (load and save) from a Pi directory to the computer
  • Serve single PRG files (load and save) from a disk image to the computer

What it cannot yet do:

  • Disk drive commands, other file types than PRG
  • Support fastloaders

As there obviously is no IEC bus connector on the Pi, an adapter must be made. The instructions are in the documents subdiredctory. As far as electronics projects go this is quite simple to build. The six pin DIN connector might be difficult to find; I happened to have an old video dubbing cable set that had those and which I dismantled and recycled for this project.

The application itself consists of two parts, a kernel module and a command-line program. Although constructing a kernel module is not trivial, it was unavoidable in this case. My first trials were userspace-only code, but the realtime demands of the bus traffic were simply too much.

The kernel module can be installed with the script. It is very simple, without error checking, but serves as documentation on how to install the module and how to make the device node accessible. The module takes one parameter, debuglevel from 0 to 3. The debug prints go to /var/log/messages.

The command line utility raspbiec is used like this:

pi@raspberrypi ~ $ ./raspbiec 
As drive:    raspbiec [serve] <directory or disk image> [<command>|<device #>]
				<command> is a computer command below applied to the disk image;
				in this way files can be transferred between the filesystem and the image
As computer: raspbiec load <filename> [<device #>]
			 raspbiec save <filename> [<device #>]
			 raspbiec cmd <command> [<device #>]
			 raspbiec errch [<device #>]

There is a binary of the kernel module compiled against an old kernel in the bin_kernel_... subdirectory. There are compiling instructions for example in, and of course more can be found with the help of your favourite search engine. The makefile expects that the variables KERNEL_SRC and CCPREFIX have some meaningful values.

I will not go into a detailed description of the implementation here, as it is quite well documented in the source files. Briefly, the kernel module translates traffic from the bus into a byte/control code stream and sends bytes/control codes to the bus as well. This is executed in a state machine reacting to GPIO and timer interrupts and device reads and writes. The userspace side reads and writes those streams via the device node (/dev/raspbiec) and implements the higher level logic.

In hindsight, this project would have been impossible without a logic analyzer. Fortunately, I had another Pi where I could run Panalyzer. The GPIO pins labeled DEBUG1 and DEBUG2 in the schematics contain extra information in addition to the bus traffic: DEBUG1 is high when the kernel module state machine is busy, DEBUG2 is high when serving a timer interrupt.