Just thought I'd share my current project: Taking an old ISA sound card I got off eBay and wiring it up to an Arduino to control its OPL3 synth from a MIDI keyboard. I have it mostly working now.

No intention to play audio samples, so I've not bothered with any of the DMA stuff - just MIDI (MPU-401 UART) and OPL3.

It has involved learning the pinout of the ISA bus connectors, figuring out which ones are actually used for this card, ignoring the standards a little (hello, amplifier chip that is wired up to the +12V line but which still happily works at +5V...)

Most of the wires going to it are for each bit of the 16-bit address and 8-bit data. Using a couple of shift registers for the address, and a universal shift register for the data. Wrote some fairly primitive ISA bus read/write code, but it was really slow. Eventually found out about SPI and re-wrote the code to use that and it became very fast. Had trouble with some timings, fixed those.

The card is an ISA Plug and Play card, meaning before I could use it I had to tell it what resources to use. Linux driver code and some reverse-engineering of the official Windows/DOS drivers got me past this stage.

Wired up IRQ 5 to an Arduino interrupt to deal with incoming MIDI data, with a routine that buffers it. Ran into trouble with the interrupt happening during I/O and needing to do some I/O inside the handler and had to set a flag to decide whether to disable/re-enable interrupts during I/O.

It looks like total chaos, but the various wires going across the breadboard are mainly to make it easier to deal with the 16-bit address and 8-bit data lines. The LEDs were initially used to check what addresses/data were being sent, but now only one of them is connected and indicates when the interrupt handler is executing.

There's still a lot to do after that though - MIDI and OPL3 are two completely different things so I had to write some code to manage the different "channels" of the OPL3 chip. I have it playing multiple notes at the same time but need to make it able to control the various settings over MIDI. Eventually I might add some physical controls to it and get a PCB made.

The fun part is, I only vaguely know what I'm doing with the electronics side of this. I didn't know what a "shift register" was before this project, nor anything about the workings of the ISA bus. I knew a bit about MIDI (both the protocol and generally how the MPU-401 UART works) along with the operation of a sound card from a driver/software perspective, but everything else is pretty new to me.

As a useful little extra, I made some "fake" components that I can build the software against on a PC, to run some tests before uploading it to the Arduino (mostly just prints out the addresses it is going to try and write to).