Well, one never gets things right the first time. Looking at that first schematic, I can see it is not the best design. For a start, it uses three chips – way too many. I also ran out of I/O pins so I had none left for the stop button or the MIDI output. I’ve redesigned it using a 7 bit bus approach.
Instead of the 74HC05 buffer, I’ve used a 74HC174 which can latch the MIDI output and allow me to share the bus with the LED display. I’ve also gotten rid of the 74HC47 LED driver, I’m now driving them directly from the PIC.
I went parts shopping today. Jaycar had everything I needed, well, except the PIC. They were out of stock. They didn’t have 8MHz crystals either so I’ll have to use a 10MHz. They also had mysteriously run out of jiffy boxes. Anyway, I got enough stuff to start building the thing.