Z80 PIO, Keyboard, Sound

This is an expansion card for my [Z80 processor board](https://oshwlab.com/peterw8102/simple-z80) and connects to that card via the 60 pin connector. Software and system programming information is [available on Github](https://github.com/peterw8102/Z80-Retro/wiki). The card is primarily provides parallel I/O but also includes a simple sound generator. Features: + Z80 PIO dual port controller for general use + Z80 PIO port A can optionally be used to drive a dual joystick interface (the port can be general I/O _or_ joystick, not both! + Separate parallel keyboard interface. Rather than design a new keyboard I've chosen to use the brilliant [Omega MSX keyboard](https://github.com/skiselev/omega/blob/master/Keyboard.md). The keyboard connector on this board should directly connect to the Omega keyboard. + Single channel sound chip [SN76489](https://ftp.whtech.com/datasheets%20and%20manuals/Datasheets%20-%20TI/SN76489.pdf) I've just ordered the PCB and I'll update this page with more information once I've got it working!