Specifically, we have used their NMIN series, which has a 68HC11 microcontroller with 24 digital I/O lines, a serial port, and 8 channels of 8 bit A/D conversion. The on board memory is small, but usable: it comes with 256 bytes of RAM and 512 bytes of EEPROM. Perhaps the best feature is a built-in, 12K Forth interpreter. You just hook the device up to a standard RS-232 port to write your program. There are two specific models to check out: the MINIMAL which has TTL level serial I/O and the NMIN-0121 which uses standard RS-232 voltages and has a 25 pin D-SUB connector. Both can run off of a 9V battery, though you will need a 5V regulator for the MINIMAL.

It is a simple matter to change the baud rate of the port (from software) to allow MIDI communication. I have a schematic for a small interface board that allows you to do MIDI input and output to the MINIMAL board. (It doesn't handle the +/-9V RS-232 from the NMIN-0121.)