Got a link to the Spectrum and Amstrad software? If the hardware is really the same it may help me figure out the chipset used. (I assume you don’t want to try to open yours up? 
)
Maybe the other system was BBC btw, I didn't find anything solid 
- Pins on the cartridge’s edge connector: /INT, /IORQ, /WR, A0-A2, A4-A7, D0-D7, GND, CLOCK, +5V, SW1, SW2. This confirms that it is I/O based. Also A3 is missing so it probably uses 8 I/O ports, and another 8 mirrored. /RD is missing but maybe it infers it from the absence of /WR?
Where have I seen that kind of mirroring before? 
Grauw, did you ever find out more about it? I just read the article in MSX Computing and now I'm more interested
It seems that almost all EMR Midi interfaces share the main box. At least the interfaces made for Z80 based computers, like the Spectrum, CPC and the MSX.
On the CPC the decoding of the port range is made on the connection box by a 74LS138B(all chips installed on the interface have the labels erased), and sent the decoded signal to the main box. F8FXh on the CPC, 0Xh on the MSX.
Then, another decoding is made on the big box. Port 0 is used to receive signals from the clock connector, optional, only used on old MIDI gear, port 2 Write is the Control register, while port 2 Read is the Status register. Port 3 is the Data I/O.
According to the MC6850 data sheet on page 8, a correct initialisation is to send 3 and 16h to the port 2, Control register.
03 means XXXXXX11b, resets the chip.
16h means 00010110:
-Bits 0 and 1, "10" means 2.000.000/64=31250 BAUDS.
-Bits 2 to 4, "101" means 8 bit word 1 stop bit.
-Bits 5 and 6, "00" means /RTS=low and interrupts disabled. "EMR MidiTrack Performer" CPC program uses "10" here, /RTS=high and interrupts disabled.
-Bit 7, "0" means Receive interrupts disabled, but the "EMR MidiTrack Performer" CPC program uses "1" when recording Midi. The EMR interfaces are provided of an /INT signal from the computer, but to use it correctly the computer system interrupt routine needs to be hooked.
The "EMR MidiTrack Performer" CPC program only weights 8KB(excluding 17KB of the static screen on a separate file), and I have it partially disassembled. I think it could be very easy to convert it to the MSX. The MSX screen I have seen looks almost the same as the CPC version, but with other colours.
