A reliable, but tedious way to tell what is what in the data stream is to output and record one data stream, then make a single change to some register or setting on your instrument, and output and record the second data stream.
Somewhere that changed setting will show up as a difference between the two recordings, and if bits elsewhere change, those might point to error correcting codes, a checksum, a parity bit, etc.
Another option is to read out and disassemble the Z80 ROM, which would allow you to figure out exactly how the data are stored. This is not particularly difficult, since you have the complete circuit diagram, but it would take a few full days of effort.