Steve at Big Mess o’ Wires writes:
So how does it work? First off, we can assume that raw data bytes are sent and received in serial fashion using NRZI encoding on the ReadData and WriteData lines of the floppy connector. While the DCD documents don’t specifically mention this anywhere, it’s a safe assumption because the floppy interface is powered by the Mac’s IWM chip (Integrated Wozniak Machine), which is hard-wired to send and receive data this way. So the main questions surround exactly what data bytes are sent and received, and when, and how the other lines on the floppy connector are used.
Lawled at “Integrated Wozniak Machine”.