Understanding of the hardware protocol used to communicate with
SmartPort devices (Like the Unidisk 3.5" or the Chinook). You can grab
this from the aforementioned reference.
Understanding of the Smartport software protocol. I found the
disassembly of the IIc ROM in any of the reference manuals post the
original an excellent guide for this.
What's not documented at all as far as I can tell is the IWM, and
that's where it gets tricky. To get further, you're going to need
either a logic analyser, or very patiently reverse engineer the
encoded packets.