My next mini-project in the 3B2/400 simulator will be emulating the hard disk. The 3B2/400 used a NEC ?PD7261A hard disk controller (PDF datasheet here), which has proved to be harder to emulate correctly than I would have liked.
So far, my hard disk controller emulation has been limited to the most minimal functionality needed to get the emulator to pass self-checks at all. Other than that, it's just a skeleton. But I believe that it's actually hanging up the floppy boot process now when UNIX tries to discover what hard drives are attached, so it's time to get serious and fix it.
My progress isn't good. I am following the datasheet to the letter, trying to give the correct status bits at the correct time, but the 3B2 just gets confused. It never even tries to read data off the drive, it just gives up trying to read status bits. So, clearly I'm doing something wrong, but I don't know what it is.
Tonight I will strap a logic analyzer to the PD7261a in my real 3B2 and see exactly what it's doing. I'll report on my findings when I have them.