The HP 9121D is a dual 3.5" drive system consisting of two Sony single-sided drives (OA-D31V-1) and a single HPIB interface. Here it is with the cover removed.



These drives are ancient - apparently the Sony OA-D30V was the original 3.5" drive.



Hint for pulling this thing apart - remove the drives (unscrew from the bottom) and then you can remove the motherboard. The fan has a single flathead screw securing it the case.



These drives use a 26-pin signal interface so some investigation was required - I wanted to see if I could replace the drive(s) with the HxC floppy emulator that uses a standard 34-pin connector.



The switch on the left is the disk select switch SW1 - disk 0 or disk 1.

I can never remember which is pin 1 - quick underneath the circuit board confirms:



All of the odd numbered pins, 1 to 25 are connected to Vss/ground. You can see the pins 2 and 4 are connected to the SW1 disk select switch.

The even numbered pins are assigned as below:

2 - disk select 0 (SELECT 0)
4 - disk select 1 (SELECT 1)
6 - direction (DIRTN)
8 - step (STEP)
10 - write data (WRTDATA)
12 - write gate (WRTGATE)
14 - head load (HDLOAD)
16 - (reserved)
18 - index
20 - track 00 (TRK00)
22 - write protect (WRTPRT)
24 - read data (RDDATA)
26 - ready

I've come up with this mapping for the 26-pin (left) to the 34-pin (right) socket on the floppy emulator:

2 <-> 10 (DS0)
4 <-> 12 (DS1)
6 <-> 18 (DIR)
8 <-> 20 (STEP)
10 <-> 22 (WDATA)
12 <-> 24 (WGATE)
14 - not connected
16 - not connected
18 <-> 8 (INDEX)
20 <-> 26 (TRK00)
22 <-> 28 (WPT)
24 <-> 30 (RDDATA)
26 <-> 34 (READY)

As I'm likely to be wrong, I've used jumper wires:



So far, so good - the HP is supplying power to the floppy emulator:



Now I just need to figure out how to configure the emulator for the HP SSDD 3.5" disk format. Worst case is I could initialise a disk on the HP Series 80 system and then use various DOS/Windows tools to image that blank disk, then copy files on to it (or something).