Connector P2 on 2040/4040 mainboard?

Hi, All,

Some recent postings on one of the CBM groups on Facebook has turned
my attention back to a bit of hardware I bought more than 30 years
ago, a bare 2040/4040 mainboard.  I've been looking over the
schematics on zimmers.net in pub/cbm/schematics/drives/old/4040 and
there's a scan glitch on one of the pin assignments for P2, the 20-pin
connector that goes to the analog board.

Here's the pinout as I've collected from the three digital board
schematic sheets:

P2
 1 ?
 2 HD SEL 1
 3 -MTR0
 4 WP0
 5 ACT LED 0
 6 WP1
 7 -MTR1
 8 ACT LED 1
 9 S1B
10 ?
11 ?
12 ?
13 ?
14 DATA OUT
15 DATA IN
16 ?
17 S0A
18 S0B
19 S1A
20 ?

and a loose tag at the lower left corner of 320806-3.gif
? READ INHIBIT

The scan glitch is I can't clearly read what pin is READ INHIBIT.  It
could be pin 16 if the 1 is totally missing and the 6 has a void
through it.

The reason for going over this in detail is to figure out if it's
feasible to cobble up a way to have the digital (main) board talk
directly to an SA390/SA400 without the CBM analog board.  This is
something I tried to do long, long ago but it was beyond my skills
when I was a teenager.  I bought the bare digital board cheap and
picked up a real SA400 and really should have just bought a CBM analog
board (tossing off the Shugart analog board) but I think the local
Commodore Dealer wanted way too much for it as a loose item, so I
never got that drive connected up.

I can always keep this digital board around as a spare for maintaining
the 2040/4040 drives I already have but I'd kinda like to push this
old project to completion.  I think it's possible to hack the Shugart
analog board on the SA400 to take the signals from P2 (stepper lines
and motor controls and LED/switch lines) because I think it's a _real_
challenge to try to hack up a circuit to turn P2 into something
resembling a Shugart 34-pin floppy interface.  Not impossible, but
probably more work than hacking the drive itself.  I'm not concerned
with trying to hack _two_ drives this way.  One is enough and was my
original goal back in the day anyway.

Yes, I have a 2031 now and an upgraded 2040 (2040 badge with 4040 DOS
v2 firmware) so this isn't about having _a_ drive on a PET, it's about
getting an old project across the finish line for the sense of
accomplishment.

Anyone here do any low-level work on these drives?  Any comments or suggestions?

Thanks,

-ethan