That was a semi-rhetorical question; after I typed it, I did some looking, and came up with the answer above, the ACC VDH-11. I did eventually find the hardcopy manuals for other ACC IMP interface products, but none of them mention the VDH11. On a hunch, I looked to see if there was a VDH11 driver for ELF, and sure enough, there was: https://github.com/pdp11/elf-operating-system/blob/master/files/kdvdh.m11%5… (If anyone wants to look at it, ktbl.sml hold the register definitions.) With no manual for the device, and no museum catalog hits to show that someone has one which hasn't been scanned in yet, that's probably a dead end, although with the two drivers, one could probably mock up a rudimentary programmming manual. I'm not sure there's any point, though; using an LH/DH interface is going to work as well, and those device are already supported. Thanks for chasing those all down; I knew the BBN system was based on the NCP Unix (called in this discussion the 'NOSC' system), so I figured it would have the missing files, in some form. Looking at a diff between the damaged impio.c in the NOSC tree, and the impio.c in the BBN tree, there are some changes (in the section where we have both versions) between the NOSC one and the BBN one, but it will probably be possible to take the missing piece off the bank end of the BBN one and tack it on to the NOSC one. Somewhere in the document scans available online from DOD, there used to be a long thing from the UIUC people who did the original NCP Unix. I don't know if it included source; it might have. Noel