Yes and no. There were two 18-bit UNIBUS devices, but they were originally done for the PDP-15 (DEC's last 18-bit machine). They were the RK11-E and the RH11-AB. When the KS10 appeared, the RH11 was re-purposed for it. (How the RH11-C in the KS20 differed I'm not sure. I know it used the M7294-YA instead of the M7294, but I'm not sure how that differed. It's the MASSBUS data buffer and control, so it's something MASSBUS related. The RH11-AB already has the 18-bit stuff; see 4.16 "Logic Diagram DBCE", pg. 4-28 (65 of the PDF) in "RH11-AB Option Description" for details; it's poorly documented.) The RH11-C seems like it's very similar to the RH11-AB, physically. Both the 'disk' and 'tape' RH11-C's seem like they are separate 9-slot system units, with the same layout (in terms of boards->slots). Both have 3 Small Peripheral Controller slots, like the -AB. The first slot is purpose-wired to hold the M7296 and M7297: https://gunkies.org/wiki/RH11_MASSBUS_controller#Backplane_layout so 'no' to that idea. Although, with the 3 SPC slots - although they are on UNIBUS A, and only UNIBUS B has the 18-bit capability - although maybe the two can be jumpered together (the way the two UNIBI in the KD11-A/D can). (IIRC, the "RH11 Peripheral Controller Course" may talk about that.) I don't think things will work with the RH11 boards pulled, unless you manually jumper whatever pins are used to feed NPG and BG? through the device. Probably the easiest thing is to change the bus address of the RH11. Depends what you mean by "full-length"; no MUD (hex) slots, but yes to SPC slots (SPC) - as above. Noel