The dq686 emulates MSCP and works fine with an old version of uda.c that
was in 4.3BSD. (Have you tried it with NetBSD/vax at a standard uda address?)

To swap the tape drives:
- set all switches to off on the M7546 (tk50 controller) and it will be at
  the mua0 address
- set the dilog tape controller to an alternate address
  - if the controller is a dilog model 57145
    you will need to do this via the onboard diagnostic BEFORE changing the
    switches on the M7546 above:
    To start up the onboard diagnostic
	>>>D/P/L 20088004 80000001
	>>>D/P/W 20001F40 20
	>>>D/P/W xxxxxxxx 3FFF
    (where xxxxxxxx is the physical address of the SA register, which is
     20001940 for the first tape address)
	>>>S 200
    Use the SET CONTROLLER CHARACTERISTICS menu option to change the address

I have lost my dq686 manual, but the diagnostic is started in the same way,
using the address of it's SA (2000146A for the default disk address).

Oops, the above SA tape address should be 20001942 (not 20001940).

Good luck with it, rick