780 UBA/DH11 query

[utzoo!decvax!watmath!dmmartindale]

Would someone who really knows the details please explain to me why
DH11's don't work on the 780 UBA?  I've heard this stated countless
times, but have never seen a description of what doesn't work right
any why.  I was generally out of touch with the UNIX community at the
time the 780 first came out, so I expect that there was a fair bit
of discussion of this which I missed.  I'm quite curious about this
because this machine has been running two DEC-supplied DH11's
on a 780 for six months with no apparent problems.  (The other
inhabitants of the unibus are one DZ11, one DUP11, and a DMA
Versatec plotter interface.)

					Dave Martindale