DH-11 on a VAX 11/780

[utzoo!decvax!duke!unc!lynn]

The DEC DH11 only partly works on a VAX 11/780.  It cannot do DMA over
a buffered data path, because it is a byte at a time DMA device.  When
it requests the seventh byte of the eight in the buffered data path, the
BDP cleverly issues the prefetch, clobbering the eighth byte before
the DH can see it.  (Due to a second piece of braindamage, the BDP only
looks at bits 3-18 of the Unibus address when it is empty or prefetching,
so it can't tell if you are doing non-consecutive DMA requests.)

The obvious "fix" is to run the DH over the direct data path, but this
puts a very heavy load on the Unibus because the direct data path ties
up the Unibus until the transaction is complete, and VAX 11/780 memory
is actually rather slow when used this way.  If you only use your
Unibus for terminals and printers you can probably get away with it,
but tapes and disks may have data late problems with a busy DH on the
bus.  The ABLE and Emulex multiplexers are do not suffer from these
problems, and are cheaper anyway if you are buying new stuff.

				Lynn TenEyck
				unc!lynn
				(919) 966-5045