On Sun, Oct 30, 2022 at 2:49 PM Wayne S via cctalk
The difference between dz and dh interfaces is that
the dh used dma instead of interrupts to get characters to the cpu. It would be
transparent to any software.
I did a write up on them 40 years ago justifying the replacement of a dz with dh saying
that decreasing interrupts would increase performance on my VAX 780. It did, but just a
bit. To make a big difference, you’d have to have a LOT of people banging away on serial
terminals and rs-232 connected printers.
When I ran Unibus VAXen in the 80s every day at work, our small
machines had 5-9 serial ports for 1-4 users and our largest machine
had 57 serial ports (multiple Emulex CS/21 and at least one DEC DMF32
plus the console) and several dozen users. We never moved to terminal
servers or other external connection management. It was all
individual serial connections routed to offices and workrooms. I
think our peak usage was 50-60 active users but at that point, the 8MB
of physical memory started to be a constraint.
I don't think the 11/750 could have handled that many users on DZ-11s.