Could it be because they all started with current loop tty interfaces?
Most of the old DEC guys started with teletypes.
Having struggled with a breakout box and different mini and micro vendors implementations of serial ports... Ugh. And in three-wire the use of Xon-Xoff varied big time. No standard was the standard. IIRC the IBM Series/1 had a different 9pin layout than the PC/AR. Why? At least DEC was reasonably consistent until they moved too the Vax modified RJ design.
I am still amazed at the number of current loop DZ11s interfaced via 4 pin square phone jacks on a wall at the back of the machine room. Seemed like 100 lines of VTs and Dec writers at one hospital in 1985.
As far as the DZ... One thing I saw at Dec to improve things was the Comm-iop DZ which managed the DZ from a KMC11 (IIRC). I think it was a CSS option on PDP11s....
Bill
Sent from pechter@gmail.com-----Original Message-----
From: Clem Cole <clemc@ccc.com>
To: Dave Horsfall <dave@horsfall.org>
Cc: Computer Old Farts Followers <coff@tuhs.org>
Sent: Fri, 28 Feb 2020 16:44
Subject: Re: [COFF] 52-pin D-Sub?
Hey, don't knock the DZ-11; if you used the right driver they worked just
fine :-) The trick was to disable interrupts and empty the silos every so
often.
DZ-11 sucked... for a number of reasons, the SW issue being just part of them, but they were short pinned and really did not do modems well, particularly high-speed ones like the Trailblazer. As you said, you could make them work, but why bother?
Unix folks figure out the best idea was to use the Able DH/DM - cheaper, only one unibus slot for 16 ports (as opposed to 2 for the DZ), fully wired on the DB25 end, hardware flow control and just worked better in that is will DMA. What was not to love...
On my long 'todo' list has been to work with Mark to get SIMH to properly support the DH in his emulation. He has some stuff in there, but last time I checked (about a year ago) it still was not right, which is a sort of a shame.
FWIW: One of the guys behind DZ (who I will leave nameless) also screwed up the first serial port on the Masscomp MC/500 after he left DEC. I got there too late to fix it in the first version of the CPU board. So it was not fixed until I tore him a new one and educated him on how RS-232 actually worked (I was the first lead for the data com group as well as 1/2 the OS team). I never quite understood why HW folks often though of the serial port as '3-wires' -- sigh.