Most of the RS232 spec seemed to be designed for Sync modems and their management.

Most machines of the mini generation seemed to use either Async or Sync interfaces. Stuff like the VT180 had a comm port that was a 8251 USART for serial comm that could be either sync or async. I don't believe Dec had anything like that in that PDP11 or early Vax days.

Anyone care to enlighten me?

Bill

Sent from pechter@gmail.com

-----Original Message-----
From: Clem Cole <clemc@ccc.com>
To: William Pechter <pechter@gmail.com>
Cc: Computer Old Farts Followers <coff@tuhs.org>
Sent: Fri, 28 Feb 2020 17:36
Subject: Re: [COFF] 52-pin D-Sub?

below...

On Fri, Feb 28, 2020 at 5:26 PM William Pechter <pechter@gmail.com> wrote:

Could it be because they all started with current loop tty interfaces?
Most of the old DEC guys started with teletypes.

Very possible...

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?

RS-232A/B/C was DB-25 P for the DTE (terminating equiment - a.k.a. terminal) and S for DCE (communications equipment - a.k.a. modem). It was standardized. At one time, I (sadly) could quote the paragraph number....

Then in 1978 #$%^& Lear Seglier put a DB-25S on a DTE (terminal). They were the cheap terminal vendor and all hell broke loose.

The PC/AT used 9 pin because the back of the unit was small and -- well there could because IBM said so .... But at least the IBM engineers kept to Plug and Sockets from the standard. I did not know the Series/1 used 9 pin. Learn something new.

At least DEC was reasonably consistent until they moved too the Vax modified RJ design.

Indeed - that was a huge issue - the modified RJ block -- sigh...