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...