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From: phco@ecsvax.UUCP (John Miller)
Newsgroups: net.micro.pc
Subject: Re: IBM AT Serial Line Pinouts
Message-ID: <311@ecsvax.UUCP>
Date: Sun, 25-Aug-85 21:40:47 EDT
Article-I.D.: ecsvax.311
Posted: Sun Aug 25 21:40:47 1985
Date-Received: Wed, 28-Aug-85 09:58:20 EDT
References: <2077@ukma.UUCP>
Reply-To: phco@ecsvax.UUCP (John Miller)
Organization: Univ. of North Carolina at Chapel Hill
Lines: 28

According to D. Gary Grady at Duke U. Comp. Ctr. (mcnc!ecsvax!dgary),
the pinouts on the AT are as follows:

DB-25     PC-AT    Signal source       Purpose
-----     -----    --------------      --------
  1          -           --            Protective ground (unconnected)
  2          3           PC            Send data
  3          2          Modem          Receive data
  4          7           PC            Request to send
  5          8          Modem          Clear to send
  6          6          Modem          Data set ready
  7          5           --            Signal ground
  8          1          Modem          Carrier detect
 20          4           PC            Data terminal ready

(PC-AT refers to the 9-pin connector on the AT, DB-25 to the other end of
the serial adaptor cable.)

I haven't torn a cable apart to verify this information, but I have successfully

connected  an AT to an HP plotter and to a PDP-11/24 based on this information,
so it seems to be correct.

-- 

                        John Miller  (ecsvax!phco)
                        Dept. of Pharmacology, Univ. of N.C.-Chapel Hill
                        Chapel Hill, NC 27514       (919) 966-4628