Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!saxony!dgil
From: dgil@pa.reuter.COM (Dave Gillett)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: UART to DB9/25 connector connections
Message-ID: <315@saxony.pa.reuter.COM>
Date: 10 Aug 90 08:05:34 GMT
References: <32464@eerie.acsu.Buffalo.EDU>
Organization: Reuter:file Inc  (A Reuter Company) Palo Alto, CA
Lines: 30

In <32464@eerie.acsu.Buffalo.EDU> axaris@acsu.buffalo.edu (vassilios e axaris) writes:
>I would like to know the connections from the UART pins to the port connectors,
>as used for COM1/2 in pcs.

Well, as I understand it the UART doesn't connect directly to the DB-9/DB-25
pins, but goes through a 1488/1489 pair which handle details like conversion
between TTL and RS-232C voltage levels.

That said, here are the 8250 pinouts that correspond to RS-232C signals, as per
page 501 of _The Programmer's PC Sourcebook_, by Thom Hogan:
     8250 pin                  Name                DB-25 pin
       11                  TxD                         2
       10                  RxD                         3
       32                  RTS                         4
       36                  CTS                         5
       37                  DSR                         6
       20                  Signal Ground               7
      ~29                  CD                          8
       33                  DTR                        20
       39                  RI                         22

I'm not positive about pin 29; Hogan calls it "No Connection", but we know
there's a bit in one of the 8250 registers that corresponds to Carrier Detect
and I don't see any other possibilities.

DB-25 pin 23, Data Rate Selector, may be in some sense analogous to 8250 pin
15, ~Baud Out.  And 8250 pin 9, RCLK, may possibly correspond to one of the
optional clock lines in RS-232, but I can't tell which one.  I've never seen
any of these used.
                                                Dave