
Date: 12-17-87 (22:56)
  To: ALL
From: BILL QUINN
Subj: CABLES-MODEM TO COMPUTER

                 Cabling the Computer to a Modem
 
At the last TELESig meeting questions were asked about the proper
cables to use when connecting a modem to a computer.  Someone
said, "Buy a standard modem cable."  One of the questions asked
was, "What is a standard modem cable?"  This will be a short
discussion on cabling your modem to your computer and a  brief
description of the RS-232 circuits used.
 
Remember that most, if not all,  modems today are wired as DCE
and most computers are wired as DTE.  Note that I said most since
there seems to be no standard from manufacturer to manufacturer
as to the true function of the various RS-232 pin number
assignments.
 
Let's take a look at the RS-232 (Serial Port) on your computer.
A circuit may be defined as a continuous wire from the computer
to the modem.  The RS-232 specification provides for 25 circuits,
but less than a third of them will normally be found on a given
cable on your computer.  There is a certain amount of confusion
associated with the names used because there are three different
naming conventions.  They are the common name, the EIA circuit
name, and the CCITT circuit name.
 
The following table represents the pins typically needed in a
cable used from a computer to a modem.  The remaining pins have
been left out of the table to avoid confusion.
 
    PIN  NAME EIA  CCITT     DTE/DCE   FUNCTION
     1   CG   AA   101       ------    Chassis ground
     2   TD   BA   103       =====>    Transmit data
     3   RD   BB   104       <=====    Receive data
     4   RTS  CA   105       =====>    Request to send
     5   CTS  CB   106       <=====    Clear to send
     6   DSR  CC   107       <=====    Data set ready
     7   SG   AB   102       ------    Signal ground
     8   DCD  CF   109       <=====    Data carrier detect
    20   DTR  CD   108.2     =====>    Data terminal ready
 
Note that the signal names are from the viewpoint of the DTE
(e.g., Transmit Data is data being sent by the DTE {computer},
but received by the DCE {modem}).
 
The direction of the arrows indicates which end (DTE or DCE)
originates each signal, except for the ground (--).  For example,
circuit 2 (TD) is originated by the DTE and received by the DCE.
 
When your modem is wired as DCE and your computer is wired as DTE
you can use a standard cable. Should your computer be wired as
DCE you would have to use the reverse cable. When connecting two
computers together via the RS-232 port you will have to use the
reverse cable configuration.  This configuration is also called a
null modem cable.  A different wiring diagram for a null modem
cable is contained in a text file on the club's BBS.  The name of
the file is NULMODEM.TXT.  This wiring scheme should be tried if
the reversed cable will not work for your application.
 
The following diagrams show the wiring needed by a standard and
reverse cable.
 
    Standard Cable                         Reverse Cable
Computer pin   Modem pin               Computer pin   Modem pin
 
     1 .......... 1                          1 .......... 1
     2 .......... 2      Note change         2 .......... 3
     3 .......... 3      Note change         3 .......... 2
     4 .......... 4      Note change         4 .......... 5
     5 .......... 5      Note change         5 .......... 4
     6 .......... 6      Note change         6 .......... 20
     7 .......... 7                          7 .......... 7
     8 .......... 8                          8 .......... 8
     20 ......... 20     Note change         20 ......... 6
 
Which cable do you need?  If your are connecting a DTE serial
port on one computer to a DCE serial port on a second computer,
use a standard cable.  If you are connecting a DCE to DCE or a
DTE to DTE, you will need a reverse cable.  (For a direct
connection to another computer you usually need a reversed cable
since you are probably connecting two like devices together - DCE
to DCE or DTE to DTE.)  To connect the computer to a modem you
usually need a standard cable since you are connecting a DTE (the
computer) to a DCE (the modem).
 
When you purchase or make your own serial cables make sure that
you have the correct male or female plugs for your equipment.
should your computer has the newer 9 pin connector that is a
different story.
 
The file RS232.ARC on our BBS is a good tutor of the operations
of a RS-232 port.  The best advise I can offer is to read your
manuals for both the computer's and the modem's specification of
the RS-232 connections.
