How to wire standard RS-232 with CAT-5 wire.

By building the appropriate adapter or directly connecting CAT-5 wire to a RS-232 connector, you can send standard RS-232 over a standard CAT-5 wiring system. Keep in mind that the CAT-5 wiring system must use all 8 wires. Older CAT-5 systems use only 4 of the 8 wires.

The RS-232 standard specifies a maximum cable length of 75 feet at 9600 bits per second. In practice, lengths of 800 to 1000 feet work just fine.

You can use your CAT-5 wiring system to connect standard RS-232 devices together

a computer to a modem                     a terminal to a modem

a computer to a printer                       a terminal to a printer

a computer to a terminal                    a terminal to another terminal

a computer to another computer      a modem to a printer

a modem to another modem

Pin connections for CAT-5 (Standard RJ-45) Type A to DB-25 or DB-9 Standard RS-232:

RJ-45      CAT-5 Color           DB-25          DB-9           Signal Function

   1           White/Green                7                  5               SG Signal Ground

   2            Green                          2                  2               TD Transmitted Data

   3             White/Orange             20                1               DTR Data Terminal Ready

   4            Blue                             4                  8               RTS Request To Send

   5             White/Blue                  5                  7               CTS Clear To Send

   6            Orange                        8                  4               DCD Data Carrier Detect

   7             White/Brown               7                  5               SG Signal Ground

   8            Brown                          3                  3               RD Received Data

Diagram of the DB-25, DB-9 and RJ-45 male connectors , front view:

 
DB-25

DB-9      

  RJ-45
                                                                                                                        

The White/Green and White/Brown wires must be soldered or crimped together in the DB-25 or DB-9 connectors at both ends.

Some Data Terminal Equipment devices require the Data Set Ready (DSR) signal. This signal is usually provided by the Data Communications Equipment device. If you have a Data Terminal Equipment device (such as a terminal) that requires the Data Set Ready (DSR) signal, wire pins 20 and 6 (1 and 6 on a DB-9 connector) together. This way, the Data Terminal Equipment device will receive the Data Set Ready (DSR) signal from itself when it asserts the Data Terminal Equipment signal.

On some printers, pin 6 of the RJ-45 female connector should be connected to the Data Set Ready (DSR) line (pin 6 on both DB-25 and DB-9). Read your printer documentation to find out if it provides useful handshaking signals on Data Set Ready (DSR) instead of Data Carrier Detect (DCD).

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Last Updated on December 30, 1997 by Darren Perkins