ePanorama.net - Local area networks

Wiring RS-232 to RJ45 connector

Compiled by Tomi Engdahl

General information

Structured cabling systems with Category 5 twisted pair cables and RJ-45 are uased almost everywhere in computerized office environment. Thais wiring is normally used as Ethernet LAN and telephone wiring, but it can be also used for many other things.

Sometimes there is need to wire RS-232 serial port signals through that wiring. Unfortunately there has not been any single standard how to do it, so the used practices vary. There are quite many de-facto standards around and I have tried to compile most of them to this document.


Wire color coding

The follwing color codes are for flat cables. They are good enough for distanced of few meters to carry RS-232 signals. It is not necessary to use real twisted pair cable for short equipment to wall plug interface cables at RS-232 speeds.

RJ-45 (8 pin modular)

1       blue
2       orange
3       black
4       red
5       green
6       yellow
7       brown
8       white

RJ-11 (6 pin modular)

1 blue
2 yellow
3 green
4 red
5 black
6 white 

Wiring standards


"Simple 8 Position Non-Syncronous Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Data Interchange" Dated: October 1990

pin     signal
1       125     Ring Indicator
2       109     Received Line Signal Indicator
3       108/2   DTE Ready
4       102     Signal Common
5       104     Received Data
6       103     Transmitted Data
7       106     Clear to Send
8       105/133 Request to Send/Ready for Receiving


"Use of ANSI/EIA-232-D with 8 position Modular Connector" Bulletin TBS-25 edited in Feb 89 by the Electronic Industries Association Suositelemat kytkennät:

"Ready Start" configuration

RJ45    BD-25
1       6       DCE Ready
2       8       Received Line Signal Detector
3       20      DTE Ready
4       7       Signal Ground
5       3       Received Data
6       2       Transmitted Data
7       5       Clear to Send
8       4       Request to Send

"DTE controlled answer"

RJ-45   BD-25
1       22      Ring Indicator
2       8       Received Line Signal Detector
3       20      DTE Ready
4       7       Signal Ground
5       3       Received Data
6       2       Transmitted Data
7       5       Clear to Send
8       4       Request to Send

Recommendations mentioned in cabling system documents

AT&T Systimax recommendation

RJ-45   BD-25
1       22      RI
2       8/6 *   DCD/DSR
3       20      DTR
4       7       CRD
5       3       RD
6       2       TD
7       5       CTS
8       4       RS
* RJ-45 pin 2 is connected to RS-232C BD-25 pins 8 and 6

The way used by "Tele Kiinteistöverkko"

This adapter was supplies with "Tele Kiinteistöverkko" for adapting some HP terminals to work with HP terminal servers through the new cabling system. Those terminals did not use any of the handshakign lines, only the data transmitting and receiving. This adapter has been successfully used with AT&T PDS cabling system.

RJ45    D25     D9
1       2       3
3       3       2
6       7       5

Wirings used in terminal servers

XYPLEX 16xRS-232 to Ethernet Multiplexer MAXserver 1600

RJ-45   BD-25
1       5       Clear To Send/Ring Indicator
2       20      DTE Ready
3       2       Transmitted Data
4       7       Signal Ground
5       7       Signal Ground
6       3       Received Data
7       6/8 *   DCE Ready/Data Carrier Detector
8       4       Request To Send

* In DCE equipment pin 7 is connected to DCD output
  In DTE equipment pin 7 is connected to DTR output (named as DSR)
Note: In DECconnect wiring system DCD signal is not supported.


Digiboard terminals use Stewart SS-641010S connector, which looks like an RJ-45 connector, but has 10 pins. The connector on the the terminal server also accepts RJ-45 connector, but in that case the two outer pins (1 and 10) are not connected. When RJ-45 plig is put to the connector only the 8 middle pins (2 to 9) connect to RS-45 pins (1-8). This is accesptable solution in cases where those RI and DCD signals are not needed (they are typically needed only for modems, nit for terminals).

1       RI      (Ring Indicator)
2       DSR     (Data Set Ready)
3       RTS     (Request to Send)
4       C_Gnd   (Chassis Ground)
5       TXD     (Transmit Data)
6       RXD     (Received Data)
7       S_Gnd   (Signal Ground)
8       CTS     (Clear to Send)
9       DTR     (Data Terminal Ready)
10      DCD     (Data Carrier Detect)

Some wiring recommended by different people

General ideas for our own wiring

There is no standard, but many applications use the pinout of the 9-pin D-connector and just forget about pin 9.

Wiring used at HUT campus network for terminal

This wiring was used in Helsinki University oif Technology campus are network "Trinet" for connecting terminals to the terminal servers made by DEC. The terminal servers and terminals are no longer in use (everythign got converterd to Ethernet). The idea and information for this wiring came from Arttu Laine.

RJ45    RS-232 (D25)
1       5
2       6
3       3
4       7
5       7
6       2
7       20
8       4

Gordon Hlavenka's idea

Gordon Hlavenka used this wiring to adapt RS-232 to RJ-45 cable. The wiring is ment to be used tih the "flat cable" style cables. The benefit of this connection in that if you turn around the RJ-45 connector on one end the cable, the normally straght cable becomes a null modem cable.

RJ-45   BD-25
1       2       TD
2       5       CTS
3       20      DTR
4       7       GND
5       7       GND
6       6       DSR
7       4       RTS
8       3       RD

Bill Mayhew's idea

RJ-45   DB-25
1       2       TXD
2       20      DTR
3       3       RXD
6       6       DSR
8       7       GND

This wiring uses the same wires as 10base-T ethernet uses for carrying data. Then it needs one extra wire for ground. The circuit is designed so that the steady handsake signals act like a "shield against interference" for the data signals.

Yost Serial Device Wiring Standard

Yost Serial Device Wiring Standard was invented at 1987 Dave Yost (folks at UC Berkeley gave the basic idea). About 1994, the scheme was picked up for the 2nd edition of the Unix System Administration Handbook, where it was given this name.

The main idea is that onto every serial port of every piece of equipment at your site, you screw down an appropriately-wired adaptor from DB-25 or DB-9 to RJ-45. Now every serial port has the same kind of socket: RJ-45 Female, regardless of whether its underlying connector is DB-25 or DB-9, DTE or DCE, male or female. Furthermore, every serial port now transmits and receives data and control on the same pins.

The descriotion of this wiring can be found at http://www.yost.com/Computers/RJ45-serial/.

RJ-11 wirings

Wirings for that 6 pin telephone connector. You can pllug 6 pin RJ-11 connector to RJ-45 wall jack, but some older wall jacks don't like that (works nicely on newer ones).

Bridge terminal servers

Some terminal servers made by Bridge used the following adapters to adapt 25 pin connector to RJ-11 wiring on the terminal end:

RJ-11     BD-25
1 blue    20      DTR
2 yellow  2       TXD
3 green   7       GND
4 red     3       RXD
5 black   7       GND
6 white   6       DSR
In the adapter you must also connect BD-25 connect the following pins to eacha other: 4 to 5 and 6 to 8.

Nathan Stratton's idea

Nathan Stratton's idea uses 6 pin RJ-11 connectors in the following way:

RJ-11   BD-25
1       4       RTS
2       1       Frame ground
3       2       TXD
4       3       RXD
5       7       GND
6       5       CTS

Straight modem to computer connection works with straght RJ-11 cable.

Null modem cable between two connectors can be made by turning around one of the RJ-11 connectors on the cable.

You can use this cable with RJ-45 wall wiring by plugging RJ-11 connectors to the RS-45 plugs on the wall.

Wiring from Circuit Cellar Ink

The circuit is from Circuit Cellar Ink magazine issue December 1991. The idea is originally taken from the Circuit Cellar BBS (203.871.1988 203.871.0549):

From: Rob Kelly To: Sanjaya Vatuk

"Using the RJ-11s for RS-232 is even less standardized ... there are basically two ways to go: 1. Wire the modular to DB-25 adapter such that flipping the flat cable to the device will produce a null modem cable. This means you wire symmetrically (i.e., TxD and RxD are the center pair, RTS and CTS are the next pair, etc). The advantage of this method is that all adapters are wired the same.

2. Wire ... the adapters as DTE and DCE

Also, if you are coming from a wall jack that also has a phone line, you can't use the center pair. ... This plan uses the first method:

 Wire in the walls     o/w  w/gn bl/w w/bl gn/w w/br
                        |    |    |    |    |    |
 RJ-45 or RJ-11 jack |_                            _|
 looking into jack     |__________________________|

                        |    |    |    |    |    |
 Flat Phone Cable       bl   y    gn   r    bk   w
 Colors may vary        |    |    |    |    |    |
 Terminal Adapter    |                              |
                        4    6    2    3    20   5
                       RTS  DSR  TxD  RxD  DTR  CTS
(looking into jack)
      ______               ______
    _|      |             |      |_
  _|    RTS |-->-     --->| CTS    |_
 |      DSR |<---     -<--| DTR      |
 |      TxD |-->-     --->| RxD      |
 |      RxD |<---     -<--| TxD      |
 |_     DTR |-->-     --->| DSR     _|
   |_   CTS |<---     -<--| RTS   _|
     |______|             |______|

(when wired as null modem between computers)

You wire the flat cable like this:

  _/__                    __\_
 (this *is* "flipped")

this flips the cable and performs the null.

author: Tomi Engdahl