Neuhaus "Standard" for Interconnection of Serial Devices

RJ45 8-pin modular connectors for RS-232

Modular jack to DB-25 plug
Modular jack to DB-25 socket
Cyberguys item no. 131 2070

MilesTek item no. 10-01045

RJ45 8-pin modular to D-sub adapters are widely available for about $2 and are convenient for RS-232 connections. If you have a number of serial devices with different pin-outs, you can make adapters so that any device can easily be connected to another device. These universal adapters are particularly handy for devices such as hubs or controllers that only occasionally need to be connected to a terminal.

All devices are wired the same at the modular jack and the flat modular cable flips the connections. This way, any device may be connected to any other device, regardless of whether it is DTE or DCE.

The RS-232 signals are assigned to pins in the modular plug and jack as follows:
PinColorSignal nameRemarkSignal source
DTE (computer or terminal)DCE (modem)
1 Blue DTR, HSKo CD Data on pin 3 is valid JackModular jack pin designations
2 Orange RTS CTS I am ready to receive on pin 6
3 Black TxD RxD
4 Red Gnd Gnd
5 Green Return, Gnd Gnd
PlugRJ45 plug pin designations
6 Yellow RxD TxD
7 Brown CTS, HSKi RTS OK to send on pin 3
8 Gray DSR DTR Data on pin 6 is valid

Photo of RJ45 to RJ45 cableFlat modular-to-modular cable
The modular plugs are installed on the cable so that the connection is "flipped." When the plugs on the two ends of the cable are held next to each other, the order of the colors is reversed.

Where two wires are connected together in these wiring diagrams, solder or crimp the connection.

PC serial connections

PC 9-pin serial PC 25-pin serial
9-pin 25-pin


MacMac using adapter for old Mac 512KOld Mac 512K

More details on making RS-232 connections to Macs.



Do not use this pin-out for connecting a modem to the AUX or console port on a Cisco router. Use this pin-out.

HP Printers and Plotters

HP printers and pltters
At the computer end of the serial cable, HP recommends that the DSR and CTS inputs of the computer should be driven from the printer's DTR output.

Wyse and DEC terminals

You can connect to a Wyse or DEC terminal if you cut the connector off a DEC MMJ cable and attach a RJ45. In this one case, the cable should be wired straight through, not flipped. The serial jack on a Wyse terminal is shown here:
Wyse terminal

Lucent MPEG-2 Digital Video System

Control connection on NIM-I/O Board
Lucent RS-232 control connection
This is an example of a device which is pinned for connection to the 9-pin port on a PC with a pin-for-pin cable.
Other equipment:

Tomi Engdahl compiled a page detailing various "standards" for the pin-out of the modular connector. One such standard is the Yost Serial Device Wiring Standard, which is similar in concept, but not pin-for-pin compatible, to what I have detailed here. The Comtrol Rocketport serial card uses the RJ-45 with another pinout which is described by Jim Price.

Made with MacintoshPlease send your comments and suggestions.
John Neuhaus, WA2JXE
john at

Last revised: Tue, Mar 27, 2007