Telco What is RJ?

In this section, we talk about the RJ terminology used to describe many of the type of cables one needs to buy to expand some DCC systems.

1) What is RJ?  
2) Modular Connector Description
3) Telephone cable wiring. (Not for DCC)
4) Data Cable wiring. (DCC Cables)

1) What is RJ?
RJ means
Registered Jack under 
FCC Part 68 Telephony rules
.  Typically you see the names like RJ11, RJ14, RJ12 and RJ45.  
The naming convention is based on the telephone industry FCC standards for a given class of phone service.   
The number that follows the RJ term simplistically defines the number of connections or phone lines supported.    Some the RJ numbers are part of the FCC standards, some are made up by the cable industry and some have changed their purpose because of the original function becoming obsolete.

The RJ service number IS NOT connector specific because the connectors used over time have changed.   For a given period of time, the best connector/cable technology available was standardized for the given type RJ service.  Anyone remember the big round/square and tall hockey puck size 4 pin telephone connector?  In some cases there was no connector at all with the wire going into a plastic wall outlet cover plate with a hole in the center of it.  In 1976 a telephone company independent connector standard was mandated by the government to allow any phone equipment vendor to connect to any telephone system.  The result was the small clear plastic rectangular MODULAR plug/connector with gold contacts is the connection system standard developed by the AMP corporation.  My point is that the service was called RJ14 back then and called RJ14 today as the connection systems changed.

Modular plugs and the associated cable/wiring is shown in the picture on the left.

A complete discussion of  what RJ mean can be found here:

2) Modular Connector Description

Cable connectors are classified using the following designations:  xPyC   


P = Positions
x = number of available Positions for Contacts in the connector which is the physical SIZE of the connector.
C = Contacts
y = number of Contacts installed in the above size connector and the number of conductors in the cable.  One wire for each contact.


4P4C is a 4 Position with 4 Contacts and wires.  This cable also goes by the names RJH / RJ9 / RJ10 / RJ22.
6P2C is a 6 Position with 2 Contacts and wires.  This cable also goes by the name RJ11.
6P4C is a 6 Position with 4 Contacts and wires.  This cable also goes by the name RJ14.
6P6C is a 6 Position with 6 Contacts and wires.  This cable also goes by the name RJ12 / RJ25.
8P8C is a 8 Position with 8 Contacts and wires.  This cable also goes by the name RJ45.

There is some confusion in the commercial labeling of telephone cables at the store.  Labels often incorrectly use the term RJ11 generically to describe RJ14 and RJ12 type of cables which clearly have different number of wires.  Below is a detail list of what the correct names are.

3) Telephone cable wiring. (Not for DCC)

The following a official FCC registered service names for telephone service.

RJ11 6P2C connector. Single line phone service using pins 3 &,4 with mirror pin reversal allowed.
RJ14 6P4C connector. Dual line phone service using pins 2,3,4,& 5 with mirror pin reversal allowed.
RJ25 6P6C connector. 3 line phone service using pins 1,2,3,4,5 & 6 with mirror pin reversal allowed. Rarely used.

The FCC wiring standards NEVER specifically allow for any wiring pin reversals/mirroring.  The original RJ service standard was connector type independent.  That did not change with the new mandated modular connector system introduction.  Pin numbers matched a wire color as shown in the above picture.   
However the signal pin assignments are set up to forgive the situation where one connector is installed backwards, fliped upside down or reversed on any end of the cable and not lose any functionality.  In other words, it permits "mirror pin" connections between both ends of the cable.  For example, the m
irroring of a RJ14 cable means Pin1 goes to Pin4 & Pin 2 goes to Pin3 between the two ends of the cable.  
It turns out that almost all telephone cables are made in this reverse format for ease of mass production manufacturing.  Naturally people think mirror wiring is normal.

Pin mirroring is is bad for DCC.  DCC needs a pin to pin "Data cable".

The 4P4C modular connector is not part of FCC standards but is used by the telephone industry for other purposes.  It goes by RJ9, RJ10, RJ22 or RJH (for "Headset" Jack).  The most popular use was the small coiled cable that ran between the telephone base and the handset you held against your ear.

4) Data Cable wiring. (DCC Cables)

The key difference between data cables and telephone cable is the issue of pin mirroring or reversals.  Here the pin outs are well defined.  These names are intended for data communication services.   

RJ12 6P6C connector.  Pin to Pin wiring.  NO mirror reversals. RJ12 works for RJ25 but not visa versa. Hence popularity over RJ25.
RJ45 8P8C connector.  Popularized by Ethernet.  Well documented pin outs.

Digitrax and NCE use RJ12 cables.   MRC uses RJ45.

NCE also used a second 4P4C pin to pin cable for the "control bus" AKA the "booster bus".