Telephone plug, jack and cord standards

On this page I will attempt to explain the connections of telephone plugs and jacks (sockets) and hopefully filter out some of the confusion that arises in this area. This page is very much a "work in progress" and any corrections and/or relevent additions (preferably with citations) would be welcomed.

The section regarding connections to a UK master socket are based on common accepted practise. This has changed recently inasmuch that 2 wire connections to the NTE are quite common now, whereas from 1982 to around 1998 this was not generally considered acceptable due to the prevelance of loop disconnect dialing 'phones.

US Connectors

Note: Although this is North America centric, these type of connectors are very common throughout the world, and thus some of this information will apply in almost every country.

Most modern US and Canadian installations will use modular jacks. These are most likely to use RJ11 (1 line) or RJ14 (2 line) wiring, although in a home office situation, RJ25C (3 lines) is possible. In office situations, a jack similar to RJ14 but with a different purpose is available. It is designated RJ12.

In all the cases considered here, the wiring is a simple parallel arrangement using the 2 line wires coming in from the telephone exchange. (Central Office).

RJ11 is the most basic and supplies one single line on 2 contacts in a shell that has sufficient room for 6 contacts. The center two in the connector are used for the line. RJ14 uses the 4 central connectors to supply 2 lines, the first line occupying the center 2 as in RJ11, and the second line occupying the next ones out. RJ25C uses all 6 contacts in the shell to allow up to 3 lines. One of the features of this modular system is that an RJ11 plug will fit an RJ11, 14 or 25C jack and connect to the first line. Other interconnections are possible, although if the equipment (telephone) has more lines than are available on that jack, obviously these lines will not function.


Styles of Plugs

RJ11

RJ11 plug: 2 Contact Modular Plug  and diagram of same: 6 Contact Modular Plug Diagram
You will see from this that the 2 center contacts are the only ones present, and carry one line. The wiring is as follows.

Pin 3=Tip, Positive wire, Green
Pin 4=Ring, Negative wire, Red


RJ12

RJ12 plug: 4 Contact Modular Plug  and diagram of same: 6 Contact Modular Plug Diagram
You will see from this that the 4 center contacts are the only ones present (outer pair missing) and carries one line, with a set of auxiliary contacts to (for example) busy out a direct line when the extension associated with it is in use.

Pin 2=A, Yellow
Pin 3=Tip, Line 1, Positive wire, Green
Pin 4=Ring, Line 1, Negative wire, Red
Pin 5=A1, Black

The use of this is as follows: If a business has outside lines that connect direct to internal PBX extensions (Direct Inward Dialing, or Direct Dial In), and an extension (say 1234) is in use for an internal call, a set of auxiliary contacts on the phone's cradle switch will connect A to A1, busying out the DDI number associated with that particular extension (say 555-1234). This will prevent people from hearing ringing tone when the phone is in fact in use. With modern electronic switching systems, this is no longer necessary.

This type of wiring is not frequently found in domestic environments, but some ADSL modems refer to the second pair on a 4 contact plug as A/A1, rather than Line 2 as shown in the screenshot from the configuration screen of my ADSL router below.

Image showing Auto, Tip/Ring or A/A1 options

This is technically inaccurate as there would be no "line" present on A/A1, it should say Line 1 and Line 2, but this is a minor point, with routers costing in the low tens of US Dollars.


RJ14

RJ14 plug: 4 Contact Modular Plug  and diagram of same: 6 Contact Modular Plug Diagram
You will see from this that the 4 center contacts are the only ones present (outer pair missing) and carry one or two lines. The wiring is as follows.

Pin 2=Ring, Line 2, Negative wire, Yellow
Pin 3=Tip, Line 1, Positive wire, Green
Pin 4=Ring, Line 1, Negative wire, Red
Pin 5=Tip, Line 2, Positive wire, Black


RJ25C

RJ25C plug: 6 Contact Modular Plug  and diagram of same: 6 Contact Modular Plug Diagram
You will see from this that all 6 contacts are present and carry one, two or three lines. The wiring is as follows.

Pin 1=Ring, Line 3, Negative wire, Blue
Pin 2=Ring, Line 2, Negative wire, Yellow
Pin 3=Tip, Line 1, Positive wire, Green
Pin 4=Ring, Line 1, Negative wire, Red
Pin 5=Tip, Line 2, Positive wire, Black
Pin 6=Tip, Line 3, Positive wire, White


In modern usage, category 3 or 5 cable is generally used, which has different color codes from the cords and older 'station wire' that was used until the early 1980s. If you have to connect the two, the color codes are as follows:

CordCableFunction
BlueGreen/WhiteLine 3 Tip
YellowOrange/WhiteLine 2 Tip
GreenBlue/WhiteLine 1 Ring
RedWhite/BlueLine 1 Tip
BlackWhite/OrangeLine 2 Ring
WhiteWhite/GreenLine 3 Ring

When a cord is made using 2 of these style plugs, the cord reverses the polarity (pin 1 goes to pin 6, pin 3 goes to pin 4 etc.) The same is true of a wiring run consisting of a jack at each end. The rule is, if the gender of the connectors is the same, the wiring crosses over, if it is different, as for example, in a simple extension cord or jack doubler, the wiring is straight through.

It seems to have become normal to misidentify all of these connectors as RJ11, independent of the number of contacts, this is in my opinion wrong, but is due to the fact they don't actually have a name, being known officially simply as 'modular connectors'. The RJ stands for "Registered Jack" and is simply a specification of the type of connector and its associated wiring.

As an aside, in similar vein, "RJ45" is a specification for a somewhat obsolescent type of connection that happens to use 8 Contact modular plugs. Thus "RJ45" has come to be incorrectly used for all 8 contact plugs and jacks.

It is recommended to use 2 pair or 3 pair cable with RJ14 jacks even on a single line installation, as this will make it a simple matter of connecting the spare pair(s) of wires if you have a second (or third in the case of 3 pair cable) line installed at some future date.


The origins of the terms "Tip" and "Ring" for the wire pair of a telephone line in North America date back to the days of manual switchboards with plugs. These plugs were similar in construction to a modern 1/4 or 1/8 inch (6.35 or 3.5mm) stereo jack plug as used on headphones. The exact construction was usually different for operational reasons.

The following diagram shows the parts of a British 316 switchboard plug, on the grounds that's what I had to hand. The exact construction may vary, but the concept is the same.

Cord-PlugBody-Sleeve-Insulation-Ring-Insulation-Tip

The third connection, or "sleeve" was generally used within the switchboard wiring to show the line was busy. This prevented another operator from plugging into a parallel jack on the same line and interrupting a call. This third wire was never extended outside of the telephone exchange (Central office).


UK Connectors

In the early 1980s, British Telecom designed a new modular jack system for the UK's telephone network. They purposely chose different connectors from the already established North American RJ system. This was not just for the sake of being different. The way the UK telephone system worked at the time required a unique ringing system to prevent distortion on outgoing dial pulse trains. Most telephone administrations in the world use AC ringing with or without a DC bias, which requires a capacitor to separate the ringing from the DC line voltage.

The North American system uses separate capacitors in each telephone, this had the advantage of requiring only 2 wires throughout the installation, but also came with the disadvantage that pulse dialing from rotary dial phones would cause 'bell tap' or 'bell tinkle' on other phones on the same line. This was not really an issue in the US as DTMF (or TouchTone) was already well advanced there. The British system at the time was still mostly Strowger based (step by step) and required pulse dialing, and in keeping with older standards, bell tinkle and impulse distortion was to be minimized if not eliminated.

The solution involved fitting the capacitor for ringing current separation into the main phone jack in the premises, rather than in the phones themselves, and extending a 3rd wire to all extension jacks and thence to the phones. A Pulse dialing phone will shunt the bell with a low resistance or short circuit it during dialing. This shunt is extended to every phone bell in the installation by the ringing wire. This is why the UK chose a unique style of jack for it's plug in telephones. This is also responsible for the fact that the first phone jack in an installation is still the property and repsonsibility of the telephone company, it more or less takes the place of the Network Interface in North American systems, and is referred to as the "Network Termination Point". Modern installations use a special type of jack called an NTE5, either NTE5A with a socket on the front, or NTE5B, which are quite rare and have a blank front with no socket, but the same internals. It provides customer wiring terminals on a removable faceplate, simplifying disconnection from the network to prove where a fault exists. If it exists within or prior to the NTP, the phone company is responsible. If the fault is within customer wiring, a call to the phone company to fix it will usually be chargeable. Previous plugin systems in the UK had used series ringing with break contacts on the jacks, but this was considered a fault liability, so parallel ringing was introuduced.

431A Plug

Plug: 431A Telephone Plug  and diagram of same: 431A Plug Diagram
Some major confusion exists over these pin numbers because the British Standard is reversed with respect to the contact numbers on telephone jacks. Why this is so, I have no idea, but I shall try to fix the problem as best I can. The pin numbers shown in the above diagrams are the British Standard ones. The designations of the pins is as below:

Plug Pin 2, Jack Pin 5, A wire, Positive, White
Plug Pin 3, Jack Pin 4, Earth, Green
Plug Pin 4, Jack Pin 3, Ringing, Blue
Plug Pin 5, Jack Pin 2, B Wire, Negative, Red

The ringing is derived from the B wire using a capacitor of 1.8 μF with a 250v rating. A resistor of 470 kΩ (½ watt) is connected between the ringing wire and the A wire for 2 reasons: to provide a path for the ringing current enabling line testing, and to prevent the ringing wire assuming a dangerous voltage in the event no telephones are plugged in. These components along with a Gas Discharge Tube type surge arrester are contained within the master socket (of which there should really be only one, but see below) or NTE5A or 5B LineBox.

Many modern phones do not require the ringing wire, and loop disconnect (pulse) dialing is becoming quite rare, thus the requirement for the ringing wire to be continuous throughout the installation is not as great as it once was. When a line is used for ADSL, the presence of the ringing wire can cause problems.

The general rule on British telephone installations now seems to be that whatever comes after the NTP is okay

Back to Home


Viewable With Any Browser  Valid
HTML 4.01!  Valid CSS!

Last Modified 2011-09-30      

Apache Webserver