Registered Jack

In the mid 1960’s the Bell System companies used the 505A plug, a round connector with four prongs.  We’ve moved to the de facto standard of Registered Jacks.

Common Registered Jacks

Code Connector Usage
RJ2MB 50-pin micro ribbon 2–12 telephone lines with make-busy arrangement
RJ11(C/W) 6P2C For one telephone line (6P4C if power on second pair)
RJ12(C/W) 6P6C For one telephone line ahead of the key system
RJ13(C/W) 6P4C For one telephone line behind the key system
RJ14(C/W) 6P4C For two telephone lines (6P6C if power on third pair)
RJ15C 3-pin weatherproof For one telephone line
RJ18(C/W) 6P6C For one telephone line with make-busy arrangement
RJ21X 50-pin micro ribbon For up to 25 lines
RJ25(C/W) 6P6C For three telephone lines
RJ26X 50-pin micro ribbon For multiple data lines, universal
RJ27X 50-pin micro ribbon For multiple data lines, programmed
RJ31X 8P4C Allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed ahead of all other equipment. (Only 4 conductors are used)
RJ38X 8P4C Similar to RJ31X, with continuity circuit. If the plug is disconnected from the jack shorting bars allows the phone circuit to continue to the site phones. (Only 4 conductors are used)
RJ41S 8P8C, keyed For one data line, universal (fixed loop loss and programmed)
RJ45S 8P8C, keyed For one data line, with programming resistor
RJ48C 8P4C For four-wire data line (DSX-1)
RJ48S 8P4C, keyed For four-wire data line (DDS)
RJ48X 8P4C with shorting bar For four-wire data line (DS1)
RJ49C 8P8C For ISDN BRI via NT1
RJ61X 8P8C For four telephone lines
RJ71C 50-pin micro ribbon 12 line series connection using 50-pin connector (with bridging adapter) ahead of customer equipment. Mostly used for call sequencer equipment.

Many of the basic names have suffixes that indicate subtypes:

  • C: flush-mount or surface mount
  • F: flex-mount
  • W: wall-mount
  • L: lamp-mount
  • S: single-line
  • M: multi-line
  • X: complex jack

T1 Termination

The RJ48C is used for T1 service.

The RJ48C is used for T1 service.

An RJ-48 plug is often mistaken for RJ-45. On the outside, the two look identical—both are housed in miniature 8-position jacks. The difference is in the wire pairing. RJ-48 connectorIn RJ-48, two of the wires are for transmit, two are for receive, and two are for the drain. The last two wires are reserved for future use

 

An RJ48X is wired like an RJ48C but shorting bars are added so when nothing is plugged into the jack, the shorting bars loop back the line toward the far end (pin 1 shorts to pin 4 and pin 2 shorts to pin 5.)

An RJ48X is wired like an RJ48C but shorting bars are added so when nothing is plugged into the jack, the shorting bars loop back the line toward the far end (pin 1 shorts to pin 4 and pin 2 shorts to pin 5.)

 

Three subsets

There are three subsets within RJ-48: RJ-48C, RJ-48X, and RJ-48S. RJ-48C and RJ-48X are very similar, though RJ-48C is more common. Both use lines 1, 2, 4, and 5, and connect T1 lines. RJ-48X connectors, however, have shorting bars. RJ-48S uses lines 1, 2, 7, and 8, and generally connects 56K DDS lines.

Here’s how RJ-48C pinning compares to RJ-48S pinning:

Pin RJ-48C RJ-48S
1 Receive ring Receive data +
2 Receive tip Receive data –
3 No connection No connection
4 Transmit ring No connection
5 Transmit tip No connection
6 No connection No connection
7 No connection Transmit data +
8 No connection Transmit data –

(*T568B is equivalent to AT&T 258A so for reasons of tradition, it’s likely the wire scheme the telco is going to drop off)

 

DS0 / DDS Termination

An RJ48S is used for subrate data services or a 56K DS0.

An RJ48S is used for subrate data services or a 56K DS0.

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