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
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
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
