RJ11 and RJ12 pinout: tip, ring and lines 1 to 3

RJ11, RJ12, 6P6C: wiring code or connector?

RJ11, RJ14 and RJ25 do not name a connector but a wiring configuration: USOC (Universal Service Ordering Codes) that a US subscriber ordered from the telephone company. The physical connector is described by its positions and contacts: 6P2C, 6P4C, 6P6C. The same 6-position jack can be wired as RJ11C (1 pair), RJ14C (2 pairs) or RJ25C (3 pairs), as the Siemon guide puts it.

The electronics "RJ12", meaning the six-wire 6P6C, is not the Bell System RJ12C. Bell System Practice 463-400-120 defines RJ12C as a single line plus the A and A1 hold-control leads of a key telephone system, on contacts 2 and 5. For a six-wire cable, the numbering that applies is RJ25.

A 6-position plug fits an 8-position USOC jack and keeps pair continuity, since pair 1 sits in the center in both. Siemon warns, though, that jack contacts 1 and 8 may be damaged by this practice.

Straight or reversed flat cord

A straight cord connects pin 1 to pin 1, 2 to 2, and so on. A reversed cord connects 1 to 6, 2 to 5, 3 to 4: the usual wiring for telephone cords. To tell which one you hold, lay both plugs side by side, contacts facing you: if the colors run in the same order, the cord is straight; if the order is mirrored, it is reversed.

On a line, a reversed cord brings pin 4 onto pin 3: tip and ring swap places, and in RJ14 so does line 2. On a data or debug interface it is not a polarity question but a pinout one: use the cord the manufacturer specifies.

Wire colors: what the sources agree on

Solid-color wiring (Bell System station wire, multi-core cable) agrees on the four center contacts: black on 2, red on 3, green on 4, yellow on 5. Green is tip and red is ring of line 1; black and yellow form line 2.

Beyond that, sources diverge. For pins 1 and 6, RS Components gives white and blue, while Siemon pairs the third cord pair as blue and orange. On twisted-pair cable, both put the white-striped conductor on tip, but not the same pair in the center: RS puts the blue pair on 3-4, Siemon’s 6-position drawing the green pair. We do not pick one: check the jack label or ring out the cord.

Microchip ICD debuggers: the RJ-11 jack on the board

MPLAB ICD debuggers connect to the target board with a six-conductor modular cable that Microchip calls RJ-11. The cable is reversed: the ICD 3 and ICD 4 guides state that the debugger and target connections are mirror images, pin 1 at one end going to pin 6 at the other. A replacement cord must keep that wiring.

On the board, the ICD 3 guide recommends a pull-up of about 50 kΩ from VPP/MCLR to VDD, and rules out pull-ups, capacitors and diodes on PGC and PGD. The VPP line can swing up to 14 V depending on the device, so nothing on MCLR may be harmed by that voltage.