Registered jack RJ21 telco connector

A cable color scheme, known as even-count color code, is determined for 25 pairs of conductors as follows: For each ring, the primary, more prominent color is chosen from the set blue, orange, green, brown, and slate, in that order, and the secondary, thinner stripe color from the set of white, red, black, yellow, and violet colors, in that order. The tip conductor color scheme uses the same colors as the matching ring but switches the thickness of the primary and secondary colored stripes. Since the sets are ordered, an orange (color 2 in its set) with a yellow (color 4) is the color scheme for the 4•5 + 2 − 5 = 17th pair of wires. If the yellow is the more prominent, thicker stripe, then the wire is a tip conductor connecting to the pin numbered 25 + the pair #, which is pin 42 in this case. Ring conductors connect to the same pin number as the pair number.

A conventional enumeration of wire color pairs then begins blue (and white), orange (and white), green (and white) and brown (and white), which subsumes a color-coding convention used in cables of 4 or fewer pairs (8 wires or less) with 8P and 6P connectors.

Dual Amphenol connectors are often used on punch blocks to make a breakout box for PBX and other key telephone systems.

25-pair telco cable pinout
A common application of the 25-pair color code is the cabling for the Registered Jack interface RJ21, which uses a female 50-pin miniature ribbon connector, as shown in the following table. The geometry of the pins of the receptacle corresponds to the pin numbers of the table. The left column of pins are the ring (R) conductors, while all tip (T) conductors are on the right.

Even-count color code

Color	Pin (tip)		Pin (ring)	Color
White/blue	26		1	Blue/white
White/orange	27		2	Orange/white
White/green	28		3	Green/white
White/brown	29		4	Brown/white
White/slate	30		5	Slate/white
Red/blue	31		6	Blue/red
Red/orange	32		7	Orange/red
Red/green	33		8	Green/red
Red/brown	34		9	Brown/red
Red/slate	35		10	Slate/red
Black/blue	36		11	Blue/black
Black/orange	37		12	Orange/black
Black/green	38		13	Green/black
Black/brown	39		14	Brown/black
Black/slate	40		15	Slate/black
Yellow/blue	41		16	Blue/yellow
Yellow/orange	42		17	Orange/yellow
Yellow/green	43		18	Green/yellow
Yellow/brown	44		19	Brown/yellow
Yellow/slate	45		20	Slate/yellow
Violet/blue	46		21	Blue/violet
Violet/orange	47		22	Orange/violet
Violet/green	48		23	Green/violet
Violet/brown	49		24	Brown/violet
Violet/slate	50		25	Slate/violet

Similar connectors
The same physical connector is used to connect Ethernet ports in bulk from a switch with 50-pin ports to a CAT-5 rated patch panel, or between two patch panels. A cable with a 50-pin connector on one end can support 6 fully wired 8P8C connectors or Ethernet ports on a patch panel with 1 spare pair. Alternatively, only the necessary pairs for 10/100 Ethernet can be wired allowing 12 Ethernet ports with a single spare pair.

The same connector with spring bail locks is used for SCSI-1 connections. Some computer printers use a shorter 36-pin version known as a Centronics connector.

Source From Wikipedia