RJ21

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RJ-21 (or RJ21) is a registered jack standard for a modular connector using 50 conductors, usually used to implement a 25-line (or less) telephone connection such as that used in the 1A2 key telephone system. It is also known as a 50-pin telco connector, or an Amphenol connector. (The latter is a genericized trademark, as Amphenol was the largest manufacturer of these at one time.)

Dual RJ-21 connectors are often used on punch blocks to make a breakout box for PBX and other key telephone systems.

The same connector (with spring bail locks) is used for SCSI-1 connections. Computer printers use a shorter 36-pin version known as a Centronics connector. These connectors are also used to connect Ethernet ports in bulk from a switch with RJ-21 ports to a CAT-5 rated patch panel, or between two patch panels.

See also:

• Key telephone system

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

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