RJ-21

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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. It's 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 is used for SCSI-1 connections. Computer printers use a shorter 36-pin version known as a Centronics connector.

USOC registered jacks   RJ9 | RJ11 | RJ12 | RJ14 | RJ21 | RJ25 | RJ45 | RJ48 | RJ50 | RJ61