66 block

A 66 block (also M-Block) is a type of punchdown block used to connect sets of wires in a telephone system. 66 blocks are designed to terminate 22 through 26 AWG solid copper wire.

The 25-pair standard non-split 66 Block contains 50 rows; each row has four columns of clips that are electrically bonded.

The 25-pair "Split 50" 66 Block is the industry standard for easy termination of voice cabling, and is a standard network termination by telephone companies–generally on commercial properties. Each row contains four clips, but the left two clips are electrically isolated from the right two clips.

66 blocks pre-assembled with an RJ-21 female connector are available that accept a quick connection to a 25-pair cable with a male end. These connections are typically made between the block and the CPE (customer premise equipment).

Use

Circuit pairs are connected to the block with a punch-down tool by terminating the tip wire on the leftmost slot of one row and ring wire on the leftmost slot of the row beneath the mating tip wire. Typically, a 25-pair cable coming from the phone company is punched down on the left side of this block in pairs. The right hand side of the block is wired to the customer premise equipment with jumper wires. Bridging clips are used to connect the two center slots bonding the left-hand side of a split block with its right-hand side, thus completing the circuit. The clips form the point of interface between the subscriber and the provider. The bridging clips can be easily removed by either the subscriber or phone company personnel for trouble isolation, allowing the ability to split a circuit and test which direction trouble may exist. An orange cover attached to a 66 block denotes its designation as a demarcation point by the local exchange carrier.

Currently, 66 blocks are considered legacy devices. They are large, compared to more contemporary wire terminating devices, and, due to their maximum 16 MHz Category 3 signaling compatibility, are ill suited for high speed data circuits greater than 10Base-T. 'Split 50' 66 blocks are still used as network interface blocks in distribution frames to interconnect circuits with bridging clips, but are primarily limited to narrowband circuits such as POTS/DSL, DS0, or DS1 circuits.

Modern 110 blocks have largely supplanted 66 blocks in new installations because the termination is Category 5 compliant, and capable of supporting 100 MHz or broader signaling.

ee also

* The 110 block is a similar device that serves the same purpose
* The 25-pair color code is used to identify individual pairs within a 25-pair cable
* The cross connect point is an Insulation-displacement connector

References

* National Electric Code