A 66 block (also M-Block or B-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).
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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 the block in pairs. The right hand side of the block is wired to the customer premise equipment (CPE) with jumper wires. Bridging clips are used to connect the two center terminals, connecting 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 determine in which direction trouble may exist. An orange insulating 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. Compared to more contemporary wire terminating devices they are physically large, and because of their maximum 16 MHz Category 3 signaling compatibility, they are ill suited for high speed data circuits faster than 10BASE-T. However, there are special Cat5e Certified 66 blocks available from manufacturers such as Siemon which meet all standards for Cat5e termination.[1]
"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 newer termination is almost always Category 5 compliant, and capable of supporting 100 MHz or faster signaling.