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A split-50 M or 66 Block with bridging clips
A 66 block is a type of punchdown block used to connect sets of wires in a telephone system. They have been manufactured in three sizes, A, B, and M. A and B have six clips in each row while M has only 4. The A blocks spaced the rows farther apart, and has been obsolete for many years. The B style is used mainly in distribution panels where several destinations (often 1A2 key telephones) need to connect to the same source. The M blocks are often used to connect a single instrument to such a distribution block. 66 blocks are designed to terminate 22 through 26 AWG solid copper wire. The 66 series connecting block, introduced in the Bell System in 1962, was the first terminating device with insulation displacement connector technology. The term 66 block reflects its Western Electric model number.
The 25-pair standard non-split 66 Block contains 50 rows; each row has four (M) or six (B) 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 (M) or six (B) clips, but the left two (or three) clips are electrically isolated from the right two (or three) clips. Smaller versions also exist with fewer rows for residential use.
66 blocks are available pre-assembled with an RJ-21 female connector that accepts a quick connection to a 25-pair cable with a male end. These connections are typically made between the block and the customer premises equipment (CPE).
66 Block B series with 6 clips in each row. Jumper wires on the left connect the top pair with the bottom pair, allowing to split up to 10 devices.
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 premises equipment 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.
Modern 110 blocks largely supplanted 66 blocks for new commercial installations at the end of the 20th century, as the capability for a circuit to carry digital data overlaid its ability to carry analog voice conversations. 110 block termination is almost always Category 5 (or higher) compliant, and capable of supporting 100 MHz (or faster) signaling. Compared to 110 and higher-density wire terminating blocks, 66 blocks are physically large; and because of their maximum 16 MHz Category 3 signaling compatibility, they are ill-suited for high speed (faster than 10BASE-T) data circuits. However, special Category 5e Certified 66 blocks are available from manufacturers such as Siemon which meet all standards for Cat5e termination.
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.
25-pair Color Code
The 25-pair color code, originally known as even-count color code, is a color code used to identify individual conductors in twisted-pair wiring for telecommunications.
With the development of new generations of telecommunication cables with polyethylene-insulated wire by Bell Laboratories for the Bell System in the 1950, new methods were developed to mark each individual conductor in cables. Each wire was identified by the combination of two colors, one of which is the major color, and the second the minor color. Major and minor colors are chosen from two different groups, resulting in 25 color combinations. The color combinations are applied to the insulation that covers each conductor. Typically, the major color was a solid, background color on the insulation. The minor color was a tracer, consisting of stripes, rings, or dots, applied over the background. The minor color always matches the major color of its paired conductor.
The major, or primary group of colors consists of the sequence of white, red, black, yellow, and violet. The minor, or secondary color was chosen from the sequence blue, orange, green, brown, and slate.
The wire pairs are referred to either directly by their color combination, or by the pair number. For example, pair 9 is also called the red-brown pair. In technical tabulations, the colors are often suitably abbreviated.
Violet is the standard name in the telecommunications and electronics industry, but it is sometimes referred to as purple. Similarly, slate is a particular shade of gray. The names of most of the colors were taken from the conventional colors of the rainbow or optical spectrum, and in the electronic color code, which uses the same ten colors (though in a different order).
When used for POTS, the first wire is known as the tip or A-leg (U.K.) conductor and is usually connected to the positive side of a direct current (DC) circuit, while the second wire is known as the ring lead or B-leg (U.K.), and is connected to the negative side of the circuit. Neither of these two sides of the line has a connection to the local ground. This creates a balanced audio circuit with common-mode rejection, also known as a differential pair. The tip and ring convention is based on the 1⁄4″ (6.5 mm) TRS phone connectors, which were employed in telephone switchboards in the 19th and 20th centuries, where the tip contact of the connector is separated from the ring contact by a spacer of insulation. The connection furthest from the cable is known as the tip, the middle connection is the ring, and the (largest) connection closest to the wire is the sleeve.
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 (right hand image) 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.