1A2 Key System - Wiring

Wiring

Each line to the telephone sets was routed using six wires:

• Two wires (one pair) carried the actual telephone line,
• Two wires (a second pair) carried control information (known as 'A-Leads') for that line,
• Two wires (a third pair) carried current to a lamp for the specific line key position on the phone.

A telephone set could contain anywhere from two to over twenty-nine individual telephone lines. Most key telephones (often called 'keysets') with up to nine line positions are connected to the system using a single 25-pair cable and an Amphenol 50-position "MicroRibbon" connector. Keysets with up to 19 line positions used a 50-pair cable, where the big sets with 29 line positions used 75 pairs (three connectors). There was even a 'Call Director' style phone made, at one time, which had over 30 line key positions, and used 100 pairs (four connectors).

Each of the keyset cables was usually run back to the wiring closet, or whatever central location where the KSU had been installed, and terminated on a connection device known as a 66 block or punch block. The blocks most often used to terminate these station cables were the type 66M1-50. Each of these blocks could accept two 25-pair cables (50 pairs, total) for termination.

Cross-connect wire jumpers, consisting of three twisted pairs (the six wires referenced above) would then be run between these blocks and the larger distribution connecting blocks within the KSU.

Very large installations of 1A2 systems had multiple wiring closets fed by branch cables extended from the central closet where the KSU was located. An example of this type of installation would be a multi-story building. The KSU and incoming lines might be in the basement, and each floor would have a branch wiring closet of its own where the phones for that floor were connected.