Panel Switch - Line Circuit

Line Circuit

As in the divided-multiple telephone switchboard arrangements with which it served until it replaced them, Panel had an originating section and a terminating one, connected by a line circuit. As in the switchboard's "A Board", the originating section was organized into "panel and jack." At first, Panel was used only to replace the "B Board". All telephone lines were connected to the incoming section for incoming calls, while operators at the cordboards continued to handle originating calls. The line circuit consisted of a Line Relay to indicate that a customer had gone off-hook, and a cutoff relay to keep the line relay from interfering with an established connection. The cutoff relay was controlled by a "sleeve" lead that, as with the multiple switchboard, could be activated by either the incoming section or the outgoing. The incoming final selector performed a sleeve test to detect a busy line. In the original "Ground Cutoff" (GCO) version of Panel, battery on the sleeve lead indicated that "busy tone" was to be returned to the caller. In the later, more fireproof and more numerous BCO offices, a grounded sleeve was the busy indicator.

Supervision or line signalling was supplied by a District Circuit, similar to the plug and light cord circuit that plugged into a line's TRS phone connector on a switchboard. It supervised the calling and called party and, when both had gone on-hook, released the ground on the sleeve lead, thus releasing all selectors, which returned down to their start position to make ready for further traffic. Some District Frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals and otherwise handle payphones.

Many of the urban and commercial areas where Panel was first used had mostly Message Rate service rather than flat rate. For this reason the Line Finder had, besides the "tip and ring" leads for talking and the "sleeve" lead for control, a fourth wire for the District Circuit to send metering pulses to control the message register. The introduction of Direct Distance Dialing in the 1950s required the addition of Automatic Number Identification equipment to allow Centralized Automatic Message Accounting.

The incoming section of the office, being fixed to the MCDU structure of the last four digits of the telephone number, had a limit of 10,000 phone numbers, but in some of the urban areas where Panel was used, even a single square mile might have three or five times that many. Thus the incoming selectors of several separate switching entities would share floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Select Tandem was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common equipment; just one stage of selectors and accepting only the Office Brush and Office Group parameters. Panel Sender Tandems were also used, when their greater capabilities were worth their additional cost.