Panel switch
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The Panel telephone switch was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s.
The system was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor, to drive its selectors by electromagnetically controlled clutches. Each selector had five brushes, any of which had 100 terminals among which it could select, arranged in groups. Pulses were sent back from the selector, rather than forward as in the SXS system, hence the signaling was called "Revertive Pulse".
[edit] Sender
While the non-director Strowger (step-by-step or SXS) switch moves synchronously with the dial pulses that come from the telephone dial, the more sophisticated Panel switch had senders, similar to the directors of later Strowger exchanges except the caller had to dial the whole phone number before connection setup could start. The sender first translated the received digits into numbers appropriate for the selectors: District Brush, District Group, Office Brush, Office Group, Incoming Brush, Incoming Group, Final Brush, Final Tens, Final Units. Decoders helped by translating the first three digits of the phone number into four "District" and "Office" selecting numbers. Auxiliary senders were added in the middle 20th Century to implement Direct Distance Dialing.
When the selector had activated the correct brush or group, the sender sent a brief open circuit signal to command an advance to the next number. District and Office parameters were variable translations supplied by the decoder, while Incoming parameters and Final Brush were a fixed translation from the Thousands and Hundreds digits of the phone number, merely to adapt efficiently to the capabilities of the Panel selector. In the 1930s when the 1XB switch crossbar switching system was introduced, it used the same Revertive Pulse Register signaling system.
[edit] Stuck Sender
Revertive Pulse was fast, but the greater advantage was when something went wrong. When a worn pawl or other problem in a Strowger selector caused it to fail to advance, nobody knew except the caller. The caller eventually lost patience, redialled, and might easily get stuck again in the same place, or another caller could get stuck there. One bad selector could block dozens of calls per hour. With RP, the pulses were going backwards to the sender, a complex and sophisticated piece of hardware. If a selector failed to advance, it stopped sending pulses to the sender. A timer in the sender detected the failure, held the switch train out of service so no other caller would use the faulty circuit, and sounded an alarm. Staff could then trace the stuck sender, and identify and repair the defect while the caller tried again and usually succeeded.
Panel was installed in cities where many people still had manual service. Calls to those required "Panel Call Indicator" (PCI) signals to tell the operator the phone number. PCI used multilevel DC pulses, for a bit to baud ratio of 2:1. PCI continued in use for tandem purposes, decades after its original purpose had disappeared. In the 1950s Auxiliary Senders were added, to allow receiving by DTMF and storing and sending by MF more than seven digits for Direct Distance Dialing.
[edit] Motor
Panel is an example of a power drive system. Strowger or crossbar systems, in contrast, use individual electromagnets for operation: the power available from an electromagnet limits the maximum size of the switch element it can move. Panel, by contrast, has no such restriction. The dimensions of the panel are determined solely by the needs of the switch and the design of the exchange, as the driving electric motor can be made as large as is necessary to move the switch elements.
A good description of the Panel switch can be found in the Survey of Telephone Switching.