The panel switching system 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 Panel and Rotary systems were developed in parallel by Western Electric Labs (later called Bell Labs) in the U.S. and International Western Electric in Belgium before World War I, and had many features in common, though the Rotary system was used in Europe.
The first Panel exchange was installed at the Mulberry Central Office in Newark, New Jersey. It was placed in service on January 16, 1915. It was a semi-automatic system using non-dial telephones. The next installation was in the Waverly C.O. June 12th. of the same year, also in Newark, NJ.
The Panel Machine Switching System (M.S.S.) was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor, to drive its (usually sixty) selectors by electromagnetically controlled clutches. The selector was similar in effect to a stepping switch though it moved continuously rather than in steps. 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 to a Register which had received the dialled digits, rather than forward as in the SXS system, hence the signaling was called "Revertive Pulse".
Telephone numbering 
As in the SXS system, each installed office could handle up to 10,000 numbered lines, requiring four digits for a line number within the office plus an office code.
The panel system was designed to interconnect the offices of a city or a local calling area. Each office had a two-digit code (later three). Callers dialed the office code followed by the line number within the office. In most situations this led to six-digit numbers (later seven). But from the beginning the panel system handled seven-digit numbers (later eight), for two reasons.
- Party line numbers were listed with a J, M, R, or W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter.
- The panel system was designed to work with manual offices of up to 10,500 lines. Callers dialed the office code followed by the line number within the office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number.
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.
The various switching entities in the building could share the same outgoing section, and this was particularly advantageous for trunks to particularly distant parts of the city, consolidating traffic that would otherwise be scattered among smaller and less efficient trunk groups or require using a tandem switch office.
Idle outgoing trunks were picked by the traditional "sleeve test" method, as lines were, except that hunting was the usual practice for trunks rather than a special service feature. The selector moved upward through twenty terminals, checking for one with an ungrounded sleeve lead, then selecting and grounding it (this in the Battery Cutoff version, which was the later, more fireproof and more widespread one). If no trunk was idle, the selector sent back an All Circuits Busy tone (reorder tone). There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones.
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 Director telephone system Strowger exchanges. 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. Decoders also determined the proper rate at which to operate the message register and gave this information to the sender, which set this rate in the District Junctor. Auxiliary senders were added in the mid 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 the selector to stop there and prepare for the next number for the next stage. 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, not only to control panel selectors but to signal within itself and with similar exchanges. Later 5XB switch, 1ESS switch and other systems included RP equipment in order to maintain compatibility, in some cases decades after the last Panel switch in the city had been scrapped.
Stuck sender 
Revertive Pulse was faster than dial pulse, but the greater advantage was when something went wrong. In earlier systems when a worn pawl or other problem in a Strowger selector caused it to fail to advance, nobody knew except the calling party, who only knew nothing was happening. The caller eventually lost patience, redialled, and might easily get stuck again on the same selector, or another caller could get stuck there. One bad Strowger selector could block dozens of calls per hour until subscriber complaints led staff to discover it.
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, returned a trouble tone to the caller, held the switch train out of service with a grounded sleeve lead 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.
Interoffice signaling 
For panel-office-to-panel-office calls, the two offices communicated by revertive pulsing. For calls within a single panel office, the two halves of the office also communicated by revertive pulsing.
For compatibility with manual offices, Panel Call Indicator (PCI) signaling was used. PCI used multilevel DC pulses, for a bit to baud ratio of 2:1. PCI signalling lit lamps on the B operator's desk at the terminating manual office. Another type of signaling, Call Annunciator, used speech recorded on strips of photographic film to announce the called number to the answering operator.
PCI continued in use for tandem purposes, decades after its original purpose had disappeared. In the 1950s Auxiliary Senders were added, to allow storing more than eight digits, and sending by MF for Direct Distance Dialing. MF was also used by Crossbar and ESS exchanges.
Compatibility with manual offices 
Panel was installed in cities where many people still had manual (non-dial) service. Calls from panel offices to manual offices required PCI signals to tell the "B Board Machine Incoming" operator or incoming operator the phone number. The number appeared on a lit display (or indicator). The operator completed the call in the usual way.
Calls from manual offices to panel offices required the "A board" or outgoing operator to request the number from the caller, connect to an idle trunk to the distant exchange and relay to the "B Board Manual Incoming Call" operator the desired number. The "B Board Manual Incoming Call" operator would then key in the desired number. The Panel machine would then setup the incoming and final frames to the called telephone number.
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 having no such restriction, the dimensions of the panel were 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. Thus, most calls required only about half as many stages as in earlier and later systems. Motors used on Panel frames operated on A.C. or D.C. however they could only be started on D.C. In the event of an A.C. power failure the motor would switch over to its D.C. windings.
- Short movie of a Panel call from Seattle Museum of Communications.
- Short movie about early Panel Archive Footage.
- Photos & descriptions last working Panel Office Seattle Museum of Communications.
- More Panel photos can be found at WECo Panel Office.
- A good description of the Panel switch can be found in the Survey of Telephone Switching.
- A Panel Final Selector frame is on display at the Museum of Communications.
- Some 1970s-era recordings of telephone calls involving panel switches by Evan Doorbell can be found at Phone Trips, including one presentation devoted exclusively to "panel pulsing".
- The sounds of a rotary dial call originating in a Crossbar office (old dialtone) going through a XB Tandem office MF and terminating in a Panel office (MP3 Format)(hear the revertive pulsing) followed by machine ringing and the clicks of the Panel test line.
- Machine-switching in 1911 Britannica
- Fagen pages 581, 607
- page 582
- Fagen, M. D. A History of Engineering and Science in the Bell System; The Early Years (1875-1925) (1975, Bell Telephone Laboratories)