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Panel switch district selector frame at the Connections Museum in Seattle

The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983.

The Panel switch was named for its tall panels which consisted of layered strips of terminals. Between each strip was placed an insulating layer, which kept each metal strip electrically isolated from the ones above and below. These terminals were arranged in banks, five of which occupied an average selector frame. Each bank contained 100 sets of terminals, for a total of 500 sets of terminals per frame.[1] At the bottom, the frame had two electric motors to drive sixty selectors up and down by electromagnetically controlled clutches. As calls were completed through the system, selectors moved vertically over the sets of terminals until they reached the desired location, at which point the selector stopped its upward travel, and selections progressed to the next frame, until finally, the called subscriber's line was reached.

History

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In c. 1906, AT&T organized two research groups for solving the unique challenges in switching telephone traffic in the large urban centers in the Bell System. Large cities had a complex infrastructure of manual switching that prevented complete ad hoc conversion to mechanical switching, but more favorable economics was anticipated from conversion to mechanical operation. No satisfactory methods existed for interconnecting manual systems with machines for switching. The two groups at the Western Electric Laboratories focused on different technologies, using competitive development to stimulate invention and increase product quality, a concept that had been successful at AT&T previously in transmitter design.[2] One group continued existing work that yielded the Rotary system, while the second group developed a system that was based on linear movement of switch components, which became known as the panel bank. As work continued, many subassemblies were shared, and the two switches only distinguished themselves in the switching mechanisms.

By 1910, the design of the Rotary system had progressed farther and internal trials employed it at Western Electric as a private branch exchange (PBX). However, by 1912, the company had decided that the panel system showed better promise to solve the large-city problem, and delegated the use of the Rotary system for use in Europe to satisfy the growing demand and competition from other vendors there, under the management and manufacture by the International Western Electric Company in Belgium.[3]

After a trial installation as a PBX within Western Electric in 1913, Panel system planning commenced with design and construction of field trial central offices using a semi-mechanical method of switching, in which subscribers still used telephones without a dial, and operators answered calls and keyed the destination telephone number into the panel switch, which then completed the call automatically.[4]

1948 Western Electric Model 302 telephone set that was installed near Newark airport, and connected to a line of the Newark exchange "BIgelow", the last existing Panel office when it was dismantled in 1983.

These first panel-type exchanges were placed in service in Newark, New Jersey,[5] on January 16, 1915 at the Mulberry central office serving 3640 subscribers, and on June 12 in the Waverly central office, which had 6480 lines. Panel development continued throughout the rest of the 1910s and in the 1920s in the United States. A third system in Newark (Branch Brook) followed in April 1917 for testing automatic call distribution.

The first fully machine-switching Panel systems using common control principles were the Douglas and Tyler exchanges in Omaha, Nebraska, completed in December 1921. Subscribers were issued new telephones with dials, that permitted the subscriber to place local calls without operator assistance. This installation was followed by the first installations in the eastern region in the Sherwood and Syracuse-2 central offices in Paterson, New Jersey, in May and July 1922, respectively.[6] The storied Pennsylvania exchange in New York City was cut-over to service in October 1922.[4][7]

Most Panel installations were replaced by modern systems during the 1970s. The last Panel switch, located in the Bigelow central office in Newark, was decommissioned by 1983.[8]

Operational overview

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When a subscriber removes the receiver (earpiece) from the hookswitch of a telephone, the local loop circuit to the central office is closed. This causes the flow of current through the loop and a line relay, which causes the relay to operate, starting a selector in the line finder frame to hunt for the terminal of the subscriber's line. Simultaneously, a sender is selected, which provides dial tone to the caller once the line is found. The line finder then operates a cutoff relay, which prevents that telephone from being called, should another subscriber happen to dial the number.

Dial tone confirms to the subscriber that the system is ready for dialing. Depending on the local numbering system, the sender required either six or seven digits in order to complete the call. As the subscriber dialed, relays in the sender counted and stored the digits for later usage. As soon as the two, or three digits of the office code were dialed and stored, the sender performed a lookup against a translator (early-type) or decoder (later-type). The translator or decoder took the two or three digits as input, and returned data to the sender that contained the parameters for connecting to the called central office. After the sender received the data provided by the translator or decoder, the sender used this information to guide the district selector and office selector to the location of the terminals that would connect the caller to the central office where the terminating line was located. The sender also stored and utilized other information pertaining to the electrical requirements for signaling over the newly established connection, and the rate at which the subscriber should be billed, should the call successfully complete.

On the district or office selectors themselves, idle outgoing trunks were picked by the "sleeve test" method. After being directed by the sender to the correct group of terminals corresponding to the outgoing trunks to the called office, the selector continued moving upward through a number of terminals, checking for one with an un-grounded sleeve lead, then selecting and grounding it. If all the trunks were busy, the selector hunted to the end of the group, and finally sent back an "all circuits busy" tone. There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones.

Once the connection to the terminating office was established, the sender used the last four (or five) digits of the telephone number to reach the called party. It did so by converting the digits into specific locations on the remaining incoming and final frames. After the connection was established all the way to the final frame, the called party's line was tested for busy. If the line was not busy, the incoming selector circuit sent ringing voltage forward to the called party's line and waited for the called party to answer their telephone. If the called party answered, supervision signals were sent backwards through the sender, and to the district frame, which established a talking path between both subscribers, and charged the calling party for the call. At this time, the sender was released, and could be used again in service of an entirely new call. If the called subscriber's line was busy, the final selector sent a busy signal back to the called party to alert them that the caller was on the phone and could not accept their call.

Telephone numbering

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As in the Strowger system, each central office could address up to 10,000 numbered lines (0000 to 9999), requiring four digits for each subscriber station.

The panel system was designed to connect calls in a local metropolitan calling area. Each office was assigned a two- or three-digit office code, called an office code, which indicated to the system the central office in which the desired party was located. Callers dialed the office code followed by the station number. In larger cities, such as New York City, dialing required a three-digit office code,[9] and in less-populated cities, such as Seattle, WA[10] and Omaha, NE, a two-digit code. The remaining digits of the telephone number corresponded to the station number, which pointed to the physical location of the subscriber's telephone on the final frame of the called office. For instance, a telephone number may be listed as PA2-5678, where PA2 (722) is the office code and 5678 is the station number.

In areas that served party lines, the system accepted an additional digit for party identification. This allowed the sender to direct the final selector not only to the correct terminal, but to ring the correct subscriber's line on that terminal. The panel system supported individual, 2-party, and 4-party lines.

Panel ground-cut-off (GCO) line finder brushes

Circuit features

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Similar to the divided-multiple telephone switchboard, the panel system was divided into an originating section and a terminating section. The subscriber's line had two appearances in a local office: one on the originating side, and one on the terminating side. The line circuit consisted of a line relay on the originating side 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 originating section or the terminating. On the terminating end, the line circuit was connected to a final selector, which was used in call completion. Thus, when a call was completed to a subscriber, the final selector circuit connected to the desired line, and then performed a sleeve (busy) test. If the line was not busy, the final selector operated the cut-off relay via the sleeve lead, and proceeded to ring the called subscriber.

Supervision (line signaling) was supplied by a District circuit, similar to the cord circuit that plugged into a line jack on a switchboard. The District circuit supervised the calling party, and when the calling party went on-hook, it released the ground on the sleeve lead, thus releasing all selectors except the final, which returned down to their start position to make ready for further traffic. The final selector circuit was not supervised by the district circuit, and only returned to normal once the called party hung up.[11] Some District frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals for handling calls from payphones.

Many of the urban and commercial areas where Panel was first used had message rate service rather than flat rate calling. For this reason the line finder had a fourth wire known as the "M" lead. This enabled the District circuit to send metering pulses to control the subscriber's message register. The introduction of direct distance dialing (DDD) in the 1950s required the addition of automatic number identification equipment for centralized automatic message accounting.

The terminating section of the office was fixed to the structure of the last four digits of the telephone number, had a limit of 10,000 phone numbers. In some of the urban areas where Panel was used, even a single square mile might have three or five times that many telephone subscribers. Thus the incoming selectors of several separate switching entities shared floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Selector Tandem was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common control 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.

Sender

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Closeup of all-relay type sender

While the Strowger (step-by-step) switch moved under direct control of dial pulses that came from the telephone dial, the more sophisticated Panel switch had senders, which registered and stored the digits that the customer dialed, and then translated the received digits into numbers appropriate to drive the selectors to their desired position: District Brush, District Group, Office Brush, Office Group, Incoming Brush, Incoming Group, Final Brush, Final Tens, Final Units.

Sender alarm and Make Busy panel

The use of senders provided advantages over the previous direct control systems, because they allowed the office code of the telephone number to be decoupled from the actual location on the switching fabric. Thus, an office code (for example, "722") had no direct relationship to the physical layout of the trunks on the district and office frames. By the usage of translation, the trunks could be located arbitrarily on the physical frames themselves, and the decoder or translator could direct the sender to their location as needed. Additionally, because the sender stored the telephone number dialed by the subscriber, and then controlled the selectors itself, there was no need for the subscriber's dial to have a direct-control relationship to the selectors themselves. This allowed the selectors to hunt at their own speed, over large groups of terminals, and allowed for smooth, motor controlled motion, rather than the staccato, momentary motion of the step-by-step system.

The sender also provided fault detection. As it was responsible for driving the selectors to their destinations, it was able to detect errors (known as trouble) and alert central office staff of the problem by lighting a lamp at the appropriate panel. In addition to lighting a lamp, the sender held itself and the selectors that were under its control out of service, which prevented their use by other callers. Upon noting the alarm condition, staff could inspect the sender and its associated selectors, and resolve whatever trouble occurred before returning the sender and selectors back to service.

When the sender's job was complete, it connected the talk path from the originating to the terminating side, and dropped out of the call. At this time, the sender was available to handle another subscriber's call. In this way, a comparatively small number of senders could handle a large amount of traffic, as each was only used for a short duration during call setup. This principle became known as common control, and was used in all subsequent switching systems.

Signaling and control

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Revertive Pulsing (RP) was the primary signaling method used within and between panel switches. The selectors, once seized by the sender or another selector, would begin moving upwards under motor power. Each terminal the selector passed would send a pulse of ground potential along the circuit, back to the sender. The sender counted each pulse, and when the correct terminal was reached, the sender then signalled the selector to disengage the upward drive clutch and stop on the appropriate terminal as determined by the sender and decoder. The selector then either began its next selection operation, or extended the circuit to the next selector frame. In the case of the final frame, the last selection would result in connection to an individual's phone line and would begin ringing.

As the selectors were driven upwards by the motors, brushes attached to the vertical selector rods wiped over commutators at the top of the frame. These commutators contained alternating segments serving as insulators or conductors. When the brush passed over a conductive segment, it was grounded, thereby generating a pulse which was sent back to the sender for counting. When the sender counted the appropriate number of pulses, it cut the power to the solenoid in the terminating office, and caused the brush to stop at its current position.

Calls from one panel office to another worked very similarly to calls within an office by use of revertive pulse signalling. The originating office used the same protocol, but inserted a compensating resistance during pulsing so its sender encountered the same resistance for all trunks.[12] This is in contrast to more modern forms of forward pulsing, where the originating equipment will directly outpulse to the terminating side the information it needs to connect the call.

Compatibility

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Later systems maintained compatibility with revertive pulsing, even as more advanced signaling methods were developed. The Number One Crossbar, which was the first successor to the Panel system also used this method of signaling exclusively, until later upgrades introduced newer signaling such as Multi-frequency signaling.

Panel was initially installed in cities where many stations still used manual (non-dial) service. For compatibility with manual offices, two types of signaling were supported. In areas with mostly machine switches and only a few manual switchboards, Panel Call Indicator (PCI) signaling transmitted the called number to the "B" Board Machine Incoming operator, which lit lamps on the operator's desk at the terminating manual office. The lamps illuminated digits on a display panel corresponding to the number dialed. The manual operator connected the call to the appropriate jack, and then repeated the process for the next incoming call. In areas with mostly manual switches, the Call Annunciator signaling system was used to avoid installing lamp panels at every operator station. The Call Annunciator used speech recorded on strips of photographic film to verbally announce the called number to the answering operator.

PCI signaling continued to be used for tandem purposes, decades after its original need had disappeared. In the 1950s, auxiliary senders were added for storing more than eight digits, and sending by multi-frequency (MF) signaling for direct distance dialing (DDD).

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 the desired number to the B Board Manual Incoming Call operator, who keyed it to the Panel machine for setting up the incoming and final frames to the called telephone number.

Motor power

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The panel switch is an example of a power drive system, in that it used 1/16 horsepower motors to drive the selectors vertically to hunt for the desired connection, and back down again when the call was completed. In contrast, Strowger or crossbar systems used individual electromagnets for operation, and in their case the power available from an electromagnet limits the maximum size of the switch element it can move. With Panel having no such restriction, its dimensions were determined solely by the needs of the switch, and the design of the exchange. 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 systems. Motors used on panel frames were capable of operating on alternating (AC) or direct current (DC), however they could only be started with DC. In the event of an AC power failure the motor would switch to its DC windings, and continue running until AC power was restored.

Maintenance and testing

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Because of its relative complexity compared to direct control systems, the Panel system incorporated many new types of testing apparatus. At the time of its design, it was decided that maintenance should be done on a preventative basis, and regular testing of the equipment would be used to identify faults before they became severe enough to affect subscribers. To this end, multiple types of test equipment were provided.[13] Test equipment generally took the form of either a wooden, switchboard-like desk, a wheeled cart, known as a "Tea Wagon", or a small box-type test set that could be carried to the apparatus that required testing. The central test location in the office was known as the "OGT Desk", or "Trouble Desk", and took the form of a large wooden desk with lamps, jacks, keys, cords, and a voltmeter. This desk served as the central point for analysis and trouble resolution.

An OGT (OutGoing Trunk test) desk at the Connections Museum, Seattle. This desk was part of the RAinier/PArkway panel office, and was installed in 1923.

Other test apparatus included frame-mounted equipment that was used to routine commonly used circuits within the office. These included an automatic routine sender test frame, and an automatic routine selector test frame. When testing was to be done manually by a switchman, he or she used a Tea Wagon, which was wheeled to the apparatus to be tested, and plugged into jacks that were provided for this purpose.

Upgrades

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A switchman works on a two-digit translator type sender.

Throughout its service time, the Panel system was upgraded as new features became available or necessary. Starting in the mid-1920s, such upgrades improved the initial design. Major attention was initially focused on improving the sender. Early two- and three-digit type senders stored dialed digits on rotary selector switches. The senders employed translators to convert the dialed digits into the appropriate brush and group selections needed to complete the call. As better technology became available, Panel senders were upgraded to the all-relay type. These were more reliable, and in addition, replaced the translator equipment with decoders, which also operated entirely with relays, rather than with motor-driven apparatus, which yielded faster call completion, and required less maintenance.

Another important improvement involved a fundamental change in the electrical logic of the switching system. The Panel originally shipped in a ground cut-off (GCO) configuration, wherein the cut-off relay had ground potential on one side of its winding at all times. A busy line condition was indicated by -48 volt battery applied to the other side of the cut-off relay winding, and thus at the sleeve lead. This would be detected by the final selector as it hunted over the terminals. Starting in 1929, all newer panel systems were deployed as battery cut-off (BCO) systems.[14] In this revision, the presence of ground and -48V was reversed. Battery was constantly applied to one side of the cut-off relay, and the presence of ground on the other side of the winding indicated the line was busy. This change necessitated a fundamental change in the design of the system, and was undertaken for many reasons. One of the most notable was that GCO offices were more prone to fire.[15]

The line finder was also improved during the system's lifetime. Originally, the line finder frame had a capacity of 300 lines each, and used 15 brushes (vertical hunting segments) on each rod. This was intended to reduce hunting time as there were more brushes hunting over a shorter distance. As these line finders went into service, however, it became evident that 15 brushes on each vertical selector rod were quite heavy, and needed springs and pulleys at the top of the frame to compensate for their mass. Later line finders used 10 brushes and rearranged the layout to accommodate 400 lines per line finder frame. This increased capacity while eliminating the need for compensating equipment.

Western Electric estimated that the design changes of 1925 to 1927 accounted for a 60% reduction in overall costs for the Panel system.[15]

The following table presents early major panel system upgrades:[16]

Year Line finder type Sender connection type Maximum number of
senders per group 		Sender type Cut-off relay type
1920 Line Switch (200-type) Sender Selector 22 Translator GCO
1920 300 pt line finder Sender Selector 22 Translator GCO
1924 400 pt line finder Sender Selector 22 Translator GCO
1926 400 pt line finder Rotary Link 44 Translator GCO
1927 400 pt line finder Panel Link 100 Translator GCO
1928 400 pt line finder Panel Link 100 Decoder GCO
1929 400 pt line finder Panel Link 100 Decoder BCO

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Panel switch

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from Grokipedia
A panel switch is an electromechanical telephone switching system developed by the American Telephone and Telegraph Company () and its manufacturing arm, , in the to automate call routing in large urban central offices. It represented a significant advancement over earlier step-by-step switches by introducing a "store and forward" mechanism, where a device called a sender temporarily stored the dialed digits before directing selectors to connect the call, enabling more efficient handling of high call volumes without requiring direct between switches. The system's development addressed the limitations of manual switchboards and rudimentary automatic switches in growing metropolitan areas, with initial semiautomatic trials conducted in , in 1915. Fully automatic installations began in , in December 1921, followed by the Pennsylvania exchange in in 1922, marking its rapid adoption for interoffice trunking and local calls. By 1930, panel switches served all central offices in , though their complexity and high cost limited deployment outside the . Key technical features included tall vertical panels—each with 500 rows of terminals across multiple banks—driven by electric motors and controlled via electromagnetic clutches for precise wiper movement. The system employed indirect control through translators, invented by Edward C. Molina in 1905, which converted dialed pulses into selector positions, and a 1916 numbering plan devised by W.G. Blauvelt to facilitate automatic dialing in large exchanges. Call setup involved multiple frame types, such as line finders for originating lines, district and office selectors for routing, and final selectors for termination, using revertive pulsing for communication between switches. Panel switches remained in service through the mid-20th century, with the last installations decommissioned in the early 1980s, paving the way for crossbar and electronic systems.

History

Development and invention

The panel switch emerged from early 20th-century efforts by the to address the limitations of manual switchboards in rapidly growing urban telephone networks. In 1903, acquired the patents for the Lorimer brothers' automatic switching system, which featured motor-driven rotary selectors and laid foundational concepts for electromechanical . By 1906, engineers formed dedicated teams to adapt and expand these ideas, resulting in parallel development of rotary and panel switch designs tailored for large-scale metropolitan use. The panel switch was developed by a team of engineers under the oversight of Frank B. Jewett, president of the company and a key figure in research. Advancements included Edward C. Molina's 1905 translator for indirect control and W.G. Blauvelt's 1916 numbering plan for large exchanges. A pivotal advancement came with U.S. 1,172,585, filed on August 23, 1915, by inventor Henry F. Dobbin, which detailed a selector switch mechanism for panel-based automatic exchanges, enabling precise vertical and horizontal movement across terminal banks via stepping magnets to connect up to 100 lines per set. This innovation emphasized layered panels of terminals separated by insulators, allowing efficient line selection without direct mechanical linkage to dials, thereby minimizing operator involvement compared to prior step-by-step systems. Prototypes were rigorously tested in laboratories starting around 1915, focusing on electromechanical reliability for replacing manual operations in high-volume environments. The initial field trial occurred with semiautomatic panel installations in , in 1915, serving several thousand subscribers using a hybrid of dials for outgoing calls and operators for incoming. Key challenges during prototyping included achieving accurate to coordinate selector movements and prevent call disconnects or misrouting, addressed through refined electromagnetic controls and motor-driven clutches. By 1921, these efforts culminated in the first fully automatic dial-controlled panel switch, installed and operational in , in December, enabling subscribers to complete local calls independently via dials on their telephones. This marked the system's commercial readiness after six years of iterative refinement from the 1915 patent, establishing panel switching as a cornerstone for urban expansion.

Deployment and adoption

The first full installation of a panel switch occurred in , in December 1921, with the system designed to handle up to 10,000 lines in a large urban setting. This marked the beginning of AT&T's Bell System rollout of the technology, which was specifically engineered for high-volume interoffice calling in growing metropolitan areas. The second installation followed in New York City's Pennsylvania exchange in October 1922, demonstrating early scalability for dense populations. Adoption accelerated rapidly through the , driven by the need to accommodate urban expansion and the economic advantages of over manual switchboards. Panel switches offered significant labor reductions—estimated at up to 30% in operator staffing for equivalent service—while enabling efficient handling of complex call traffic without proportional increases in personnel. A notable early example was the deployment in , where semiautomatic panel variants had been tested since 1915, evolving into larger full-dial installations by the mid-1920s that served as benchmarks for major urban upgrades. By the end of 1928, the had 128 panel offices operational across key U.S. cities, including New York, , and . Rollout challenges included the high upfront costs of and existing to integrate with legacy lines, as well as the need for specialized technician training to manage the system's electromechanical complexity. Despite these hurdles, the technology proved economically viable for long-term operation, with total panel-served lines reaching approximately 1.6 million stations by the end of 1928. The Bell System planned full conversion of major panel areas by 1942, prioritizing regions where justified the investment. International adoption remained limited in the late , with no widespread exports to or due to the system's high maintenance demands and costs, which favored simpler alternatives abroad.

Evolution through the mid-20th century

In , panel switches underwent enhancements to support more complex arrangements, including toll for long-distance connections, which improved efficiency in urban exchanges handling high volumes of interoffice calls. These modifications allowed panel systems to manage extensive toll networks, with designs emphasizing large panels for better trunk selection and reduced blocking in metropolitan areas. By 1930, every telephone connected to a central exchange in operated via panel switches, marking widespread adoption in major cities like New York and . During , the shifted significant resources toward military production, including adaptations of panel switching technology for reliable communications in defense installations, with Western Electric's government sales reaching $2.3 billion by war's end. This period emphasized durability and redundancy in panel designs for 1940s deployments, supporting wartime needs amid material shortages and heightened demand for robust telephony infrastructure. Post-war expansion in the saw panel switches reach peak usage, serving millions of lines in large urban areas where their capacity for complex routing remained advantageous, contributing to the Bell System's growth to over 68 million telephones by 1961. Variants like the No. 5 crossbar, introduced in 1947 and first installed in 1948, offered higher capacity alternatives, gradually supplementing panel systems in metropolitan-adjacent regions. By the mid-1950s, competition from step-by-step and crossbar systems—such as the No. 1 crossbar deployed since 1938—began eroding panel switch dominance due to faster call setup times and lower maintenance costs in crossbar designs. Installations continued in some areas into the early , particularly where existing infrastructure favored electromechanical reliability, but the transition accelerated with the 1956 regulating AT&T's operations and paving the way for electronic alternatives. Decommissioning of panel switches commenced in 1965 alongside the rollout of the first (No. 1 ESS), marking the end of major reliance on this technology.

Design and Components

Overall architecture

The panel switch telephone exchange employs a consisting of specialized panels organized into functional frames, enabling scalable automatic switching for urban networks. These include line finder frames for connecting subscriber lines, district and office selector frames for initial digit routing, incoming selector frames for interoffice trunks, and final connector frames for completing connections to called lines. Each frame houses vertical panels made of insulating materials, with electromagnetic drives and motor mechanisms for selector movement, supporting a hierarchical structure that accommodates up to 10,000 subscriber lines per exchange. Core components facilitate reliable latching and control within this . Relays, often multi-contact types, provide latching functions for circuit states across up to 50 leads in decoder and connector units, ensuring stable connections without continuous power. Markers serve as temporary control elements, interfacing with senders to store dialed information and direct routing through the panel train, distinct from the permanent switching paths. The layout arranges these in racks of , with each selector panel featuring 500 terminals divided into five banks of 100 positions, driven by continuously rotating motors and clutches for precise brush alignment. The logical flow follows a multi-stage hierarchical selection , beginning at the line finder panel where an originating call seizes a district selector, progressing through office and incoming selectors for digit-by-digit guidance, and terminating at the connector panel for the final line. Signal paths conceptually traverse these stages via dedicated links, with revertive pulsing coordinating movement without direct progressive control, optimizing for high-density metropolitan traffic. Typical exchanges start with primary units handling 600 lines but expand to full configurations supporting 10,000 lines, as seen in early 20th-century installations like those in .

Panel mechanisms and selectors

The panel mechanisms in the panel switch system utilized electromechanical selectors mounted on rigid steel frames to physically route calls by establishing metallic contacts between subscriber lines and trunks. These frames supported multiple banks of terminals, with insulated wiring connecting the contact points, ensuring reliable in noisy urban environments. Initial 1921 installations were designed to serve 600 subscribers, expandable to up to 10,000 through additional frames and selector groups, allowing scalability for large metropolitan exchanges. Vertical selectors handled line scanning within subscriber groups, employing motor-driven equipped with multiple wipers—typically five brushes per rod, one per —to access groups of 100 lines. The s consist of 100 rows of terminals with 30 columns, enabling up to 30 selectors per side to scan the same 100 lines for . These wipers moved continuously upward along the vertical axis under the control of electromagnetic clutches, which engaged to position the brushes precisely at the desired row based on decoded dial pulses; latching relays then held the position to maintain the connection during the call. For , the design incorporated redundant wipers across frames, enabling any available selector rod to access the same terminal leads if one failed, thus minimizing service disruptions. Office and incoming selectors managed trunk and selection, operating on similar vertical principles. Group selection was achieved by pre-assigning specific selector rods to particular outgoing trunk groups, with the vertical movement determining the specific trunk within the group. Revertive pulsing via brushes provided real-time feedback on wiper location to the control circuits. The overall build emphasized , with double-sided terminal banks constructed from insulated metal strips and supports to withstand the mechanical wear of continuous operation in high-traffic exchanges. A key distinction from step-by-step systems lay in the panel's pre-digit selection approach, where all digits were registered and selectors positioned in advance using common control circuits, rather than the progressive stepping method that advanced switches incrementally with each dial . This enabled more efficient handling of complex urban trunking but required sophisticated for coordination.

Sender and control systems

In panel switch systems, the sender serves as a critical auxiliary component responsible for registering the pulse-dialed digits from the calling subscriber and translating them into instructions for the panel selectors to establish the connection. Upon seizure of a line finder, an idle sender is automatically attached, providing dial tone to the caller and capturing each digit as a series of interruptions in the loop current, typically up to seven digits for local and toll calls. This digit storage enables a "store and forward" mechanism, allowing the system to process the full number before initiating switching operations, which improves efficiency over direct pulse-following methods in earlier step-by-step exchanges. Senders primarily use relay registers to store dialed digits and incorporate rotary selectors in associated translators to convert office codes into routing instructions, supporting the seven-digit numbering plan. These devices convert the incoming dial pulses—short DC interruptions—into appropriate control signals for the panels, ensuring reliable digit interpretation even with minor variations in dialing speed. The sender then communicates with the switching fabric through revertive pulsing, where it sends digit information to distant offices and receives feedback pulses to confirm selections. , developed by Edward C. Molina, convert the dialed office code digits into the physical positions required for selectors, using prewired or early rotary mechanisms. Control functions are managed via markers, which are temporary assemblies that seize control upon receiving from the sender, interpreting the stored digits, and directing the selectors to the correct positions. These markers perform pulse-to-DC conversion by transforming the pulsed input into steady DC levels that drive the motor-driven panel wipers, completing the translation in approximately 300 milliseconds using pre-wired lookup tables for office codes. This centralized control decouples the switching hardware from direct pulse dependency, enhancing reliability in high-traffic urban environments. Integration of senders occurs early in the call setup, with each line connected to a pre-selection frame via line finders that route the call to an available before engaging the district selector, allowing for efficient handling of multiple lines without per-line dedicated equipment. A typical panel exchange provisions over 100 senders in multiple groups, supporting numerous simultaneous call setups by pooling resources for reuse after digit registration. Unique to these systems is built-in error detection for digit reception, which tolerates widths of 50-70 milliseconds and discards malformed digits to prevent errors, ensuring robust operation amid variable subscriber dialing.

Operation

Call routing process

The call routing process in a panel switch begins with when the calling subscriber lifts the , closing the line circuit and operating the line relay. This triggers the line finder—a scanning mechanism that identifies the calling line among multiple subscribers and connects it to an idle district selector, establishing the initial path into the switching network. An idle is then automatically seized through a sender link and selector, providing to the caller to indicate readiness for dialing. As the subscriber dials the number, the sender registers and stores the trains corresponding to each digit, beginning with the first three digits that specify the office code within the . The sender's decoder translates these digits into routing instructions, determining the appropriate paths through the switch. With the digits registered, initiates by using revertive pulsing—a feedback mechanism operating at approximately 30 pulses per second—to position the wipers of the district selector on an idle trunk group leading toward the desired destination. It then sequentially controls the office selector to select a specific frame or inter-office trunk if required, followed by the incoming selector to reach a group of final selectors. The sender directs the final selector (also known as the connector) to the precise called line by positioning its wipers accordingly, seizing idle selectors along the way to avoid conflicts and ensuring a continuous metallic path. For intra-office calls, the entire sequence remains within the local exchange, directly linking the calling and called lines after the office selector. In contrast, inter-office calls involve selecting a trunk from the office selector to an incoming selector at the distant panel office, where the process continues to the final connection. Upon reaching the called line, the system tests for availability: if idle, the final selector applies a ringing signal (typically 20 Hz at 90 volts) to the called telephone, while sending ringback tone to the calling party. Answer supervision is provided through detection of loop current when the called party lifts the handset, which closes the circuit, stops the ringing, and fully establishes the two-way talking path. If the called line is busy or no path is available, the sender generates and applies a busy tone—an intermittent 480/620 Hz signal—to the calling party, and the connection attempt is released. The sender is then disconnected and returned to idle for reuse. The mechanical positioning of selectors via motors and clutches, combined with electrical translation, completes the core switching in fractions of a second per stage, with digit decoding taking about 300 milliseconds; however, the full process from off-hook to conversation readiness, including digit inter-dialing pauses, typically spans 2-5 seconds.

Telephone numbering and addressing

The panel switch system utilized a 2L-5N numbering plan (two letters plus five numbers), corresponding to a three-digit office code followed by a four-digit subscriber line identifier in large urban areas, with two-digit office codes in smaller areas; this facilitated local call routing within the network and supported up to 10,000 lines per exchange. For example, a number like KL5-1234 translated to the numeric equivalent -1234, where the first three digits () selected the exchange frame. This structure preserved compatibility with manual switchboard practices while enabling automated selection. Subscribers dialed using letter-number combinations on rotary dials marked with letters adjacent to digits 2 through 9 (e.g., and both on 5), allowing mnemonic exchange names like "KLondike" to be converted to numeric pulses by the sender equipment. The panel system interpreted these impulses through decoders that mapped letters to corresponding digits, ensuring seamless translation without altering printed directories significantly. In terms of addressing logic, the first dialed digit determined the vertical position on the selector panel; the second digit specified the vertical position on the selector, and the third digit the horizontal position on the selector, grouping lines into of 100 or 200 subscribers. The fourth digit guided the vertical position on the final selector, the fifth the horizontal position, with the sixth and seventh digits selecting the precise subscriber line via numerical sequencing, with senders temporarily storing pulses to coordinate this multi-stage selection. The numbering evolved from earlier five-digit local formats (two letters plus three numbers) in the to the standard seven-digit structure (two letters plus five numbers) by , accommodating urban growth and increased demand in cities like Newark and Omaha where panel switches were first deployed. This shift supported up to 10,000 lines per exchange without requiring a complete renumbering, as the system handled optional eighth digits for extensions or party lines exceeding 9,999 subscribers. Panel switches integrated with the (NANP), introduced in 1947, by adapting existing office codes to the new three-digit area codes and seven-digit local formats, allowing direct dialing across regions while maintaining for intra-city calls. Unique to the panel design were aspects such as the absence of a in local numbers—reserved instead for operator assistance—and specific toll access codes decoded by senders to route long-distance calls via trunk selectors, often prefixed with "0" to connect to manual toll boards.

Signaling protocols

Panel switches in the Bell System employed electromechanical signaling protocols based on pulses and battery supervision to manage call setup, digit transmission, and supervision between subscribers and the exchange, as well as between offices. These protocols relied on ground and battery connections over twisted-pair lines to transmit control information without voice path interference, ensuring reliable operation in urban networks from the 1920s onward. Pulse dialing formed the primary method for subscriber input, where the generated ground pulses by momentarily interrupting the loop current for each digit, typically at a rate of 10 pulses per second as standardized by the in the 1920s. For the digit "1," one was sent after an initial off-normal signal, while "0" produced ten pulses, with each pulse train lasting approximately 32-40 milliseconds make (loop closed) and 60-68 milliseconds break (loop open) to maintain compatibility with panel selectors. Supervision signals complemented this, including 20 Hz ringing voltage superimposed on -48 V DC battery to alert the called party, applied for two seconds in initial ring bursts followed by four-second silences. Inter-office signaling utilized E&M leads on trunks for two-way transmission between panel exchanges or to other switch types, where the "E" lead provided earth (ground) for signaling from the originating end and the "M" lead handled battery or ground from the terminating end to indicate seizure and answer. Loop start supervision was standard for subscriber lines, with ground start used for certain trunks, requiring the calling party to ground the tip lead to seize the line in those cases, preventing glitches on idle lines, while disconnect was achieved by opening the loop (removing battery) to release the connection. These methods operated at -48 V DC nominal voltage, with pulse durations aligned to the 10 pulses per second rate for consistency across the network. Control signals between internal components, such as the and sender, employed revertive pulsing—a reverse-action protocol unique to panel systems—where the sender initiated by applying battery to start selector movement, and the marker responded with reverse battery pulses to confirm digit reception and handshaking, at 30 pulses per second. Error conditions triggered supervisory tones, like the 120 interruptible per second reorder tone on the voice path to indicate busy lines or equipment faults, ensuring operators or senders could abort failed attempts. These protocols adhered to specifications from the , emphasizing -48 V DC levels for all battery signaling and standardized pulse timing to support scalable urban deployments.

Features and Integration

Circuit capabilities

Panel switches utilize revertive pulsing as a fundamental signaling technique to facilitate call routing and operator assistance. In this method, the originating transmits a start signal to the distant selector, which then advances and sends back intermittent pulses—known as marks—at a rate of approximately 30 pulses per second, corresponding to the desired position on the bank. The counts these pulses to halt the selector precisely at the target level or rotary position, enabling accurate digit-by-digit control even in operator-assisted calls where human intervention is required for complex routing or . This reverse-direction pulsing distinguishes panel systems from forward-pulsing alternatives and ensures reliable inter-switch communication. Traffic handling in panel switches incorporates grading across trunk and selector groups to balance load distribution, where incoming lines are allocated to multiple parallel selector banks via decoders that interpret office codes and optimize path selection. This hierarchical grading reduces congestion by spreading calls evenly, with systems engineered for a low blocking probability during peak busy-hour loads to deliver acceptable grade of service. Audio quality in panel switches is preserved through transformer-coupled transmission paths in line and trunk circuits, which provide electrical isolation and attenuate crosstalk to levels below perceptible thresholds. These paths support a standard voice bandwidth of 300 to 3400 Hz, ensuring intelligible speech transmission with minimal across the network.

Compatibility with other exchanges

Panel switches in the were designed with trunk interfaces that enabled interoperability with manual switchboards through tip-ring signaling, primarily utilizing the Panel Call Indicator (PCI) system over two-wire circuits. This setup allowed outgoing calls from panel offices to manual positions, where operators received the called number via a display panel activated by PCI pulsing, facilitating manual completion without verbal exchange. For incoming calls from manual offices, cordless "B" boards or call distributing switchboards were employed to handle trunk assignments and route connections efficiently. Interconnection with step-by-step exchanges relied on revertive pulsing protocols, where the panel sender directed the distant selector's movement by sending pulses and receiving feedback to confirm positioning, differing from the direct dial control of step-by-step systems. Interworking challenges arose from differing numbering plans, necessitating digit conversion by panel senders to translate dialed impulses into the non-decimal selector paths required for to non-panel offices; for instance, four-digit local numbers were reformatted as station (STA), thousands (TH), hundreds (H), tens (T), and units (U) codes. In , adapters such as enhanced PCI systems and key indicator setups addressed these issues in mixed networks, enabling seamless operation across urban installations with varying exchange types. For toll connectivity, panel switches supported long-distance trunks via offices, incorporating analog signaling that aligned with emerging international standards by the , allowing integration into networks through standard two-wire transmission paths. However, limitations persisted due to the system's analog nature, precluding direct compatibility with emerging digital exchanges and requiring intermediate analog gateways or protocol converters for later interoperations in the post-1950s era.

Power supply and motors

Panel switch exchanges utilized centralized DC power systems to ensure reliable operation of relays and associated electromechanical components. The standard nominal voltage was -48 V, supplied from lead-acid battery plants, which powered customer loop supervision and signaling functions, accommodating loop resistances up to 1300 ohms. These batteries served as the primary energy storage, providing reserve capacity for outages; typical configurations offered 3 to 8 hours of in central offices, extendable to 24 hours in remote installations, with cell ratings from 100 to 1680 ampere-hours and an expected service life exceeding 15 years. During normal conditions, commercial AC utility power was rectified to DC to support equipment loads and recharge the batteries, forming an integrated power plant that minimized disruptions. The mechanical actions in panel switches, including selector movements and wiper positioning, were driven by as part of the gross-motion electromechanical . For extended outages beyond battery reserves, engine-alternator sets—typically diesel or units—could generate up to 2000 kW to sustain operations, though these were supplementary to the primary DC infrastructure. Power distribution occurred via dedicated frames in the central office, delivering DC to line circuits and other components through tip and ring conductors, with design constraints based on path resistance and loop lengths ranging from 3.0 miles (26-gauge wire) to 14.5 miles (19-gauge wire). Overall, these systems prioritized continuity of service, though the motor-driven elements contributed to elevated maintenance demands from mechanical wear compared to later electronic alternatives.

Maintenance and Upgrades

Testing and troubleshooting

Routine maintenance of panel switches involved daily inspections to monitor system performance and prevent disruptions. Traffic load was assessed using peg counters, which registered incoming seizures, outgoing seizures, and overflows to gauge usage and identify overload conditions. Relay chatter inspections checked for unstable contacts in relays, a common issue in electromechanical systems that could lead to signaling errors, using auditory and visual checks during operation. Tools such as voltmeters for voltage measurements and tone generators for signal testing were essential for these daily routines, allowing technicians to verify circuit integrity without full disassembly. Troubleshooting followed a step-wise isolation process to diagnose faults like persistent busy signals. For instance, panel seizure tests involved using portable test sets, often mounted on a "tea wagon," to simulate call attempts and check if line finders or selectors were properly seizing trunks, isolating issues to specific components like faulty or trunks. Common problems included wiper misalignment in selector switches, where the moving brushes failed to align with bank contacts due to mechanical wear; this was typically resolved by manual reset and adjustment during maintenance. Relay failures, such as non-operating contacts in markers containing up to 1,500 , were addressed by cleaning eroded contacts and verifying operation with test keyboards. Bell System procedures, as outlined in early manuals, emphasized specific diagnostic tests. For example, 1920s documentation specified lamp tests on selectors to confirm status indications, such as guard or busy lamps lighting correctly during trunk hunts. Annual overhauls included motor lubrication for the continuously driven shafts in selectors and sequence switches, using prescribed oils to reduce friction and extend component life, performed during low-traffic periods. Safety protocols were critical due to the high-voltage components in panel switches. Technicians followed precautions like de-energizing circuits before inspections and using insulated tools to avoid shocks from the -48V DC battery supplies and higher voltages in ringing circuits. These practices contributed to reliable operation of electromechanical systems like panel switches through proactive testing.

Major modifications

In , panel switch systems received key retrofits to support the transition to 7-digit local dialing, which standardized addressing with a 2-letter, 5-number local format equivalent to seven digits. These modifications primarily involved upgrading the senders and district selectors to the additional digits efficiently, enabling larger urban exchanges to handle expanded local traffic without full replacement. Upgraded were added to interpret the dialed codes more accurately, improving compatibility with emerging features. During the 1940s, hybrid configurations combining panel switches with crossbar technology emerged to address toll expansion needs, particularly for long-distance routing. The #4 Crossbar toll switch, introduced in 1943, integrated with existing panel offices to provide faster revertive pulsing and reduced setup times for inter-city calls, marking a significant step in modernizing the Bell System's network for growing interstate demand. These hybrids replaced some mechanical components with crossbar matrices, enhancing overall switching speed and scalability. Post-World War II upgrades focused on enhancing reliability through selective relay replacements, where worn or less efficient electromagnetic s in selectors and connectors were swapped for more durable variants with smaller contact motions, reducing wear and failure rates. Capacity expansions were also pursued, with some large installations achieving support for up to 60 trunks per operator position on cordless "B" boards, allowing better traffic distribution in high-volume urban areas. For instance, offices incorporating these changes handled increased loads into the 1950s, delaying the need for complete overhauls. Further retrofits in the early enabled panel switches to support (DDD), initiated with trials in 1951, by incorporating pre-translators and additional sender capacity for 10-digit national calls including area codes. However, these electromechanical systems proved unable to integrate with precursors to electronic switching systems (ESS), such as early stored-program controls, due to inherent limitations in relay-based logic and speed. This incompatibility contributed to their phased obsolescence, with most panel offices converted to crossbar or ESS by the mid-1960s.

Decline and legacy

The phase-out of panel switches began in the 1950s as crossbar systems, introduced by in 1938, gained prominence for their greater reliability and lower maintenance costs in urban environments, gradually supplanting panel installations in major cities. By the 1970s, the advent of electronic switching systems (ESS), pioneered by with the first commercial installation of the No. 1 ESS in 1965, accelerated the replacement process due to ESS's superior scalability, reduced mechanical wear, and ability to handle higher call volumes without the physical selectors of panel technology. The final panel switch in the United States, located at the Bigelow central office in , was decommissioned in 1983, marking the end of widespread panel operations after over 50 years of service. Panel switches left a profound legacy in by enabling the first large-scale of urban , transitioning cities from operator-dependent manual boards to dial-based systems that supported millions of subscribers without proportional increases in labor. This influenced the development of standardized telephone numbering plans, such as the , by necessitating consistent digit lengths and formats compatible with panel's translating selectors, which handled two-letter-five-number addresses in early deployments. Additionally, panel systems played a key role in the 1960s rollout of (DDD) pilots, where retrofitted panel senders facilitated customer-dialed long-distance calls by interpreting multi-digit sequences, paving the way for nationwide DDD by 1965. Preservation efforts have ensured that examples of panel technology survive for educational and historical purposes, with operational restorations highlighting the ingenuity of electromechanical design. The Connections Museum in , operated by the Telecommunications History Group, houses one of the world's last fully functional panel switches, originally from the Parkway exchange (in service 1923–1970s), where volunteers demonstrate call routing to illustrate the system's relay-driven logic and motor-powered selectors. Other restoration projects, including those at the Museum of Communications, have meticulously rebuilt -era panel frames using original components to document the technology's role in early automation, allowing public interaction with dial telephones connected to live switches. Modern analyses underscore panel switches' efficiency gains over manual switchboards, where reduced the need for young female operators by 50–80% in affected areas between 1920 and 1940, as mechanical selectors handled routine connections and freed labor for other roles, though this pales against contemporary VoIP systems that eliminate physical infrastructure and operator intervention entirely for scalable, software-defined .

References

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