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".
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.
Supervision or line signalling was supplied by a Junctor Circuit, similar to the plug and light cord circuit that plugged into a line's TRS 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 Junctor Circuits 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 junctor 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.
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.
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.
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.
For compatibility with manual offices, Panel Call Indicator (PCI) singalling 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 signalling, 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 receiving by DTMF, storing more than eight digits, and sending by MF for Direct Distance Dialing.
Calls from manual offices to panel offices required the "A board" or outgoing operator to request the number from the caller, connect to a selector on an incoming frame in the distant office, and set up the call through the incoming and final frames to the called telephone.