Editing Crossbar switch (section)

===Electromechanical switching in telephony===
A telephony crossbar switch is an [[Electromechanics|electromechanical]] device for [[telephone switch|switching]] [[telephone]] calls. The first design of what is now called a crossbar switch was the Bell company [[Western Electric]]'s coordinate selector of 1915. To save money on control systems, this system was organized on the [[stepping switch]] or selector principle rather than the link principle. It was little used in America, but the [[Televerket (Sweden)|Televerket]] Swedish governmental agency manufactured its own design (the [[Gotthilf Betulander]] design from 1919, inspired by the Western Electric system), and used it in Sweden from 1926 until the digitization in the 1980s in small and medium-sized A204 model switches. The system design used in [[AT&T Corporation]]'s [[1XB]] crossbar exchanges, which entered revenue service from 1938, developed by [[Bell Telephone Labs]], was inspired by the Swedish design but was based on the rediscovered link principle. In 1945, a similar design by Swedish Televerket was installed in Sweden, making it possible to increase the capacity of the A204 model switch. Delayed by the Second World War, several millions of urban 1XB lines were installed from the 1950s in the United States.

In 1950, the Swedish [[Ericsson]] company developed their own versions of the 1XB and A204 systems for the international market. In the early 1960s, the company's sales of crossbar switches exceeded those of their rotating 500-switching system, as measured in the number of lines. Crossbar switching quickly spread to the rest of the world, replacing most earlier designs like the [[Strowger switch|Strowger]] (step-by-step) and [[Panel switch|Panel]] systems in larger installations in the U.S. Graduating from entirely electromechanical control on introduction, they were gradually elaborated to have full electronic control and a variety of [[calling feature]]s including short-code and speed-dialing. In the UK the [[Plessey]] Company produced a range of [[TXK]] crossbar exchanges, but their widespread rollout by the British Post Office began later than in other countries, and then was inhibited by the parallel development of [[TXE]] [[reed relay]] and electronic exchange systems, so they never achieved a large number of customer connections although they did find some success as [[tandem switch]] exchanges.

Crossbar switches use switching matrices made from a two-dimensional array of [[Electrical connector|contacts]] arranged in an x–y format. These switching matrices are operated by a series of horizontal bars arranged over the contacts. Each such select bar can be rocked up or down by [[electromagnet]]s to provide access to two levels of the matrix. A second set of vertical hold bars is set at right angles to the first (hence the name, "crossbar") and also operated by electromagnets. The select bars carry spring-loaded [[wire]] fingers that enable the hold bars to operate the contacts beneath the bars. When the select and then the hold electromagnets operate in sequence to move the bars, they trap one of the spring fingers to close the contacts beneath the point where two bars cross. This then makes the connection through the switch as part of setting up a calling path through the exchange. Once connected, the select magnet is then released so it can use its other fingers for other connections, while the hold magnet remains energized for the duration of the call to maintain the connection. The crossbar switching interface was referred to as the [[TXK]] or TXC (telephone exchange crossbar) switch in the UK.

[[File:Crossbar-hy1.jpg|thumb|Western Electric 100-point six-wire Type B crossbar switch]]
However, the [[Bell System]] ''Type B'' crossbar switch of the 1960s was made in the largest quantity. The majority were 200-point switches, with twenty verticals and ten levels of three wires. Each select bar carries ten fingers so that any of the ten circuits assigned to the ten verticals can connect to either of two levels. Five select bars, each able to rotate up or down, mean a choice of ten links to the next stage of switching. Each crosspoint in this particular model connected six wires. The vertical off-normal contacts next to the hold magnets are lined up along the bottom of the switch. They perform logic and memory functions, and the hold bar keeps them in the active position as long as the connection is up. The horizontal off-normals on the sides of the switch are activated by the horizontal bars when the butterfly magnets rotate them. This only happens while the connection is being set up, since the butterflies are only energized then.

[[File:Crossbar-mini-hy2.jpg|thumb|Late-model Western Electric crossbar switch]]
[[File:C Xbr sw HON jeh.jpg|thumb|upright|Back of Type C]]
The majority of Bell System switches were made to connect three wires including the [[tip and ring]] of a [[balanced pair]] circuit and a sleeve lead for control. Many connected six wires, either for two distinct circuits or for a [[four wire circuit]] or other complex connection. The Bell System ''Type C'' miniature crossbar of the 1970s was similar, but the fingers projected forward from the back and the select bars held paddles to move them. The majority of type C had twelve levels; these were the less common ten level ones. The [[Northern Electric]] ''Minibar'' used in [[SP1 switch]] was similar but even smaller. The ITT Pentaconta Multiswitch of the same era had usually 22 verticals, 26 levels, and six to twelve wires. Ericsson crossbar switches sometimes had only five verticals.