The crossbar "format" is also being used in some experimental high-density memory devices (see nanotechnology). Here the "bars" are extremely thin metal wires, and the "switches" are fusible links. The fuses are blown or opened using high voltage and read using low voltage. Depending on the design, the devices may be Programmable read-only memory or reprogrammable.
At the 2008 NSTI Nanotechnology Conference a paper was presented discussing a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation.
Crossbar switches use switching matrices made from a two-dimensional array of 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 electromagnets 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 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 fingers to close the contacts beneath the point where two bars cross. This then makes the connection through the switch to connect the telephone call. 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 switch (Telephone eXchange Crossbar) - in the UK.
The Bell System Type B crossbar switch of the 1960s was the most produced type. The majority were 200 point switches, with twenty verticals and ten levels of three wires, but this example is a 100 point six wire switch. Each select bar carries ten fingers so 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. Note the Vertical Off-Normal contacts next to the hold magnets, 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.
The majority of Bell System switches were made to connect three wires; many six. 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.
The crossbar switch itself was simple: exchange design moved all the logical decision-making to the common control elements, which as relay sets were themselves very reliable. The design criterion was to have two hours of "downtime" for service every forty years, which was a huge improvement on earlier electromechanical systems. The exchange design concept lent itself to incremental upgrades, as the control elements could be replaced separately from the call switching elements. The minimum size of a crossbar exchange was comparatively large, but in city areas with a large installed line capacity the whole exchange occupied less space than other exchange technologies of equivalent capacity. For this reason they were also typically the first switches to be replaced with digital systems, which were even smaller and more reliable.
Two principles of using crossbar switches were used. One early method was based on the selector principle, and used the switches as functional replacement for Strowger or stepping switches. Control was distributed to the switches themselves. Call establishment progressed through the exchange stage by stage, as successive digits were dialled. With the selector principle, each switch could only handle its portion of one call at a time. Each moving contact of the array was multipled to corresponding crosspoints on other switches to a selector in the next bank of switches. Thus an exchange with a hundred 10x10 switches in five stages could only have twenty conversations in progress. Distributed control meant there was no common point of failure, but also meant that the setup stage lasted for the ten seconds or so the caller took to dial the required number. In control occupancy terms this comparatively long interval degrades the traffic capacity of a switch.
Starting with the 1XB switch, the later and more common method was based on the link principle, and used the switches as crosspoints. Each moving contact was multipled to the other contacts on the same level by simpler "banjo" wires, to a link on one of the inputs of a switch in the next stage. The switch could handle its portion of as many calls as it had levels or verticals. Thus an exchange with forty 10x10 switches in four stages could have a hundred conversations in progress. The link principle was more efficient, but required a more complex control system to find idle links through the switching fabric.
This meant common control, as described above: all the digits were recorded, then passed to the common control equipment - the marker - to establish the call at all the separate switch stages simultaneously. A marker-controlled crossbar system had in the marker a highly vulnerable central control; this was invariably protected by having duplicate markers. The great advantage was that the control occupancy on the switches was of the order of one second or less, representing the operate and release lags of the X-then-Y armatures of the switches. The only downside of common control was the need to provide digit recorders enough to deal with the greatest forecast originating traffic level on the exchange.
The Plessey TXK1 or 5005 design used an intermediate form, in which a clear path was marked through the switching fabric by distributed logic, and then closed through all at once.
In some countries, no crossbar exchanges remain in revenue service. However, crossbar exchanges remain in use in countries like Russia, where some massive city telephone networks have not yet been fully upgraded to digital technology. Preserved installations may be seen in museums like The Museum of Communications in Seattle, Washington, and the Science Museum in London.
Changing nomenclature can confuse: in current American terminology a 'switch' now frequently refers to a system which is also called a 'telephone exchange' (the usual term in English) - that is, a large collection of selectors of some sort within a building. For most of the twentieth century a 'Strowger switch' or a 'crossbar switch' referred to an individual piece of mechanical equipment making up part of an exchange. Hence the pictures above show a 'crossbar switch' using the earlier meaning.
As computer technologies have improved, crossbar switches have found uses in systems such as the multistage interconnection networks that connect the various processing units in a Uniform Memory Access parallel processor to the array of memory elements.