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. Crossbar exchanges remain in revenue service only in a few telephone networks. Preserved installations are maintained in museums, such as the Museum of Communications in Seattle, Washington, and the Science Museum in London. Semiconductor Semiconductor implementations of crossbar switches typically consist of a set of input amplifiers or retimers connected to a series of interconnects within a semiconductor device. A similar set of interconnects are connected to output amplifiers or retimers. At each cross-point where the bars cross, a pass transistor is implemented which connects the bars. When the pass transistor is enabled, the input is connected to the output. 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. Arbitration A standard problem in using crossbar switches is that of setting the crosspoints. In the classic telephony application of crossbars, the crosspoints are closed, and open as the telephone calls come and go. In Asynchronous Transfer Mode or packet switching applications, the crosspoints must be made and broken at each decision interval. In high-speed switches, the settings of all of the crosspoints must be determined and then set millions or billions of times per second. One approach for making these decisions quickly is through the use of a wavefront arbiter. See also Matrix mixer Nonblocking minimal spanning switch - describes how to combine crossbar switches into larger switches. RF switch matrix References Further reading External links Images on an Ericsson ARF crossbar switch Switches Telephone exchange equipment Electronic circuits