crossbar switch
A matrix of switches with multiple input and output lines that form a crossed pattern between which a connection may be established by closing a switch located at each intersection.
crossbar switch: the telephone exchange's routing matrix
A crossbar switch is an electromechanical or electronic device that connects any one of multiple input lines to any one of multiple output lines by activating a switch at the intersection point of a selected row and column. The earliest crossbar switches, deployed by telephone exchanges from the 1920s onward, used electromagnetic relays arranged in a grid pattern: an incoming call on line A could be routed to any outgoing trunk line by energizing the relay positioned at their intersection. This eliminated the need for manual cord-board operators to physically patch connections.
The classic electromechanical crossbar used stepping switches or rotary selectors to scan the grid. A subscriber dialing a number sent pulses that drove a shaft through successive switch positions until it landed on the desired output terminal. The relay at that intersection would close, completing the circuit. Larger exchanges stacked multiple crossbar units vertically to handle hundreds of simultaneous connections. A typical telephone exchange crossbar might have 1000 input lines and 1000 output lines, though many were smaller.
Variants and failure modes
Electronic crossbar switches replaced electromechanical ones from the 1960s onward, using semiconductors or integrated circuits instead of relays. A solid-state crossbar built from transistor arrays or diodes could switch faster and required no mechanical moving parts. However, electromechanical versions remained common in telephone networks into the 1980s because they were robust and required minimal electrical power to hold a connection once made. The main failure mode was mechanical wear: the stepping mechanism would jam, or relay contacts would corrode, breaking the connection path or creating noise on the line.
The crossbar switch's design has survived well beyond telephone switching. Modern digital multiplexers and packet switches in data networks use the same conceptual grid: inputs at one edge, outputs at another, with the ability to route any input to any output through a central matrix. The difference is that selection is now done by software-controlled logic gates rather than mechanical stepping.
The term 'crossbar' itself refers to the physical bars that formed the grid in early designs, with one set of bars running one direction and another set crossing them perpendicularly. Early inventors, including Almon Strowger in the 1890s, patented designs that relied on this crossed-conductor principle. Though the underlying technology is obsolete, the crossbar switch remains the conceptual ancestor of every routing matrix in modern communications and computing.