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Crossbar switches replace operators

ARM is a crossbar system for transit and tandem stations.

An automatic exchange can be compared to a robot that replaces an operator in a manual station. It takes messages from human beings, makes certain connections according to these orders and then sends messages back. The crossbar switch is such a robot.

The crossbar switch derives its name form the fact that it is based on the principles for a rectangular coordinate system. In an automatic telephone system, the objective is to find the right line - telephone number - based on impulses from a rotary dial. In a crossbar switch, each such position can be specified using coordinates.

The switch is based on 10 holding or vertical magnets and 10 selecting or horizontal magnets. The contact field thus consists of 100 contact points. Connection to the correct line is accomplished by the selecting magnets, which are activated by each pulse from the dial up to the selector corresponding to the first digit in the telephone number. Each selector has thin piano wires that control a row of springsets to which the subscriber line is connected.

When the next digit is dialed, the holding magnet's armature is activated by each of the dial pulses up to the armature corresponding to the digit. When armature is activated, contact is established at the crosspoint between the horizontal selecting finger and the vertical holding armature.

In a telephone station with thousands of subscribers, a few crossbar switches are not sufficient. Instead, a register must be used that stores digits and incrementally selects the group of 100 subscribers to which the number being dialed belongs. This is accomplished by a group switch that uses the telephone number digit-by-digit to select the correct 100-group. A final selector then identifies the desired telephone number.

Apart from switching, an automatic telephone station must have several other functions. When the person who wishes to make a call lifts the receiver, a line finder identifies the telephone number from which the call is being made. The number is marked as busy and a register stores the desired telephone number for the following connection procedure. Signaling tones must be sent (dial tone when the receiver is lifted and dialing can begin, a ring signal when the connection has been established, and a busy signal as appropriate). The dialed subscriber must also receive a ring signal, which is immediately terminated when the receiver is lifted, and impulses to the caller's call meter must be generated. When the call is terminated, everything returns to the original state.

Author: K V Tahvanainen


ARM is a crossbar system for transit and tandem stations.

© Telefonaktiebolaget LM Ericsson and Centre for Business History

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