Stepping switch

In electrical controls, a stepping switch is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. They could step on one axis (called a uniselector), or on two axis (called a Strowger switch). Stepping switches were invented by Almon Strowger in 1888. The major use for these devices was in early automatic telephone exchanges (commonly called "Strowger" or step-by-step exchanges or "steppers") to route telephone calls.

Basic operation of a stepping switch

A basic stepping switch has a single input terminal (the stepping terminal) and multiple output terminals. Connection from the input terminal to the outputs is controlled by an internal rotary contact, or wiper, which can rotate, rather like the hand on a clock, so as to connect the input terminal (electrically connected to its central axis) to whichever output terminal it is currently touching. The position of the wiper is controlled by an electromagnet. Each time an electric pulse is received at the stepping terminal, the electromagnetic ratchet is activated, causing the rotary contact to advance one position, and connect the input terminal to the next output.

In most applications, such as telephony, it is desirable to be able to return the rotary contact to a "home" position. Some stepping switches would rotate continuously back to the "home" position as soon as they reached the last position, while others had a separate "reset" coil and a return spring, and still others would rotate forward rather than back, to the home position.

More complex switch configurations

One development was a type of stepping switch which had several rows of contacts, with one rotating wiper per row. All the wipers were mechanically coupled so that, as one rotated, the others remained aligned with it. Hence, one input with multiple wires could be connected to one of multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts looked, in general, somewhat like the head support arms in a modern rigid ("hard") disk drive.

Separate sets of cam-operated contacts on some switches moved when the rotor was at its home position, so that all of the bank of selectable contacts could be used for other purposes.

Although not as common, there were bidirectional stepping switches with two coils that could rotate the moving contacts in either direction, one coil for each direction.

Two axis stepping switch

Slightly more complicated was the "two axis stepping switch", also called "Strowger switch" or "Step by Step" in Britain), which had two stepping coils, one to raise it to successive banks of contacts, and one to rotate the wiper. These were commonly used in telephone switching with ten banks of ten contacts. These were driven by the electrical pulses (opens) generated from a telephone dial: On a selector switch, as each digit was dialled, the wiper would step up the banks, then rotate automatically until the wiper found an "unused" set of contacts to the next switch. The last two digits dialed would operate the connector switch. The second to last digit would cause the shaft to move up and the last digit would cause the wipers to rotate. If the line was idle then ringing generator would be applied to the called line and ring back tone was sent to the calling line.Another variant of the two-axis switch was the Stromberg-Carlson X-Y Switch which was quite common in telephone exchanges in the western USA. It was a flat mechanism, and the moving contacts moved both sidewise, as well as to and fro.

Applications of stepper switches

Listen
filename=Sxs intra to busy.ogg
title=Stepping switch in telephony
description=The sound of a step-by-step call reaching a busy circuit.
format=OggAs well as the decoding of pulses from telephones, stepping switches could be used for a variety of purposes, depending on how they were wired. By connecting several in series with the highest output of one going to the stepping contact of the next, a counter could be constructed. Or by feeding the stepping contact with an endless pulse train via a relay, and controlling the relay from the switch's own output, it can be made to automatically hunt for the first unpowered line (or powered, depending on whether the relay is normally open or normally closed). They could also be used as a demultiplexer, so that two input lines could control a number of output devices. One input line steps the switch until the correct device is selected, and the other than powers that device. Many other applications are possible.

Such switches were used in a series of Japanese cypher machines during World War 2: Coral, Jade, PURPLE (the names were American). Some of the equipment used to break the Enigma machine code also used many such switches, which some observers called the "Machine Gun" for the loud noise.

See also: Panel switch, Strowger switch

External links

* [http://www.technology.niagarac.on.ca/people/mcsele/TelephoneSwitch.html Explanation of the way a stepping switch works]
* [http://web.ukonline.co.uk/freshwater/shared/auto1.htm British SXS telephone switches]