Lever dial
The Strowger switch - with only one switching level rotary knobs - is an electromagnetically driven step switch and was formerly known as electromechanical coupling element in the telephone - exchanges used. The selector connects an input (feeder line) with one of several possible outputs (customer line). The connection is made by electrical sliding contacts.
The reverse principle with several inputs and only one output is used for the call seeker .
control
Rotary dials and lever dials are controlled by electrical pulse trains. In the case of telephones with a rotary dial , these were generated directly by their contacts closing according to the selected number. The dial pulses excite a magnetic coil in the rotary selector similar to a hinged armature relay, the armature of which advances one step with each pulse via a latch.
With the rotary switch, two coordinates are operated one after the other - a vertical (lifting - it selects one of the following switching levels) and an angular coordinate (step switch):
First, one of ten height steps is set by lifting a switching arm set. A row of ten or eleven contacts is permanently mounted in a block for each step in height. After the desired height step has been reached, a certain turning step is set with the next pulse sequence. The switch arms slide over the lamellae of the contact block. If three and four impulses are fed to the rotary selector one after the other, the input is electrically connected to output 34 (out of 100 possible).
The sequence controller performs a relay - logic circuit which is permanently assigned to each Strowger switch. After the end of a connection, the switching arms are rotated over the remaining contacts in a row in order to fall back to the lowest output row. Then the switching arms in this row are turned back to the starting position. The steps of lifting, turning, spinning, falling, turning back describe a square on the contact block, which is why this selector is also known as a square selector.
Coupling system
According to the point of the coupling system of an exchange in which the rotary dial was used, a distinction was made between group dialers and line dialers. Group dialers processed one digit, line dialers the last two digits of the destination number.
In the case of group dialers, the height step corresponded to a number, it was dialed with direct, forced dialing. The subsequent turning steps, on the other hand, were freely chosen, the turning process was ended when an unoccupied selector for the next coupling stage was found. In the case of the line voter, both the lifting and the rotating movement were carried out in “forced selection”. He was the last voter to set up the connection and had other functions such as applying ring voltage , sending dial tone , initiating billing and the like. a.
During the rotary movement, the contacts of the lever-operated rotary selector rubbed against the contacts of the contact bank multiple and were therefore heavily stressed. A bronze alloy was used as the material for the contact surfaces, since the switch arms of the selector could be replaced much more easily than the rigidly mounted multiple contacts. The contact surfaces had to be cleaned at regular intervals.
history
The lever dial was invented by the American Almon Strowger . Early Strowger voters had 100 contacts in a row. The workflow was slightly different from that described above. These dialers were called triangular dialers, because the switching arms covered the same path when the connection was disconnected as when it was set up.
In the second half of the 20th century, the rotary dials in the Federal Republic of Germany were predominantly replaced by so-called precious metal motor rotary dials (EMD). At the Deutsche Post of the GDR , switching systems with rotary dials were in use until the end. So was z. In Schwerin, for example , the system with rotary dials installed in 1935 was in operation essentially unchanged until it was replaced by a digital system in 1995.
The precious metal motor coordinate selector (EMK) was an intermediate step, but it was only used in one office in Berlin-Moabit. The following improvements have been introduced compared to the rotary dial: vertical contact set, therefore less contamination, motor drive and therefore adjustment-free, and contact establishment of the speech wires only after the setting process has been completed.
In the meantime, coordinate switch systems (System64 and System 65) were also used at the Deutsche Post in the GDR . The crosspoints were established by these crossbars. These were controlled by flat relays and reed relays. Reed relays have their contact in a vacuum flask. The contact is operated directly by magnetic force. The reed relay is not sensitive to dust by this design and wear of the contact. Due to their low mechanical movement, these coordinate switch systems required significantly less maintenance and were more reliable.
Since the introduction of the digital switching centers in the 1980 / 1990s, the electromechanical components are no longer used in public switching technology. The Deutsche Bahn ( BASA network) had this technology until 2001. In 2005 there was still an active EMD office there. It was the last office of its kind in operation in Germany.
Others
In the Museum of Communication in Nuremberg , a small local exchange with mechanical dialers can be viewed and - using the connected telephones - observed in action.
Two voter systems can also be viewed in action in the Heinz Nixdorf Museum Forum Paderborn . These differ in that one is a pre-selection system, the other is a caller system. Another small system is on display and in operation in the Jever Fire Brigade Museum.
A small local exchange with both pulse dial-controlled rotary dials and a modern variant of selector-free switching technology will also be demonstrated in the Deutsches Museum in Munich as part of a tour of the information and telecommunications technology exhibition .
In the part of the exhibition “Das Netz. People, cables, data streams ”in the German Museum of Technology in Berlin is also a small local exchange with mechanical dialers. This can be operated by the visitor from various telephones. This permanent exhibition has been on Ladestrasse since September 2015.
literature
- Handheld dictionary of electrical telecommunications ; Volume 2 (G-P); 2nd Edition; P. 777
