Click here to walk around a PABX 3

This page attempts to explain in, simple terms, how a call is routed through a Strowger (step by step) automatic exchange.  As the Telephone File normally deals with Customer equipment, the description shall be centred around the PABX, in particular the PABX 3 as this is the simplest exchange to describe and the nearest to a proper public exchange (this example could also be used for a small Non-Director Exchange - Director exchanges were only found in big cities like London).  In this explanation the caller dials extension 2369 and gets answered.

Getting Dial Tone
The caller picks up the telephone handset and this puts a loop condition on the line, which energises a couple of relays on the extension line circuit.

These relays are part of a Linefinder and this allocates the Subscribers Uniselector, which operates and finds a free 1st Group Selector.  A Uniselector is a mechanical device which rotates a set of springs around a bank of 25 or 50 contacts.

Once a free 1st Selector is seized then dial tone is returned to the caller.

First digit is dialed
On receipt of Dial Tone the caller dials the first digit (2 in this case).  The 1st Selector steps up 2 levels vertically and then rotates freely horizontally until a free 2nd Group Selector is found.  Selectors have a set of springs connected to a shaft which can be stepped vertically and then horizontally.

Second digit is dialed
By now the caller is dialing the second digit (3) and the 2nd selector steps vertically 3 levels and then rotates freely horizontally to find a free Final Selector.

Last two digits are dialed
The caller dials the third digit (6) and the Final Selector steps vertically 4 levels and then steps horizontally according to the forth digit dialed (9).  Once the Final Selector has stopped on the required outlet, the outlet is tested for three conditions, these being: busy, free or unobtainable.  

Ringing the called extension
If free, the Final Selector connects ringing current to the called extensions line and gives the caller ringing tone.  Should the extension line be busy then busy tone is returned to the caller and if unobtainable or in line lockout condition (the extension has left their handset off the hook and the linefinder has parked the extension line) then NU tone.  The tones are fed from a ringing machine known as a Dynomotor, or in the case of very small exchanges, a vibrating relay.

The called extension answers
When the called extension answers the Final Selector removes ringing current and ring tone and connects the speak path via a transmission bridge.  The two extensions are now free to talk.  The Uniselector and all the selectors are held whilst the call in in progress.  So for one call, four pieces of equipment are used.

Clear down
When the calling party hangs up all the selectors restore to normal.  They are automatically driven clockwise to the end of the bank of contacts, the shaft then drops down and finally rotates anticlockwise, under the bank, to the parked position.  The selectors in the picture are shown in the parked position.


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Last revised: October 30, 2022