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The 7A Rotary Telephone Exchange


The goal of this section is to describe the process flow for making a call using a 7A Rotary exchange.



Throughout this site, many aspects of telephone systems technology are discussed.  Each facet is a part of a big jigsaw puzzle. When the pieces are locked together, an automatic exchange starts to breathe – see image. The “outside plant” cables, poles, and conduits are not covered here but this is a big cost in servicing each subscriber.







Each of these pieces is discussed in the Exchange Anatomy (EA) and associated sections. So, their detailed operation won’t be repeated here. If you are new to exchange workings, and want a basic understanding, it may be best to review the EA sections before continuing here. Interesting historical aspects, notable people and intriguing inventions are also covered under EA. 

Of relevant importance are the 7A rotary switches and the 7A sequence switches with diagrams and explainer videos.  

the telephone exchange puzzle diagram

7A Rotary workings – making a call

This section outlines how the 7A rotary system establishes a call when one subscriber dials another in the same office.

To get a feeling for the view inside an exchange building, see Fig 1. This is a small section of a 7A Rotary system on display at [Ferrymead]. It is hi-rez, so may be slow to load. The individual racks are ~11.5’ tall and hold Line Finders, Selectors, Registers, sequence switches, and relays. These are tied together with miles of wiring and rely on other puzzle pieces to build a working exchange.      

Fig 1, 7A Rotary office, partial view at Ferrymead

Row of selector bays in 1930 7A rotary exchange

Fig 2, Row of selector bays [Miller] circa 1930

Hague, NL,  West-General View of Automatic 7A Rotary Switch Room

Fig 2A. Hague, NL,  West-General View of Automatic 7A Switch Room 1925 [Turkhud]

wiring complex, Hague, NL,  West-General View of Automatic 7A Rotary Switch Room

Fig 2B. Hague, NL,  West-General View of Automatic 7A Switch Room. Top rack wiring complex. 1925 [Turkhud]

A typical exchange houses thousands of switches, and a single photo cannot capture the intricate layout of frames and support systems.  It takes an immense infrastructure to support even 4K subscribers. See Appendix A below. 

The rotary system is based on the high-level diagram shown in Fig 3. This template can be tweaked and applied to the panel and crossbar systems; they share many similarities. The Strowger-type, step-by-step, system is the exception. The example to be presented does not show interoffice trunking, but this is vital in any practical system. 

7a rotary system overview diagram

Fig 3, 7A Rotary system overview

The following provides a simplified call flow based on Figure 3:

•    An “off-hook” (#288) subscriber triggers the line circuit causing the concentration stage switches (Line Finders) to search and find this sub. 
•    The caller is forwarded to an idle Register via the Reg switch. This switch's operation is more complex and simplified here. The talk path output of the concentration stage is connected to the expansion stage input. 
•    The Register is the brains during the calling phase and provides the functions listed in the figure.
        --Based on the dialed digits (calling #315), the Register, via the Reg switch, closes the talking path switches in the expansion stage. 
        -- Next, the Reg switch flips causing the Register to disconnect from the call.  With a negative busy test, the ringing cycle starts and talking commences if answered. 
•    Upon call end, the talking path switch chain resets and is now available to service new callers.  

The video below expands on Figure 3, adding detail and color.  It’s an overview of a 7A exchange and includes the process steps for connecting two subscribers. The system workings rely on a "reverse dial" on each subscriber's phone. You may want to review Appendix B before watching the video. 

The focus is on basic concepts, it’s not a click-by-click description of operations. 

The 7A design scales to 2,000,000 subscribers! No other exchange type was designed to this scale. [Deakin] reports on a system supporting 560,000 subscribers. 

The 7A rotary and its successors left a legacy of durability and reliable service. It holds a prominent position among large system designs during the golden age of electromechanical exchanges.

References and Acknowledgements 

Ferrymead Post and Telegraph Historical Society, Switch Room, Christchurch, New Zealand. This site houses a well-preserved and operational 7A rotary system. The exhibit is supported by volunteers. I am indebted to Brian Cameron for providing raw exchange videos and many vital insights into the 7A's workings. Thanks Brian!   

Deakin, Gerald, ELECTRICAL COMMUNICATION, a Journal of Progress in the Telephone, Telegraph and Radio Art,  No. 7-A Machine Switching System – Rotary System, Jan 1925.

Turkhud, B.A.,  ELECTRICAL COMMUNICATION, a Journal of Progress in the Telephone, Telegraph and Radio Art, 7001 TYPE OF AUTOMATIC PRIVATE BRANCH EXCHANGE, April 1926.

Miller, Kempster, Telephone Theory and Practice, 1933, New York, McGraw Hill. 

     Appendix A
How many switches?

This Appendix assumes a basic knowledge of the various types of switches used in a 7A system. 

Below is an inventory of switch types to support 4,000 7A rotary lines (5 digits required). The arrangement supports a max of 200 interoffice conversations (5%) at the same time plus other subs connected to remote exchanges.  

  • 40 bays of 1st Line Finders (LF), 15 x 40 = 600 switches

  • 40 bays of 2nd Line Finders. This is 540 switches, for reasons not stated here.

  • 50 bays of 1st Group Selectors = 540 switches. Each is paired with a 2nd LF. 

  • 20 bays of 2nd Group Selectors = 200 switches

  • 20 bays of Final Selectors = 200 switches

  • 32 Registers with associated sequence switches and relays. 

The total is 2,080 rotary switches. This is an apples plus oranges total because the switch types do differ in design. Nonetheless, each needs a home in a rack and requires a control means. Add to this total about 1,700 sequence switches (including 540 Register Choosers) and the extent of the equipment required comes into focus. 

Appendix B

The Reverse Dial 

The 7A rotary system required a "reverse dial" to operate. For the earliest rotary exchanges the numerals on the dials were arranged 0, 1, 2. ..8, 9 in a clockwise direction
(reversed numbering). Over time, the rotary exchange logic was modified to accept standard-dial numbering 0, 9, 8. ..2, 1.

telephone with reverse dial used in 7A rotary systems

Telephone with reverse dial

Why the reverse dial? The electromechanical method of recording digits inside the exchange was simplified IF the subscriber dial had reversed numbering. The subscriber was likely not aware of this oddity. For more on the reasons see Appendix B on the Sequence switch pages.

Even at this early stage of dial automation selecting a reverse dial was problematic since the de-facto world standard was a normal dial. In time, this made calling interop a major issue and begged for the rotary system to change the design to accept a normal dial.  

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