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The Sequence Switch


"The electromagnetic switch is the cornerstone of any automatic exchange. There are many variations including relays, Strowger type, crossbar and other “talking path” switches discussed on this site. This section covers the sequence switch and some derivative switches. The sequence switch is a type of auxiliary switch used to assist during the calling process. Its usage in panel and the 7A rotary exchanges is considered.

This switch type was invented by Frank McBerty of Western Electric. He was granted US patent 1,009,080 in 1911. The opening statement of the patent reads:

The present invention provides a simple means of bringing about any desired sequence of electrical operations while permitting any desired degree of external control.

The device is a sequencer of operations. Next, let’s see why this type of operation was of value in a telephone exchange.

State Machines

Making a call involves several discrete steps. A state machine, which advances step by step, is beneficial for managing call operations. 

For example, consider a simple state machine, 5 states total, with output wires A, B, C, D available to signal or control external devices. Leads A-D may exhibit different conditions for each state.

State 1- Home state; A, B grounded  

State 2- Caller off-hook; start the line finder, A, B open, C&D closed

State 3- Provide dial tone; A&B closed, C, D open

State 4- Record dialed digits; C&D closed, A, B grounded
State 5- Ring called sub; C, D grounded

One way to build such a machine is using relays. Indeed, small state machines are often relay based. The panel and the rotary systems need very complex state machines during call setup. For this purpose, Frank McBerty’s rotary design group, circa 1908, developed the sequence switch to serve as a state machine. It was used extensively in rotary and, to a lesser extent, in panel exchanges.  

The next three figures show images of sequence switches used in the panel exchange. The Fig 1 diagram shows it is attached to a motor driven shaft and shares the drive train with selectors and other devices. Notice the 3 cams (24 possible) and separate brushes for each cam. "The “R drive magnet” briefly engages the shaft as required. Notice “cam A” and see the 18 rotational state positions marked. 

A panel exchange sequence switch with 3 cams and two brushes per cam

Fig 1, A panel exchange sequence switch with 3 cams and two brushes per cam

Each cam had metal traces riveted onto an insulator base. The metal pattern determined if a brush would make contact or not at a given position. The cams advanced together, during a call, moving from sequence position 1 to 18 and back to 1 at the end of the call. With four brushes per cam and 24 cams there are 96 available leads to signal or control external devices for each state!  Often a few positions were not used (spares). 

Fig 2 pictures 24 cams with 18 possible states of rotation. Each state position locks solidly per step by the “A cam”. This example is from a panel exchange in the 1920’s and is from [Goldsmith]. Incidentally, the sequence switch state machine design has the constraint that all the states must proceed in order 1 to 18, although some states may be “don’t care.”


A sequence switch is very energy efficient, in contrast to a relay, it requires no electric power to hold contacts open or closed.  Power is required only to step the spindle from one position to the next. See Appendix A for more on how a positive step is always achieved. Next, an example of how a sequence switch is used in a panel exchange.

Parts breakout for panel sequence switch

                                Fig 2, Parts breakout for panel sequence switch 

For a panel selector switch, there are 30 selector rods on both the front and rear sides. Figure 3 shows five sequence switches, each associated with a panel selector rod [CMoS]. Although not fully visible, there are 30 sequence switches total for the front side and 30 on the rear side.  

Rods of a panel selector switch (left side) with five associated sequence switches 

   Fig 3, Rods of a panel selector switch (left side) with five associated sequence switches 

Next, the discussion will focus on the sequence switch as used in the 7A rotary system.

Sequence switch applications in a 7A Rotary exchange

Putting things into perspective, in the panel exchange, relays were mostly used to record dial pulses and to orchestrate processes during call setup. Sure, panel systems used the sequence switch but to a lesser extent than the 7A did. 

McBerty’s 7A design group applied sequence switches (SS) in place of relays where they could. It was likely a McBerty design imperative.  As noted elsewhere, the panel design group was in a sort of design competition with the rotary designers to see who could develop a new exchange type for metro areas. 

Fig 4 is a high-level view of some different use cases for sequence switches in a 7A exchange. 

Sequence switch usage in a 7A rotary exchange 

                           Fig 4, Sequence switch usage in a 7A rotary exchange 

The rotary and panel sequence switch designs differed in structure but often shared the same purpose. The 7A rotary design used cams with individual switches riding on each cam’s notches. 

Fig 5 is associated with a 1st Group Selector switch. It has 18 index steps, as with the panel version.
 Figs 5-8 are from [Ferrymead]. 

Sequence switch associated with a 1st Group Selector switch rotary 7A

              Fig 5, Sequence switch associated with a 1st Group Selector switch

At the very top is the index wheel indicating position 1, home state. There are 18 cams each with an associated switch. Also notice the clutch electromagnet at the very bottom. 

A sequence switch is associated with each Group Selector and each Final Selector and this is the lion’s share of usage in a typical exchange.  

-The Register  

The Regis
ter (Fig 4) is also a major consumer of sequence switches. The Register is the master controller for call establishment.  It provides dial tone, records the dialed digits, and directs the talking path switches to build connections to the called subscriber. The Register is released from the call once established. It is then available to service a new caller.  

When a subscriber goes off-hook, the LF goes into action. Immediately after the caller is found, they are directed via the 1st Group Selector SS, to the first idle Register by way of the R3 Register Chooser.  It’s called a Chooser since it may have six of more Registers to choose from. This switch, in Fig 6, is nearly identical to the sequence switch shown in Fig 5. A difference is there are only 10 steps for this case not 18. This can be seen by observing the top index label. 

7A rotary-  R3 Register Chooser sequence switches

Fig 6, four R3 Register Chooser sequence switches

Another use case is the Register digit recorder.  For this, the sequence switch is modified to be a digit counter. When a subscriber dials, say a 5, the digit recorder rotates five steps and stops. Fig 7 shows two rows of 5-digit registers (count to 99999). In some panel and all crossbar systems, relay logic is used to count the dial pulses. This is a designer’s choice. 

7A rotary excahnge- Two separate 5-digit Register digit recorders as part of R3 Registers

Fig 7, Two separate 5-digit Register digit recorders as part of R3 Registers

It’s obvious that this design is a smaller version of a normal sequence switch. It has only 6 cams and associated “following switches.” Interestingly, there are 20 steps for 360 degrees of rotation. Why? A cycle of operation is performed in 10 steps, half a full revolution. So, the mechanism moves less to record a digit. This is good for reliability among other advantages.


Finally for consideration, are the R4 and R5 sequence switches (Fig 4). Fig 8 pictures R4/R5 as part of a Register. The dust covers hide control relays that work together with R4/R5.

 7A Register controller showing the R4 and R5 sequence switches 

Fig 8, a 7A Register controller showing the R4 and R5 sequence switches 

Switch R4, along with relay logic, manages the recording of dialed digits such that each digit is recorded on the corresponding digit recorder. It has 18 steps and acts like a state machine.

Switch R5, along with relay logic, uses the just recorded digits to advance the rotary switch chain composed of 1st, 2nd Group Selectors, (or more) and the Final Selector.  It has 18 steps and acts like a state machine. Both dialed digit recording and talking path Selector control run in parallel but are asynchronous operations.  


Switch demos

Next, two videos showing rotary sequence switches in action. First, a caller dialing the final two digits (tens, units) into a 7A rotary system. The dial sends 4 pulses (tens) and then 5 pulses (units). Notice the switches advancing first to record a 4 and then to record a 5 then both resetting to “0” before the caller answers.

The digit recorder story doesn’t end here. See, “The Mystery of the Reversed Dial” in Appendix B.   

Second, let’s see the Final Selector (FS) and its Sequence Switch (SS) in action, completing a call.  The video shows a “round trip” demo of both switches. Watch the FS and SS going into action once the last digit is dialed and concluding with the caller disconnecting and the switches resetting.  The demo assumes the called telephone is not busy. If it was, a slightly modified variation would occur.  

References and Acknowledgements

Ferrymead Post and Telegraph Historical Society, Switch Room, Christchurch, New Zealand. This site hosts a beautifully preserved, working, 7A rotary system. The exhibit is supported by volunteers. Thanks to Brian Cameron for many pictures, raw videos and vital insights.

CMoS: Connections Museum of Seattle. YouTube, panel explainer video by Sarah Autumn, Jun 30, 2020.  See the panel exchange section for more information.

Goldsmith, Elsworth, The Panel Type Dial Telephone System, 1926, New York Telephone Company. 


                                       Appendix A

                                  Self-Timed Advancing

McBerty sequence switch patent figure

In 1911 Frank McBerty of Western Electric was granted patent US1,009,080 for an “Automatic Switching Apparatus.”  This was the seed patent for what became several versions of “sequence switches”, applied in rotary and panel offices.

One aspect worth exploring is the self-timed advance concept. In most cases, the switch advances, under the direction of an external controller, from one position to the next as one positive step. Remember, the mechanism is motor driven so a clutch engages for just the right amount of time to achieve a single step action. How does this happen without some timing means?

Self-timed stepping

In the patent figure above note cam 16. If there are 18 possible rotary positions, this cam has 18 notches to match each step position. When an external signal pulse (EP) energizes magnet 12, the shaft starts to rotate and switch 20 closes as it rides cam 16. Signal EP goes off quickly by design but switch 20 is wired to keep magnet 12 excited. When cam 16 enters the bottom of the next notch, switch 20 opens and magnet 12 disengages. This process produces one positive step for each EP short pulse.

It would be difficult to reliably assure that one EP pulse by itself could always produce one positive rotary step. There are many variations related to motor speed, friction, switch differences and so on. This is yet another reason why the McBerty design had such staying power.

The panel system sequence switch was somewhat different in design but still needs a positive single step means. See Fig 1 (in the main body above) and the “A cam” with brushes, together they act like switch 20 in purpose.

Appendix B

The Mystery of the Reversed Dial

The 7A rotary system requires a peculiar “reversed rotary dial” on every telephone. It is reversed from the dial most of us are familiar with. Here is a picture of the dial [Ferrymead].

reverse dial appendix B
A reversed dial - 7A rotary system

A reversed dial (7A rotary system)

The objective of dialing is to progress the rotary selector switches, establishing the talking path. A reverse dial is a required element to reach this goal in the 7A rotary system. 

Let’s go into the weeds a bit here… On a reverse dial, dialing a 9 generates 1 pulse on the line and dialing a 1 generates 9 pulses. So, the number of dial pulses generated = (10 - digit dialed). Hence the name, reverse dial.

Note: This scheme was not compatible with all other exchange dialing methods that used "normal dialing." So, a user needed operator assistance to dial outside of the 7A system's boundary. Over time, the reverse dial was dropped in favor of the normal type for interop purposes. 

Also, the 7A rotary system uses common control.  The dialed digits do not directly control the selector switches (talking path). The Register circuit records the dialed digits and then asynchronously controls the selector switches to forge the talking path.  So, how does a reverse dial help? 

Assume the subscriber dials a 6 (4 pulses generated). The digit recorder records a 4. Soon after, the Register circuit advances the target selector switch and the digit recorder one step at a time. When the digit recorder reaches its end point, always index position 10, the counting stops. During the count up to 10, the selector switch has also advanced 6 steps and this is the end goal. Position 10, also called "0", is the home state.  

It may seem natural for the digit recorder, after it counts a digit, to count down (6 up, 6 down) instead of up to advance a selector. Then, no reverse dial would be needed. However, the digit recorder only rotates in one direction (simplifies design, improves reliability) so counting up to 10 is a better than counting down to zero (regular dial use case). This "up counting" method requires a reverse dial. 

Given this introduction, it might be beneficial to revisit the initial video in the 'Switch demos' section above. 

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