How do switches respond when more than 10 dial pulses are sent?

Question

Forgive me if this is off topic but this is actually about circuitry design:

My understanding is that electromechanical Strowger SxS switches have 10 "steps". What would happen if 11 pulses were dialed, either from a phone that happened to have 11 holes in the dial and could send 11 pulses or by tapping and depressing the switchhook/hookswitch rapidly 11 times in succession?

Similarly, what about 12, 13, etc. pulses in succession?

I'm not sure if 11 pulses would dial 10+1 or simply 10 and then stop listening.

I've tried and failed to test this out, even though I have 2 PBX systems at home. I can dial the lower five digits easily using the switchhook but I fail more than half the time simply dialing 0 using the switchhook. And because the PBXs are electronic, they may automatically recognize 11 as invalid and default to a reorder signal.

If there is a difference between how electromechanical switches and electronic switches would respond, please contrast them. (I don't know how many people just have a SxS switch at home to test with though...)

Answer 1

The extra pulses will simply be absorbed, except for the final digit which will be lost.

The 1st, 2nd etc. group selectors only accept a single digit, then hunt into the bank to find a free outlet to the next group. Once the carriage steps up to the top level it can't go any higher, so it just hammers into the backstop.

Final selectors step up for the penultimate digit, then step into the bank for the final digit. An extra pulse in the final digit will cause the carriage to step all the way around the bank and out the other side, where it will 'drop out' back to level 0 and start stepping up again.

Another selector type was the Discriminating Selector Repeater (DSR) which was designed to drop out when reaching certain levels, in order to absorb digits that weren't need in the local exchange. This monster had 29 relays, some with 2 or 3 independent coils. Here's a schematic - see if you can figure out how it works!

Answer 2

Old SXS switching centers were electro-mechanical and in addition to the step switches used several different types of relays. The rotary dial was designed to pulse on the release of the dial, the number of pulses was determined by which hole you placed your finger in. Tapping out the digits is possible (I've done it) but tricky and doesn't work every time unless you have a very steady hand. If you had the proper pause between the 10th and 11th digit, the 11th digit would become a one for the next digit in sequence. Each step switch in sequence has a different function as you progress throught the digits, 1st selector, second selector, line finder and so on. I'm trying to remember as much as possible as it's been since the early 1970's since I worked a SXS office. A PBX will convert dial pulse to DTMF (not to be confused with MF). The PBX/keysystem will also store digits and send them all at one time.

Answer 3

If you did this to some SxS switches, the wiper would fall off the top or right side of the bank of contacts depending on if you do it for the final number or somewhere before. The selector was then out of service until someone at the switching office put it back.

A friend of mine asked himself the same question many years ago as a child in a small town. He pulsed the hook 11 times and heard nothing more until the phone engineer who lived nearby scolded him for doing it. It was very clear to the engineer who did it because he frequently answered my friend's many questions on how the phone system worked.