2
\$\begingroup\$

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...)

\$\endgroup\$
  • 1
    \$\begingroup\$ To the best of my knowledge, the extra pulses would be ignored. The ratchets simply run out of teeth. \$\endgroup\$ – Dave Tweed Apr 14 '17 at 0:53
2
\$\begingroup\$

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.

enter image description here

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!

| improve this answer | |
\$\endgroup\$
1
\$\begingroup\$

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.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ I've done it, too. The phone in my university's amateur radio club station had a lock on it to prevent casual use (this was 40 years ago!), but we'd occasionally use the hook switch to dial out. It helped that we were all ham radio operators that had experience sending Morse Code with a straight key! \$\endgroup\$ – Dave Tweed Apr 14 '17 at 0:59
  • 1
    \$\begingroup\$ Experience with Morse Code would give you an advantage back then. Today interdigit timing is used for different dial paterns and would make it nearly impossible to replicate it. If I knew what type of PBX, (or more likely a KSU) you had, it may be one I'm familiar with and could give you more information. \$\endgroup\$ – MEZLAW Apr 14 '17 at 2:14
1
\$\begingroup\$

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.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ The switches I worked on were supposed to simply drop out the right side and return to home, but sometimes the wipers would catch on the edge of the bank, or the spring return mechanism didn't rotate the carriage fully home when it got to the bottom. If this happened the stepper magnet coil would remain operated with 50W of power going through it. As this was a potential fire risk we had to immediately respond to the alarm signal no matter what time of day or night. Being woken up at 2am and having to drive 40 miles to a rural exchange to reset it was not fun! \$\endgroup\$ – Bruce Abbott Apr 5 at 5:46
  • \$\begingroup\$ I emailed my friend asking for a retelling of this story. Stand by. \$\endgroup\$ – Frotz Apr 6 at 7:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.