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How does digit absorbing work?

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For some reason, digit absorbing in SxS seems intriguing. I've been trying to find out how it works, but I can't find any resources that really detail how digit absorbing selectors work.

For example, from: In order to accommodate inbound DDD, it was essential that every number have a 7-digit format. But SxS switches couldn't accommodate 7-digit dialing, so telcos faked 7-digit numbers by prepending dummy digits. Local calls continued to be dialable with only four or five digits; however, if a local caller actually dialed all seven digits, the prepended digits were absorbed by "absorbing selectors" -- i.e. ignored.

Here is an example of digit absorbing in action: (right at 12 minutes). Phil explains at 12 minutes that the digit 7 will continue to be absorbed.

And a great technical explanation is available here:

From this, I've grasped that there are special selectors that will absorb a specific digit. In Phil's case, his exchange was 377 and to absorb the 7s, the fourth(?) selector is "programmed" to absorb the digit 7 (once? twice? forever?). The rest still seems pretty ambiguous though. For instance, what is the mechanism that unlocks the selector? Can anyone offer an explanation, or point me to some resources (i.e. BSPs) that offer one? Thanks!

Alex G. Bell:
Look in the TCI library for Automatic Electric Training Bulletin 808, 945-808 or 948-808 titled "Selectors".  It explains digit absorbing selectors from A to Z.

Victor Laszlo:

Well, I've spent the last hour and a half downloading Automatic Electric PDFs. I have 320 more MB now that I didn't wake up with. I found 945-800 and 945-801. Some of the other 945-series bulletins just had the 8xx number, and I renamed these so they all had standardized file names. I'm currently missing 803, 805, 808, 809, 811, 812, 814, 816, 818, 819, and 821.

Yes, I've spent a significant amount of time scouring the Internet for the others.

Are these available somewhere, or lost to time? 808, "Selectors", is one of the bulletins recommended to me, and also one of the ones I currently do not have. And typing in "automatic electric bulletin 945-808 selectors" does not bring it up.


--- Quote from: bellsystem on July 23, 2017, 07:09:21 PM --- In Phil's case, his exchange was 377 and to absorb the 7s, the fourth(?) selector is "programmed" to absorb the digit 7 (once? twice? forever?).

--- End quote ---

There were lots of options.
Absorb once and then cut in on the next digit even if it were the previously absorbed digit.
Absorb repeatedly.
Other scenarios such as absorb twice and then cut in may have been possible.
At least some of the absorbing was triggered by what we called a "comb" on the top of the selector shaft that had 10 "teeth" on each side of it representing the ten levels that could be dialled. A contact set on each side of the comb would be operated if a tooth were missing triggering either absorb once or absorb repeatedly depending which side of the comb triggered it (I's been a long time). Early combs were metal and a tooth could be bent out of the way thus it could be returned to normal in the future but later combs were plastic (white plastic in my experience) where a tooth was broken out so it wasn't reversible in the future but would just be replaced.



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