Old analog telephone system question. (Comkey 416) has a strange capability

Technology was quite a bit more advanced in the 1970s than most people real ize.

I'm guessing it detects the DC level on the line. Not much other way it cou ld work eh ?

No matter what kind of phone it is, it works by modulating DC, so therefore it must present a constant load, kinda like a class A amplifier.

When I was playing around with phones I found like 45 volts DC when hung up and usually 6 or 7 when talking. Even if the phone co regulates that volta ge it can't be a high current source. When the other party hangs up, even i f the voltage settles to the same level there is going to be quite a positi ve going transient on the line. It probably detects that.

Honestly I am no expert, just fooled around with it a bit in younger days. But really I can see no other way it could do that than to detect that tran sient.

There is a disconnect signal that is (at least in the US) a 750 ms reversal of the talk battery. If you've ever held on when the other party got disconnected and there was a loud thump-thump and then you got dial tone back, that was the disconnect signal. Apparently, your KSU detects that signal.

Jon

In movies etc. I have seen where when the other party hangs up you automatically get a new dial tone. Not so here, it just sits there.

But if it does do that, then it is a simple matter to detect the dial tone.

Thats interesting Jon. I'm going to look for that. Wouldn't have thought th at such an old system would have recognized anything like that. And Jurb I too have never seen where you get dial tone back when someone hangs up. I t hink its purely theatrical. Lenny

I think #5 ESS goes back to dial tone when the line is disconnected from the calling party. Sometimes I hang on for a while to see if a disconnected call call from a family member can reestablish the connection.

(And, I THINK we are still on #5 ESS, but the phone co. could have very well swapped out the switch at some time.)

I know that my old answering machine will drop the line when it detects this signal. I think my NEC PBX will drop hold on the line, too.

Jon

On Monday, March 21, 2016 at 4:59:55 PM UTC-5, snipped-for-privacy@gmail.com wrote :

hone system in my house. With two primary sets and the remainder being slav es the system can handle 4 lines and 16 extensions. The sets all connect on to 66 blocks with 25 pair cable. Yes it's antiquated but it does music on h old and intercom and serves our needs.

rty and I put them on hold the system will stay on hold indefinitely. Howev er if I put them on hold and they eventually hang up my system will eventua lly go back on hook by itself. How can it do this? As far as I know this is just a "dumb system. How could it know that the other party hung up and th en to release the line. I seriously doubt that they had the technology to d o this in 1975. Maybe someone (like an Ex telephone guy) might know if West ern Electric could have built anything that sophisticated into this system in 1975 to detect anything like a kiss off, (I'm guessing) from the CO. Tha nks, Lenny

I worked for a Bell System company in the '70's and '80's. There was (and s till is) a signal called "answer supervision" that is pretty much a battery reversal as Jon Elson described earlier. All but the cheapest PBX and key systems of the time would recognize that signal when a party answered and h ung up their phone.

An abandoned call...

When the far end abandons the call, that CO tells your local CO. It in turns drops [not reverses] the DC loop current for [ISTM]

This caused grief later when Caller-ID arrived. On a 1AESS, it would announce such with a short DC glitch as it disabled incoming audio, poked in a BEEEEEP, and reconnected the audio. While the drop was far shorter, some cards would drop off hold. Newer 400G & 400H cards would not.

On Monday, March 21, 2016 at 5:59:55 PM UTC-4, snipped-for-privacy@gmail.com wrote :

I remember I was watching an old "Columbo" detective movie called "Death Le nds A Hand" (1971) where the guest star "Robert Culp" kept bragging about h ow much better private investigative organizations were at (what was I gues s) highest-end listening device technology than the police dept was.