[VoIP] Spotted On Ebay

[Mad Mark]

I won the items, and they should be arriving in a few days, if all goes well. Sounds like that S17 is the perfect novelty piece for a display. Now I understand why someone said it was rather unusual.

Anyway, I'm going to copy/paste an E-Mail I sent to Ian and John. Perhaps you can help me too.

*** Begin message ***


I’ve been reading up on EM
switching gear from the Automatic Telephony book by Arthur Bessey Smith and
Wilson Lee Campbell, and have a couple of questions about the line and cut-off
relays when using a line finder.



First, can you confirm my
understanding of this. When the customer goes off-hook, the Line-Relay (LR)
actuates, connecting the motor-magnet (MM) of the line-finder to the private
wiper (PW) and off into the rest of the exchange. If the PW is grounded by the
rest of the exchange, the current flows via the MM through the PW and to ground
via the rest of the exchange, advancing the line-finder (because the MM is
basically a chopper circuit).



When the wipers rest on an idle
trunk, the PW is no longer grounded, so the MM doesn’t have a ground connection
for power so can’t actuate. However, the cut-off relay (COR) has enough current
via the MM to actuate, cutting the customer loop through to the idle trunk, and
connecting the PW to ground via the LR, busying the trunk circuit. Because the
COR has now actuated, the LR is cut off and slowly releases, being a slow
release relay.



Here’s the part I don’t quite
understand. After the LR has released, it disconnects the ground from the COR
relay. What is now supplying the ground connection to keep the COR actuated? Is
it something at the other end of the trunk? Or am I missing something?



The following are assumptions
based on the circuit diagram I have. When the called party disconnects, the
ground is removed from the Trunk Release line and PW, thus removing ground from
the COR, returning the relay to the normal state and connecting the customer
line back onto the LR. The LR is getting power because the customers handset is
still off-hook, so it actuates again, and causes the COR to actuate,
reconnecting the line to the trunk, and cutting power to the LR. The LR
releases AGAIN, causing the COR to release, etc… One huge chopper circuit is
happening. Or, maybe the trunk circuit sees the momentary ground on the PW and
applies its ground to the PW as if the customer had just picked up their
handset again. I am most probably misunderstanding this greatly, as I don’t see
this happening as being a good thing. 



And I am assuming the connector
switch or some circuit further up the trunk handles all call disconnects…



Also I’m assuming the line-finder
rotates only in one direction and has two sets of wipers, one set coming into
use at position 0 when the other set (positioned at 180 degrees) leaves
position 24 (or whatever the maximum is) and rotates into dead air. Is this correct?



Can you give any clarification on
these points?

> Date: Mon, 4 Feb 2008 16:31:39 -0500
> From: martin at Princeton.EDU
> To: voip at ckts.info
> Subject: Re: [VoIP] Spotted On Ebay
> 
> Mad Mark wrote:
> > Someone on the CNET-UK-I mailing list spotted these items on E-Bay:
> > 
> > 
> > 130193124276
> > 
> > 130193124377
> > 
> > 130193124440
> 
> The first appears to be a standard BPO type 1 uniselector.  The third is 
> a BPO "miniature' uniselector (type 4 if memory serves.)
> 
> The second item is rather interesting.  I *think* it's a Siemens 
> digit/pulse counting switch, developed by them for their number 17 
> system (the one that used motor uniselectors.)  Unfortunately, from the 
> pictures it appears to be missing it's wipers, so it's no ore than a 
> display piece.
> 
> S17 was never used in public service in the UK as far as I am aware, but 
> maybe it was in Oz...  One of the big oil companies, I forget which, had 
> a big S17 PBX installation in their London office.
> 
> Martin
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