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Old 22nd Jul 2021, 6:55 pm   #6
Pellseinydd
Heptode
 
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 707
Default Re: Shared Service (Party Line) emulation

Quote:
Originally Posted by Dave Moll View Post
Yes, the phones (designated "X" and "Y") were indeed wired with their speech circuits in parallel as Peter describes, though the "Y" is connected with "A" and "B" legs reversed to achieve the ringing between the appropriate leg and earth.

In order to make a call, the handset is lifted and a check is made that the other party is not engaged in a call. If the line is free, the "call exchange" button is pressed to obtain a dial tone and the outgoing call then proceeds as normal.

On receiving a call (with ringing on ones appropriate ringer), I seem to recall that one simply had to lift the handset to be connected to the caller. I don't recall having to press "call exchange" in this instance, though it would probably simplify the circuitry if one did.

My question was really to avoid "reinventing the wheel" if someone has already designed a circuit, either by the traditional GPO way using relays or with solid-state electronics, before I try to work out something to make a pair of telephones behave as described above.

And yes, the (no doubt apocryphal) story about the "psychic" dog does crop up from time to time, and is quoted in one of the early issues of the Telecommunications Heritage Journal.
You did not need to press the 'Call Exchange' button when answering the telephone. Normally there was battery on both the A & B legs when normal - via the two line circuits 'L' relays. Pressing the 'Call Exchange' button operated the L relay for the appropriate line which then operates the K relay for that line circuit and the extended 'loop' then seizes a group selector which returns an earth to hold the K relay (which has connected the meter appropriate to that calling line). Call then progresses as normal. This setup was introduced just after WW2 with the introduction of the Tele 310 & Bellset No 41(to prevent bell tinkle when the other party was dialling). However with the introduction of the Thermistor a couple of years later, the Tele 310's in the stores were modified to Tele 312's and the BS41's sold off which is why they are so common on eBay and Tele 310's are so rare! This system was known as 'Shared Service, Separate Metering' whereas previously there was an earlier automatic system known as 'Shared Service, Common Metering' often seen on older N diagrams. Often disputes between parties as to made the calls! Sale of this system was discouraged but lack of copper after WW2 forced the 'Separate Metering' version being introduced.

Must be nearly ten years ago, we had some variations of party lines working on CNet. With one setup there were just two phones on the same pair each with its own number and they were rung with distinctive codes. In fact we replicated the system used in New Zealand on UAX13s where there were up to six phones on the line, one with normal double beat ringing, and the others were rung by codes - Morse letters repeated A (short long) , D (Long short short), M (long long), R (short long short) and S (short short short).
It was even possible to dial any of the other numbers on the same line . However as CNet has no metering, the setup didn't use the 'Call Exchange' button.

But it should be possible to knock up a small 'box' to go between two PBX extensions that needed the 'Call Exchange' button to initiate a call and to ring the two phone individually via one leg to earth.

With reference to 'odd situations' with party line - this is one I know to be true as I was there! Just after the conversion from a CBS2 exchange to a UAX13 at Tarporley in Cheshire, the firemen from the 'retained' fire station kept getting call outs. It took a while until we tracked the problem down. The fire-bells were operated by the 'WRU' (Who are You) mechanism being operated on the teleprinter in the fire station after the message giving details of the fire location had been sent by Fire Service HQ in Chester. What was happening here was the party line earth from the Tele 312 had been connected to the fire-bell earth (fire-bells were a single wire to earth system). The problem was when one of the firemen made a call by pressing the 'Call Exchange' button - the 50 volts from the phone line was finding a shorter route to earth via the other fire-bells (all connected between the fire bell single wire & earth) rather than via the local earth! A very interesting fault.
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