At 23:49 29/08/99 +0100, you wrote:
>Dennis wrote:
>
>>> Unipolar?
>Let me see a Telex machine is a two wire device correct! No! It is actualy
>a three wire device with an earth return (I may get this wrong it has been
>a very very long time). <<
>
>I think we may be in a bit of a mix-up here.  "Telex" is an international
>service, which is delivered at the customer's premises using a
>"teleprinter".

Telex is a service correct, but it does specify a format that it provides,
and the teleprinter prints out "TELEX MESSAGE". In Australia it is known as
Telex now just like names are associated with other things.

>But teleprinters are used for purposes other than telex.  In
>particular they were used on point-to-point circuits and broadcast services.
>For example, in the UK there was a continuous newsfeed service which most
>radio stations and newspapers subscribed to.  Also the weather bureau and
>police had their own private teleprinter networks.

Yes, but still printed on paper that says "Telex message"

>
>Many of these ran at 75 Baud, rather than the international standard of 50
>Baud for telex.

Simply a speed increase as the machines got better.

>
>To the very best of my knowledge, all genuine "telex" circuits are two wire.
>In recent years audio frequency tones - to represent the marks and spaces -
>have been transmitted over the local loop.  But until the 80's telex was
>always delivered using DC signalling on a *two* wire circuit.
>

Yes we here are a *two* wire circuit with an earth return. You don't call
AC power distrobution 1 wire do you?

>At least, that's the way it was in the UK.
>
>One wire was always at earth potential, the other switched between +75V
>and -75V.  (I think 75V is correct; it's certainly pretty near).
>

In this case it is a 72V battery, the old ones that used to be used for the
valve equipment in the long line rooms. The equipment that youre describing
does not require an earth stake as does ours. I was going to put up this
type also but did not wish to confuse people. Note that in this case the
same lines will work on either telex machine, just add a diode!

The +/- 72V is not something that is liked by exchanges as this means that
the battery is floating nominaly somewhere above/below ground for some
peroid, and makes it a hazard that is unknown (VS a known hazard). It
creates other problems when fautlts occur in that some lines are at -ve and
others are at +ve, and means that the battery banck can not be tied to the
exchange earth grid.

To say that one line is at earth potential all the time is not entirly
correct, this is the return line for the battery, thus +ve for marks and
-ve for a space.

Dennis



>In other words, I don't think current loop signalling was used on the telex
>service in the UK (at least not in recent decades), although it was most
>certainly used in some of the other teleprinter-related services I mentioned
>above.

Current loop signaling was used until the early 90s as exchanges received
work orders on them

>
>Incidentally, I believe the telex service is still plugging away, albeit
>with precious few subscribers these days.  There were two things which kept
>it going longer than you'd expect.  Firstly, even the most undeveloped of
>countries had a telex service, so it really did have the widest reach of any
>data (as opposed to voice) service.  Secondly, because of the difficulty of
>faking, telex messages had a special status legally.  I don't know much
>about law, but basically a telex message was considered to be a legally
>binding document.
>
>Of course, all this is long before any one had heard of the Internet!
>

There are also other factors, one little unknown is that a fixed copy can
be generated at both ends. If you use the service correctly you generate
the puch tape to create the file to send off line, then run it through at a
latter period (So the sender has a transcript!). The other is that it
created more than one copy (A carbon copy as well). But you are right about
the far reaching bit of the service, this format would almost go through
mud, and the human can decipher the words with errors whereas modem could
not (OK so you can flame me on that I know that there are some holes in it)


>Steve
>
>Steve Thackery
>Suffolk, England.
>Web Site: http://www.btinternet.com/~stevethack/
>
>

Dennis