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".  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.

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

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.

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 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.

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!

Steve

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