The Double Current Interface

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M1ECY
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The Double Current Interface

#1

Beitrag von M1ECY »

Hi All,

I have been building some systems here for loan to Museums in the UK, one of the problems for I-Telex in the UK is the lack of Double Current signalling as an "Off The Shelf" solution.

UK specification teleprinters are generally found with a 3 wire double current or V.21 signalling system - the Analogue Card takes care of the latter, and with a UT55A converter it is possible to run a DC machine with V.21 interface, however these modules are rare, and ones that do exist often need a lot of component level repair and refurbishment - not something that all UK enthusiasts are completely happy to do.

There have been some versions of the DC interface card designed by Fred over on the Sourceforge pages, but most were concept ideas, or prototype designs, as such not tested fully.

Henning and Fred are still working on the latest version of this interface card.

Being a little impatient, last year I worked with Jeff Angus, to come up with an external solution - this will allow UK spec machines to interface with a Single Current Control unit, and drive a DC Teleprinter - this is still in testing and development (Sorry, lots going on here!) In parallel with this innovative solution, I also had some of the I-Telex interface boards made - A very early DC board, and the later 1.50 revision.

The earlier version lacks the galvanic isolation of the later design, and will probably be tested at a very low voltage to work on a third type of UK machine, which works with a +/-6V signalling system - more to follow on this version in another thread.

Onto the Version 1.50 Card.

This is designed as a universal board - it can be built for single or double current use - being a prototype, there are some errors that have been corrected in the later design, this is a fairly minor issue, some component changes, and one addition that is easily placed into the board.

I have built a couple of these cards, and over the past few days, with a lot of help from Fred remotely have managed to get a working interface to I-Telex.

The signalling current is using a much lower loop voltage, which for short connections between I-Telex and the Teleprinter is completely acceptable - the 40V supplied by the original I-Telex Power Supply board works well in the short testing completed yesterday evening.

One of the museums that I am working with is situated in a "Nuclear Bunker" this is an underground structure over several floors, as such the previously installed telex system has many kilometres of wiring - to keep the loop current to a high enough value to drive the printer it is envisaged that the higher voltage will be needed - this will be tested once the interface card is proven completely.

So far everything looks promising - I have to discuss a small software modification with Fred, due to the way the UK Control Unit is working, hopefully this is a simple change to make, and then for the fist time since the closure of the UK Telex system there will be a viable solution to allow these machines to join in.

More work to do on this, but I thought an early report of the work so far would be good for any of our UK members to see that there is good progress, and perhaps this will allow them to begin the experience of joining the I-Telex network.

All of this would not be possible if it was not for the help and support received from Fred, and Henning - Thanks both - really appreciate what you are doing, and apologise for all the silly questions :)


Will continue to add to the thread as more progress is made,

Cheers
Sean
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459724 SEAN G - Te Ka De FS200 - Working 24/7
Creed 444 Hopefully on line soon........
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Topic author
M1ECY
Rank 10
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Beiträge: 364
Registriert: Fr 11. Aug 2017, 13:13
Wohnort: Hitchin Hertfordshire, UK
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Re: The Double Current Interface

#2

Beitrag von M1ECY »

Time for an update.

Once again thanks to Fred for help and support with some adjustments to the software for the interface card.

Some small changes were needed to allow the Control Unit (FSG) to perform correctly - In Telex service from the old exchanges in the UK there was a need for the exchange to return a negative pulse to the Teleprinter to indicate that the exchange was ready to accept the dialled number.

With this in place calling out is now a one handed operation, as it should be, and the system seems to perform very well.

There is one unavoidable detail when "Clearing Down" the call, which is a function of the Double Current system - when the line clears, the Teleprinter will print one random character, due to the change of state when the line drops - this is unavoidable, and isnt really much to worry about.

I hope to get the new "Development" system onto the server soon, then I can test incoming calls as well, but I expect this to work without fault due to this being something that is already half tested.

Exactly one year since getting this Creed 444 into the collection, and it is working properly again.

Looking forward to getting some two way use soon.

Cheers
Sean
459724 SEAN G - Te Ka De FS200 - Working 24/7
Creed 444 Hopefully on line soon........
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