Solving the double current FSG problem

[M1ECY]

Morning all,

Some of you will know that the English telex system is a little different to the German Single Current system - over this side of the channel our machines use Double Current signalling.

This presents a problem in two areas

1, The need for a Double Current interface

2, The need for a Double Current FSG (Known as a signalling unit)

Problem 1 has a solution in place - Fred has a prototype.

Problem 2 is more difficult to solve.

Because nearly all of the machines that are available or are in private hands in the UK were used for RTTY, the signalling unit was discarded - no need for them when hooked up to a terminal unit for radio.

There is some information available that covers the earlier signalling units - for things like the Creed 7B and that sort of machine - this relies on a lot of relay logic to control the line and motor circuits.

I have looked into the possibility of recreating these signalling units using off the shelf parts, two problems become immediately apparent. Firstly, modern relays are not really suitable for line monitoring due to the relatively low impedance of the coils. secondly, multi pole changeover relays and switches are extremely expensive - recreating a signalling unit to be a close replica of the original will cost around £400!

So, I think the only sensible option is to look to a software driven solution. Im not going to bluff here - I am very much a hardware kind of person, so this is going to be a massive learning experience for me!

I've got an Arduino development kit here, as well as a pi4 - my preference is to stick with Arduino - the controller is the same as is used in ITelex, and I plan any positive outcome of this to become another part that can be offered as a system accessory


Moving onto more technical things now.

The signalling unit needs to do a few things.

1, Control the motor supply of the teleprinter, both for local and remote operation. Local is fairly easy - a digital input triggers a digital output which turn a relay on and allows power to flow to the teleprinter. A seperate digital input acts as a clear signal, this switches the output off -simple.

The second function of the motor control circuit is more difficult - again to initiate a call a digital input is taken and this will switch the motor on as above. What about receiving a call? There is a change in the line conditions that is supposed to call the motor start relay into action - for this my idea is to use the current limiting resistor network as a way of generating a voltage for use at the analogue input of the arduino - set a level that will see the line current change as a signal to run the motor. when this voltage is switched off, a timer triggers to hold the connection for a few seconds and then switches the motor off.

2, Dialling - the ability to dial a number from the signalling unit once the line is in the right state, there needs to be a dialling signal to the system - it is fair to say that getting a mechanical dial in the UK is not that difficult, but getting one that will work across a double current line may be harder. At this point is a pushbutton dial a better idea? How is this implemented in software?

Is it better at this point to use the FSG-O-FSG firmware in the I-Telex interface, and use the keyboard for dialling?


Going to be a lot more questions than answers, but for the system to thrive in the UK a solution needs to be found - any suggestions from others more experienced in programming?

[detlef]

I've got an Arduino development kit here, as well as a pi4 - my preference is to stick with Arduino - the controller is the same as is used in ITelex, and I plan any positive outcome of this to become another part that can be offered as a system accessory

...

There is a change in the line conditions that is supposed to call the motor start relay into action - for this my idea is to use the current limiting resistor network as a way of generating a voltage for use at the analogue input of the arduino - set a level that will see the line current change as a signal to run the motor.

...

2, Dialling - the ability to dial a number from the signalling unit once the line is in the right state, there needs to be a dialling signal to the system - it is fair to say that getting a mechanical dial in the UK is not that difficult, but getting one that will work across a double current line may be harder. At this point is a pushbutton dial a better idea? How is this implemented in software?

I think Arduino is a good choice for this project. For all realtime control applications where you do not need much Internet communication or process large amounts of data the Raspberry Pi is oversized (especially the Pi4). Who want's to wait serveral seconds until the FSG has booted it's operating system?

You could connect the mechanical dial to a seperate digital input. That would not simulate the original dialing but it would look so.
Keyboard dialing could be implemented as an option. Like in the i-Telex.

Did the English telex system use reverse polarity for signaling incoming calls like in Germany?

[FredSonnenrein]

I don't think that another microcontroller is necessary for these functions. All of them could be placed in a variant of the existing "TW39" firmware.

Dialling is possible by "keyboard dialling". Local operation by a "special call number" or by typing the letter L instead of the number.

"Call" and "Clear" buttons are presumably possible by extra buttons (or one button) in the signal lines.

Remaining Problem is the motor control. This can (must) be solved by an external relay, which can be controlled by the i-Telex board itself.

What's "visible" at the printers "send" wire when the printer is idle? And what's if switched off?

[FredSonnenrein]

Did the English telex system use reverse polarity for signaling incoming calls like in Germany?

It's like ED1000: "Motor off" is signalled by constant space polarity. Motor on by constant mark...

[M1ECY]

I am still trying to understand the English signalling method - the double current system makes life more difficult - there are three line conditions to consider - Mark -80v, Space +80v and Idle.

When the system is idle, it seems that a Space condition is maintained - when a call is placed either locally or from the exchange the condition changes to Mark - so yes, there is an inversion of signalling polarity.

I like the idea of connecting a dial to the Arduino digital input - this would make any device that is built a lot more useful, and will mean standard firmware can be used in I Telex.

So, I need to go shopping for an old British Telefone

I think wit a little thought the hardware needed will be very small - going to need a lot of help on the software - my expertise so far with Arduino is at beginner level

[M1ECY]

What's "visible" at the printers "send" wire when the printer is idle? And what's if switched off?

This is a good question Fred - I do not have a machine here that I can check - the model 75 I have is receive only. I need to find a full double current machine to work with (and some space!)

[M1ECY]

Some behind the scenes stuff has been going on. With some considerable help from Jeff in Texas, a design for a Single Current to Double Current interface is coming together.

This will be in the form of a separate module that will also include the line current equipment to suit the British system.

Input will either be straight from I-Telex TW39 card, or from the output of a TW39 compatible FSG - this will solve the UK wide problem of availability of suitable Telex control devices.

More to follow once the prototype has been tested on a real machine, and a small number will be produced initially for use within the community.