Solving the double current FSG problem
Verfasst: Mo 25. Mai 2020, 09:42
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?
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?
