FrSky PPM output from Rx and Taranis Trainer port - Solved

[seabee]

I am trying unsuccessfully to get a FrSky Rx to mimic the output of the Taranis Trainer port in Jack/Slave mode.

Why? Well I guess a few like me have some perfectly functional 35MHz equipped planes but would like to use OpenTx on a Taranis to control them. By bridging the trainer switch on the 35MHz transmitter to permanently on it works well with a conventional buddy lead. However putting a receiver outputting CPPM on the end of the buddy lead not surprisingly doesn't.

Is there any not to complicated way to make the Rx CPPM signal look like that from the Taranis trainer port?

[Kilrah]

Probably just the signal level being too low, the RX puts out a 3.3V signal but old 35MHz radios expected more like 9-12 (battery voltage).

[seabee]

Is there a simple way to amplify the signal? As the Rx and Tx are powered independently and connected only by the signal cable and a common ground would reversing these and adding the Tx battery +ve to the signal cable work without destroying anything. Would a diode be necessary to stop the reverse 9V feeding back to the Rx? In my simplistic view this would provide a signal between 9V and 6V?

(Electronic skills limited to a multi-meter, soldering iron and thermionic valve theory)

[jhsa]

Probably just the signal level being too low, the RX puts out a 3.3V signal but old 35MHz radios expected more like 9-12 (battery voltage).

Well, doesn't the Taranis also output 3.3V level PPM? [emoji14]
My guess is that the receiver inverts it, and that is probably causing the problem. But yes, 3.3V level could also be a bit too low, but not the cause of the problem.

João

[MikeB]

The Taranis trainer output is 3.3V, so is the same as the receiver.
Possibly, the receiver output is inverted compared to the Taranis. With erskyTx firmware on the Taranis, you have an option to invert the trainer output signal, maybe openTx can do the same, so you might be able to see if it only works with a specific polarity.

If you power the receiver from its own supply, separate from the target transmitter, the you may invert the trainer signal from the Rx by connecting the ground from the Rx to the signal and the signal from the Rx to the ground of the target Tx to see if that works.

Mike

[seabee]

Measured with a digital multi-meter:
Voltage across buddy lead from Taranis: 1.00V, and when plugged into 35MHz Tx everything works!
Voltage across buddy lead from R-XSR in CPPM mode: 14mV max, when plugged into 35MHz Tx connection not detected.
Reversing polarity of R-XSR buddy lead makes no difference.

The R-XSR is configured to use 4 channels and the voltage cycles round to a maximum of 14mV
The Taranis outputs a steady 1V (but that may be due to the limitations of my meter).

I appreciate that my DC voltage readings are not the true signal voltages but being orders of magnitude different is possibly significant?
Is there some simple circuitry that could boost the R-XSR signal?

[jhsa]

The Taranis trainer output is 3.3V, so is the same as the receiver.
Possibly, the receiver output is inverted compared to the Taranis. With erskyTx firmware on the Taranis, you have an option to invert the trainer output signal, maybe openTx can do the same, so you might be able to see if it only works with a specific polarity.

If you power the receiver from its own supply, separate from the target transmitter, the you may invert the trainer signal from the Rx by connecting the ground from the Rx to the signal and the signal from the Rx to the ground of the target Tx to see if that works.

Mike

Mike, I believe that inverting the PPM signal on the Taranis will not affect the PPM sent by the receiver. The receiver might produce its own PPM signal with its own polarity.
I know for example that the Frsky D4R-II (I think this is the name) outputs a negative PPM signal.

I think the only solution here is to try inverting the signal with some inverter circuit.

João

[Kilrah]

You might need a pull-up resistor from the receiver supply to the PPM line then, maybe start with 10k.

[jhsa]

1 transistor inverter. Perhaps something like this??

If the radio PPM input has already a Pullup resistor on it's PPM input, you might be able to just leave R2 out. And the signal should now also be at the right voltage level..

João

[seabee]

@João As I understand it the problem is that Rx signal is incompatible with the 35MHz Tx input so what the Taranis is doing is largely irrelevant other than it works with a cable.

I will try Kilrah's suggestion of a "pull up resistor" and if that doesn't work and you put some component specifications on your diagram I will try that.

I have just discovered that while the Taranis is communicating with the 35MHz Tx the voltage across the buddy cable oscillates between 2.92V and 2.93V so it would appear that the R-XSR signal needs to be in that region.

Colin

[jhsa]

The problem is not the voltage.. As far as I understand it is the signal polarity..

João

[jhsa]

solution

If you power the receiver from its own supply, separate from the target transmitter, the you may invert the trainer signal from the Rx by connecting the ground from the Rx to the signal and the signal from the Rx to the ground of the target Tx to see if that works.

Mike

I didn't read this well before, sorry Mike

As Mike said, if you are not powering the 35Mhz radio and the receiver from the same source, you can try what Mike suggests here.. This would probably the easiest solution. But they MUST BE powered separately.

João

[seabee]

solution

If you power the receiver from its own supply, separate from the target transmitter, the you may invert the trainer signal from the Rx by connecting the ground from the Rx to the signal and the signal from the Rx to the ground of the target Tx to see if that works.

Mike

I didn't read this well before, sorry Mike

As Mike said, if you are not powering the 35Mhz radio and the receiver from the same source, you can try what Mike suggests here.. This would probably the easiest solution. But they MUST BE powered separately.

João

Done that, tried it both ways and neither work so I think it must be a voltage issue. The Rx is powered by a 4.8V NiMH, the TX by a 9.6V NiMH. However I am now pretty certain that the signal voltage needs to be about 3V and I don't understand why what I am seeing from the R-XSR is so low at 14mV. Hopefully the pull-up resistor may help.

I also don't understand why the voltage at the Taranis trainer port is about 1V (open circuit) but jumps to 2.92V once cable communication is established. (if the buddy cable is unplugged the Taranis trainer port stays at 2.92V) Could there be some kind of protocol negotiation necessary for communication to start which the R-XSR is incapable of doing?

Colin

Colin

[jhsa]

No, it is not a voltage problem. the rx outputs exactly the same voltage as the Taranis.
I think you still didn't say what 35Mhz system that is. I still think it might be a polarity problem, but it could be also be that the receiver is not outputting any PPM signal. Are you sure you are not trying to feed an SBUS signal to the 35Mhz radio? SBUS is not the same as PPM. Please make sure the RX is configured to output PPM and not SBUS.

João

[seabee]

No, it is not a voltage problem. the rx outputs exactly the same voltage as the Taranis.
I think you still didn't say what 35Mhz system that is. I still think it might be a polarity problem, but it could be also be that the receiver is not outputting any PPM signal. Are you sure you are not trying to feed an SBUS signal to the 35Mhz radio? SBUS is not the same as PPM. Please make sure the RX is configured to output PPM and not SBUS.

João

I have checked multiple times that it is set to PPM and not SBUS and switched them back and forth to be sure that the Rx is functional. I have also confirmed that the S-PORT functionality is working as well. Unfortunately I do not have anyway of checking if the PPM output is actually working but I have no reason to believe that it is not.

The 35MHz Tx is a JR 2610

Colin

[jhsa]

Well then you need to try either Kilrah's idea and the transistor inverter.
For the transistor circuit try something like 10K for R1 and 2.2K for R2.
The transistor itself can be any general use NPN transistor. For example BC546, BC547, 548, 549..

João

[seabee]

Post by Kilrah » Mon Jan 13, 2020 2:01 pm
You might need a pull-up resistor from the receiver supply to the PPM line then, maybe start with 10k.

This is indeed the case!

A single 10K resistor causes the LCD screen on the 35MHz JR Tx to pulsate "Master" as if the signal is not quite strong enough. A servo on the 35MHz Rx twitches but is not controlled. However I think having the JR recognise that there is a "Trainer" signal of sorts is significant progress!

Do I go up or down from the 10K resistor?

[Kilrah]

Down might help.

[seabee]

I have tried reducing the 10K to 5K and 5.5K (with what I had available) but it killed the pulsating "Master" indication. I tried reversing the polarity of the jack as per MikeB's suggestion but that also killed it from which I deduce the phase of the signal is correct. I think that also confirms that the R-XSR is outputting a recognisable (though possibly incompatible) signal.

[EDIT: Changing the value of the "pull up resistor" to anything other than 10K doesn't improve things neither does changing the R-XSR supply to 7.4V. I will stick with the cable until I can get hold of some decent test gear to see what is really going on. Thank you all for your suggestions.]

[michel49]

You could try to use some PC oscilloscope free software...

[seabee]

[Edited]

In an attempt to find out what was going on with these signals I bought a JYETech DSO150 oscilloscope - OK not a precision instrument - but better than a multi-meter.

This confirmed that the R-XSR output was orders of magnitude smaller than that coming from the Taranis trainer port.

Out of desperation I re-flashed a D8R-II Plus with D8R-XP firmware and configured it to output CPPM. Amazingly the scope showed an almost identical signal to that from the Taranis trainer port. It almost worked when wired to the trainer lead and plugged into the JR2610. Putting a 10K resistor in the signal lead and it worked perfectly as I had hoped. I can now use my 35MHz PCM receivers with the facilities of Open-Tx !

So a huge amount of time and some expense was expended on this project owing to the peculiarities of the R-XSR.

Why the CPPM output from the R-XSR is so different from that of the D8R-XP is a mystery. To me it makes no sense.