DIY Arduino buddy box and Taranis X7S

[fanfanlatulipe26]

The context: I am developing a buddy-box (with a 4 axes single stick) based on an Arduino Pro Mini 5V and generate the PPM signal for the RC master transmitter.
It seems that RC transmitters use an open collector arrangement for trainer port management in order to prevent voltage mismatch and so I generate the PPM signal with an open collector/drain emulation on the Arduino: PPM signal is only pulled low (output pin defined as output and set low) or float high (output pin defined as input).
My buddy box works perfectly with a Flysky TH9X/Turnigy 9X running ER9X, using a mono or stereo cable.

It fails with a Taranis X7S running opentx-x7 2.3.15-otx: as soon as the cable is plugged in the X7S the PPM signal remains stuck to low.

On the other hand the Taranis X7S and the TH9X work perfectly together each one as slave or master (with mono or stereo cable, positive or negative PPM …) and so I think that it is not a simple issue with software settings.

Is there some specific issues with the trainer port handling on Taranis X7S ?
Is the port “open collector” ??

[MikeB]

The X7 trainer input is to the base of a transistor (via resistors) and requires a positive voltage (>1 volt) to make it work.

Mike

[fanfanlatulipe26]

Thank you.
I forget the "open collector" and just drive high/low the Arduino output. I added a resistor voltage divider to limit the output to 3.3v.
It seems OK with both the Taranis X7S and the TH9X.
Next target will be the Hitec of a buddy as master ... Going on in the PPM port / buddy cable jungle ...