How to put it all together with basic telemetry

[redear]

Hello, I have a few unrelated questions which I tried to find answers for, but it is hard for me to go through all the material and I have to admit, I understand like one hundredth of it... I have some general idea how it fits together. Can someone help or confirm my thinking please?
My HW and SW:
- 9XR Pro with ersky9xr, r218, no module
- FlySky TH9X with original FlySky module and original FW (I can flash it to opentx if required)
- few electric airplanes and a 450 heli with flysky 9x and turnigy 9x receivers today

Now I am before buying a new model that will require 9-10 channels and is kind of expensive, so I am eyeing to go to something better than the original flyskyRM002 AFHDS module and rx for my 9xrpro.
My choice is the FrSky XJT as it has up to 16ch and telemetry, and is compatible with D8 as well as D16 RX (as I am planning to equip some of my smaller models with basic D8 based RX, such as Frsky DIY RX-F802 due to the lower price, and some cheaper ACCST 4ch rx as well for indoors. For channels 9+ I understand I need an X8R RX with SBUS decoder.
- question: switching between D8 and D16 modes on the XJT tx module is only done with the dip switch, nothing else? Am I correct to think I will set the PXX modulation just once for the module itself in the radio setup (as I read XJT needs PXX for D16mode, right?), and it is then compatible with both D8 and D16 modes when selected on the module by the dip switch, and further, with any FrSky D and X receiver? Anyone switching regularly - does it last everyday usage?
- I will then fly my new aircraft with X8R RX with sbus in 16ch, and all the others in 8ch D8 mode by toggling the dip switch. OK?

Telemetry question - I have never used telemetry before, but what exactly do I need to get RSSI strength signal and onboard battery voltage (alarm is sufficient) transmitted to my TX? Not interested in altitude, RPM, GPS and other data.
- I believe for RSSI, with all D8R and X8R receivers, as well as the DIY RX-F802, nothing is required as the RX reports it itself, and the 9XR Pro with the XJT is capable to show/alarm it as is. Correct?
- for onboard voltage and low voltage alarm, do I need some sensor/wiring to get it back or does the RX report this itself as well? If something is required, what? My new aircraft will be 2x3s lipo or 1x6s lipo based. All others are 2S-3S lipo based.
Last but not least:
Today I am using the Th9x as a buddy box for my son, so I have the module plugged into my 9xrpro and have him on the cable. With the XJT module and RX's as above, is any modification required to the Th9x tx? does it need to output something like fe. PXX as well? I am lost here. If yes, is it solved by flashing it with opentx and seting it in the menu?

if you have read this far, thank you very much. If you have any help or answer for me, thank you even more. regards and thanks, Redear

[MikeB]

With the XJT you don't need to use the dip switches to change the mode, it is all handled by the PXX protocol and automatically switches to the mode set in each model.
For more than 8 channels, you may use either a X6R or an X8R with an SBUS decoder, or TWO X6Rs (12 channels) or TWO X8Rs (16 channels).
RSSI is automatically available.
With a D8R type receiver you get two analog inputs (A1 and A2). You may easily use one of these, with a couple of resistors to scale the voltage from the flight battery, to measure the battery voltage.
The X8R doesn't have any external analog input.
The X6R has a single analog input that by default is linked to the receiver voltage, but may be use as an external voltgae input up to 13.2V with no external resistors.
You may use the buddy box connection as is regardless of the protocol you are using to the Tx module.

Mike.

[redear]

Wow. Thank you VERY much for a detailed, quick and structured reply to all my questions. I could not hope for a better answer.
So with the X6R, I could scale the 6S lipo voltage by resistors to one third, and feed this directly to the analog input of the X6R to get the reading on the screen of the 9XR Pro? Do I understand it is that easy? thanks, regards, redear

[MikeB]

The X6R has an internal divider (4:1) so handles up to 13.2V (4*3.3). For a 6S, you need to be able to handle up to 25.2V. For that you need an 8:1 divider. THis could be achieved jusy by having a single resistor, of the correct value, to make the divider into a 8:1 divider.
Off hand, I don't know what the value would need to be, I'd need to do an experiment to find out what resistors are used for the existing 4:1 divider, or open up a X6R and read the values from them. on the internal measurement, but has NO divider if you remove the link.
For a 6S battery pack I suggest a 27K and a 3K9 divider. This gives 3.3V across the 3K9 with an input voltage of 26.15V.
Yoy might get away with a 22K and a 3K3. This gives 3.3V across the 3K3 with an input voltage of 25.3, just high enough for a 6S pack as long as the resistors are 1% tolerance. With 5%, resistors you may exceed the 3.3V input voltage limit.

Mike.

Edited to correct error.

[jhsa]

No, you do that and you blow the receiver up.
The maximum voltage the analog port on the receiver can handle is 3.3v. So you have to calculate the voltage divider so 25.2V from the 6s lipo at the divider's input has maximum 3.3v at the output.
There are some voltage divider calculators online..

João

[redear]

No, you do that and you blow the receiver up.
The maximum voltage the analog port on the receiver can handle is 3.3v. So you have to calculate the voltage divider so 25.2V from the 6s lipo at the divider's input has maximum 3.3v at the output.
There are some voltage divider calculators online..

João

Thanks to both of you. Idea - wouldn't 27k and 2k7, or 33k and 3k3 or whatever like this work as a 10:1 divider - so from the analog reading I have always low enough and correct value, only the decimal point is wrong. Or am I wrong? Long time since my electronics lessons...

[MikeB]

27K and 2K7 or 33K and 3K3 are 11:1 dividers, not 10:1. The ratio is 2K7/(2K7+27K).

Mike.

[redear]

27K and 2K7 or 33K and 3K3 are 11:1 dividers, not 10:1. The ratio is 2K7/(2K7+27K).

Mike.

Oh yes. I told you it is a long time I learned electronics and physics thanks

[redear]

And my last question as the googled resources are contradictory... If I have a XJT flashed with the EU / EU-LBT firmware and set to PXX, will the D8 based rx(namely the RX-F802 and possibly d8rii ) still work without issues? Thanks again, redear

[MikeB]

I tested a XJT, flashed with the EU-LBT firmware, with a D8R-II and it worked OK. As always with something like this, do a good range check before flying.

Mike.

[redear]

Great. Thanks a lot for your help and time. This closes all my questions. Just ordered a xjt, one x6r, one d8rii, one rx f802, one frsky sbus to pwm converter. When it comes, i will start testing:) thanks again, redear