Switching power and ppm between modules

[Crucial]

I have been trying to find explanations for why it is important to switch both the ppm and power lines when running more than one module.

Can anyone here give any reasons why you should or if it doesn't really matter?

[cre8tiveleo]

I don't think it matters. (you're just running two modules, so you are taxing the battery a little more with two modules running)

[Kilrah]

Some modules will send data even when they don't receive a proper signal, so you might not want to pollute the band for nothing. It could actually cause trouble if you're using one that has telemetry downlink and there's another module sending in the same frequency band right next to it, and reduce the downlink range.

If you don't care about the extra signal, why cut the PPM in the first place, you could simply be always running both modules together.

[pmackenzie]

I would not want two high power RF sources operating so close together. Lots of possibilities for cross talk, intermodulation or other bad stuff.
Similar concern about driving two loads from one pin, but this should be less of an issue.

So switching power and PPM would be my first choice, but at a minimum I would suggest switching power.

In the case of a PPM and DSM module on the same signal line I guess the difference between the two data streams would prevent the "wrong" module from transmitting.
But I would still want to switch the power lines since this off-design operation is not defined anywhere so we can't be sure what they would actually do.

Pat MacKenzie

[Kilrah]

In the case of a PPM and DSM module on the same signal line I guess the difference between the two data streams would prevent the "wrong" module from transmitting.

I have tested that with what I had. The DSM module doesn't transmit when it receives PPM, that's OK. The FrSky stuff however transmits as soon as there's power applied to it, whether there is a valid PPM signal, an invalid one or none at all.

[pmackenzie]

An even better reason to switch the power lines then

[Crucial]

I decided to switch the power only on my low power module modded 9x. My buddy did the same on his but he has mostly DSM rx's and won't be flying many planes using the stock module. For now that is just removed from the radio.