This is something I wonder about very often.

FrSky transmitters produce digital outputs on their channels, which then go to digital transmitter module, which sends it over RF digitally, which is then received by RX and transmitted to the FBL over a digital protocol (S.BUS, F.PORT...).

So why do we need these numbers? Why are they different for each channel, and even "zero" needs trimming?





And even if there would be some analogue part in the chain, then these numbers would need to be different depending on something (FBL unit, receiver, transmitter). But they need to be same for every model. but different for different channels.

Why is this?

Question is of course, because it makes model setups 2x more complicated.
Or maybe I'm doing sth wrong?

What version of firmware are you running? The latest version is supposed to resolve the max and min issue...mostly.
You should not set the subtrims(center) on the radio - this should be done in the FBL, if needed, and your mechanical setup looks to be pretty close in most channels from the small amount of trim I see...

I believe spirit uses the old servo format where 1520 is centre, and max min are about 82.

DONT use substim in the radio.

Set your servo outputs to -82 1520 +82 and it'll be reasonably close.

Then tune the endpoints by viewing in spirit software until they show -100 +100 on the diagnostic screen.

I can double check tomorrow, but I'm pretty sure I need to use subtrim to get 'zero' in spirit diagnostics screen.

Just to clarify, I don't compensate any mechanical things here. I need min, max and subtrim on these channel just to align thing so Spirit sees -100 when I output -100 in radio, and +100 when radio sends +100, and same with zero. This is super weird to me, considering everything is digital here.

This is exactly what I do (tune on Diagnostics screen) and I end up with different values for each channel, even to get zero. Weird.

With centre, should it be 1500 or 1520?

BTW. Screenshots I posted are from OpenTx, but I now use Ethos. But exactly same issue there.

Running latest one (3.5). I'm updating all my models as I fly them for the first time this season. But no changes in this area.

i mean, everything works, I use Spirits for years and it works. But I wonder where this problem comes form, because it does make stuff so much harder when setting up models.

I use openTX and also get different values for Max/Min, usually ranging from 82 -> 85. I use 1520us centres for all channels.

I have no idea why every channel is different, and why I can't just use -100 +100 and 1500us centres in the radio - if I had to guess, i'd say it's something to do with historic receiver output values?... just a guess.

As you said it's digital, but I'm sure there's a logical explanation, but only Tomas will know the reason.

always check your Channel Monitor. Midpoint has to be 1500us. EdgeTX, OpenTX, Ethos
Primary setup with 0% Expo and mixer off

But it doesn't work like this. That's the whole point of this thread.
You can have perfect zero in channel monitor, but in Spirit diagnostics you see +3%. or -3%. Or +1.5%. Different numbers for different channels.

Same with min / max -- they are different for different channels.

but your pics are showing subtrims not zero