12 Channel PPM Question?

[DarbleyJ]

I am working on a project at the moment using an rc transmitter to control a system on an R/C Heli. Currently I am using the PPM input of a Futaba 9C radio to give me 8 channels, generating the PPM stream using a PIC, but unfortunately I need more channels. An option may be to use the 12 channel FG.

I know by looking in the user manual of the 12FG that it is possible to use the trainer input of this radio for all 12 channels (pg29), but is someone able to tell me what the format is of this 12 channel (PPM?) signal?

In anticipation,

Brad

[Malc C]

Brad, I'm no expert, but from my limited knowlege on PPm signals the pulse is 1 - 2 ms repeated on a 20ms time frame. Assuming the 12FG follows the same standard it would be impossible to fit all 12 pulses at max throw in the same 20ms time frame. I would therefore assume that the protacol uses a larger time frame to fit all these pulses in ?

Like I said I'm no expert, so maybe someone else could chime in here

[pilotpete2]

Hi Brad,
According to the reviews that I have read, the 12FG only does 8 channels in PPM, this seems to be the max with Futaba PPM. In 1024PCM is does 9 and 14 in G3 2048 PCM, yes it's 14 channels not 12, 12 proportional, 2 non- proportional :)
I have no idea as to the trainer function, got me there :o
Good luck,
Pete

[DarbleyJ]

Thanks men.

I'm very familiar with the 20ms pulse train that is required for 8ch PPM, and you are correct Malc, 12 channels could not possibly fit in there with the same pulse timings. I have successfully used more than 8 servos with a 30ms refresh rate directly controlled by the PIC, no R/C transmitter in the loop though. It would be nice if we found it was that simple.

The whole PPM - PCM thing has me a bit worried, it makes sense that it has to be the PCM signal being used because the master transmitter in trainer mode just acts as a pass through for the signal from the buddy box straight to the master Tx module (Give or take, computer radios these days are able to pick and choose) So if the buddy box output signal is a PPM signal, as you mentioned Pete, 8 channels max, if the signal is PCM2048 then its 14 channels maximum. I am thinking that there has either been a modification to the standard timings of the PPM signal to allow the extra few channels, keeping within the 20ms window that the servos require, this signal possibly then being decoded somehow to then create the PCM required for the Tx module, or, the 3 radios with 12 or more channels use their PCM signal while being used together in a buddy box configuration, the latter sounds more realistic to me.


It would be great to be able to plug one in and check it out first hand, however I don't have that option at the moment. Maybe I need to sus out whether I'm able to generate some PCM signals with my PIC.

Anyways thanks again. Any more for any more?

Brad

[Malc C]

Brad, have you tried reading the signals via a scope (true or windows -soundcard based) to see what it looks like ?

[Acetronics]

Hi, Darbley

When using PPM, the last channels are multiplexed at a slower rate than normally ... so, they are supposed not to be quick response ones ...

so, you transmit

1st frame : 1 ....7,8
2nd frame : 1.....7,9
3rd frame : 1 ....7, 10

and the decoder outputs 3 times or more the same signal on the lasts channels ...

or ... more than one channel is outputted at the same time ...

ex : Ch 1, 5 and 9 can have synchronous leading edge ( but not trailing Edge ... LOL ! )
: Ch 2, 6 and 10 ..............d° ......................

etc. etc.

so, signal outputting is much shorter than reading the serial signal ...

I think I saw in those columns a DIY decoder working like this ... from somewhere in New Zealand !!!

Alain

[DarbleyJ]

Thanks for the input Alain, I can see how both of the methods you mention would work. Do the 12/14 channel futaba radios use either of these methods on their trainer/buddy box input?

Malc, unfortunately I don't have access to a 12FG/Z/14MZ to be able to hook it up to a CRO. It would be nice to be able to do that and would surely sort me out.

Maybe someone has done this already?

Brad