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rf communication

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tomas632

New Member
Hi Guys,
I have a project which involves a remote controlled helicopter, the idea is to use a pc to control it rather than the remote that is supplied with it. However I have no information with the controller that tells me what signals it sends out to communicate with the helicopters receiver and our tutor wants us to find this information ourselves, is this communication pretty standard or would it differ with different applications? I am just trying to find something that shows me things like the frequency used, what bit pattern represents up/down etc. any help would be greatly appreciated
Thanks
Tom
 

Sceadwian

Banned
Before anyone here responds, I would recommend you start using Google search. I'll give you a tip. Analog RC communication uses something called a PPM baseband signal, each of the servo's are encoded in serial on this baseband signal and sent over RF. PCM is an all digital version which you may or not want to find out about. Google should take it from there. TONS of information about this on the net.

If you still have trouble after trying to do the research yourself please ask any questions that come up in here, and keep researching, I'll help at that point =)
 
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tomas632

New Member
hi, thanks for the quick response. well i have looked for pulse position modulation and i wouldnt say there is tons of info on it unless i'm looking in the wrong place. what ive found so far is that the signal frequency stays constant, but the position of each pulse is at different places hence the name ppm. is this correct? If you could offer any more info then that would be great as i've not managed to find much more than this, also if you know where there is a diagram showing this then that would be excellent.
 

tomas632

New Member
just this minute found something else, hope you can tell me if im on the right track...

to make it simple lets say we have 1 controlling "stick" that can move in 4 directions, this means there will be 4 pulses sent? the more the stick is pushed in one direction the wider the pulse is and vice versa. at the rx end this tells the servo how much to rotate, and the position of each pulse in the sequence tells you which servo needs to be driven? there is then a gap of a certain time length that means the string of pulses has finished and the next set for each servo can be received?
 

Sceadwian

Banned
Look up RC ppm (Google is VERY keyword sensitive)

1 controlling stick generally only can move in 2 directions, horizontally or vertically, the center spring makes it seem like four directions. You would get two signals out, 1 for the left/right, one for the up/down. Yeah you pretty much have it right, it's relatively flexible. Basicaly the servo signal itself is 1-2ms in length 1.5 being center. I'm not sure what the minimum limit on the blanking period between pulses is but it is probably very small. Any pulse over the maximum servo length is received as a reset pulse so the receiver goes back to the first servo and goes down the line again sequentially. This is why a glitching RC setups can act very oddly as applied noise or reception issues can cause it to desync with the frame pulse and send either noise or the wrong servo pulse to a specific servo. PCM is a digitally sent version of the same signals however it will tolerate noise up to a certain point and then glitch horribly badly and then you get nothing at all. Many RC'ers still prefer PPM because by judging the reaction of the vehicle to their inputs they can determine the signal quality.
 
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