Hacking receiver for PPM - signal inverted??

[weg22]

Hi all,

I opened up my Futaba receiver in search of the PPM signal. However, when reading it with an oscilloscope as shown on the following web page:

http://www.nobugs.org/engineer/uav/futaba-rx.html

I see the 9 pulses which correspond to my 8-channel receiver, but when moving one of the sticks on the transmitter the low portion of the pulse extends/contracts and not the high portion. It seems like an inverted PPM? Does anyone know what's going on? Is my interpretation of PPM (I thought it should show the high portion of the pulse extending/contracting) incorrect?

Thanks in advance,
-weg

[Mr.RC-CAM]

The demodulated and conditioned PPM's logic polarity, inside the Rx, can be whatever the responsible engineer needed to satisfy the input requirements to his decoder. This is NOT a FFM RF shift issue, but is a data slicer circuit thing that is determined by the overall design. If the observed signal is inverted from what you need, then just flip it over in your external circuit or accommodate it in your software.

[weg22]

What exactly do I need to "flip it over"? Some sort of resistor-transistor circuit?

Thanks in advance,
-weg

[Mr.RC-CAM]

Any inverter configuration you wish to use. Could be as basic as a NPN operated as a saturated switch. The options are endless. There may even be a compatible signal already on your Rx board.

[sarabi]

Hi all,

I opened up my Futaba receiver in search of the PPM signal. However, when reading it with an oscilloscope as shown on the following web page:

http://www.nobugs.org/engineer/uav/futaba-rx.html

I see the 9 pulses which correspond to my 8-channel receiver, but when moving one of the sticks on the transmitter the low portion of the pulse extends/contracts and not the high portion. It seems like an inverted PPM? Does anyone know what's going on? Is my interpretation of PPM (I thought it should show the high portion of the pulse extending/contracting) incorrect?

Thanks in advance,
-weg


see this Thread
http://www.rcgroups.com/forums/showthread.php?p=13911694#post13911694

[Tom Harper]

The signal is the time between the pulses not the pulse width. Demodulation is done by using the pulses to clock a single bit through a shift register.

[zlite]

We at DIY Drones sell a $25 board (http://store.diydrones.com/ProductDetails.asp?ProductCode=BR%2DPPME) that does this all for you, with no need to hack into your Rx. You can invert the signals, too.

[bmw330i]

PPZUAV has the orig. design (25.00): http://ppzuav.com/osc/catalog/product_info.php?cPath=13_11&products_id=91

The design and code that runs on it was created and donated by "hendrix" here on RCGroups. It works extremely well.

Latest Code, Eagle design files and other details here: http://paparazzi.enac.fr/wiki/Other_Hardware#PPM_Encoder_Board

-David

[Joshbb]

Hi can we get these in Europe?

cheers

Josh
PPZUAV has the orig. design (25.00): http://ppzuav.com/osc/catalog/product_info.php?cPath=13_11&products_id=91

The design and code that runs on it was created and donated by "hendrix" here on RCGroups. It works extremely well.

Latest Code, Eagle design files and other details here: http://paparazzi.enac.fr/wiki/Other_Hardware#PPM_Encoder_Board

-David

[rbeall]

Simple transistor circuit to flip the output. If you are clean with it you can just wrap the transisitor and resisitor up in shrink wrap and its an inline fix.

[bmw330i]

Hi can we get these in Europe?

cheers

Josh

Of course.

-David