Using PPM External modules

I'm trying to get my frsky external module to work. All I should need is power and PPM, but I can't seem to get PPM out of my devo12... even though PPM is working very nicely for phoenix. I just can't seem to get a plug working.

Anyway I was thinking we could discuss external modules in one thread and that we could let others know how/what we got working. And where you got the ppm signal from, trainer port or internal pin...

Ok, so I take it that the PPM signal has to be grounded back to the same power source? I'm not really sure about this stuff so any help would be appreciated.

If I only connect PPM wire to the DHT from a 9x and power the DHT with a battery I get no PPM control, but if I power the DHT from the 9x it works fine. So if I need to power the module from an external battery, (becuase the 12s is only 3.3v) how to I ground it so that PPM will work?

If you want to send digital data from tx to module over wires you do this with a voltage difference which is nearly 0V (what means a logical 0 or 1 depending on the definition) or quite different from 0V so it can be recognized (and what represents the other logical state). Also PPM is a kind of analog signal, the values on the line are digital.
To measure / read them you need a reference point. In almost any cases ground is used for this.
So you will have to make a connection between the tx-ground and the module-ground. If you have this, they have their common reference point and a voltage on either side can be measured against ground on the other.
Short: yes, you need a connection between the grounds and one for the data.

Hi Hex

To add what to what RBE said about references - you can use the external battery as you described, but then you have to connect both PPM and the battery negative line (the DHT module reference) to the negative of the TX (the TX reference).

(This is normally safe to do with battery equipment, be careful however when one or both are mains supplied.)

WooHoo! It works, I'll post some pictures when I have it looking a little better.

Hi Hex, I don't have a 12s.

Do you perhaps have a multimeter, a volt or ohm meter would do to determine which is which. The metal can of the Rx module would be negative or ground - using the voltmeter and 5V range on the meter, you should see no voltage difference between the rx can and ground, while you would see some voltage between the PPM signal line and the rx can. (Obviously only when the tx is on)

Thanks... Yep, the tip is signal as I thought. So I'm going to wire up the any link, and an frsky homemade anylink style module.

.... My anylink / frsky cable.