As most of you may or may not know, I've been a big proponent of using PPM Sum (aka CPPM) with the Ardupilot since I first figured out how to make it work. The simplicity of just running a single wire from the Rx to the APM is very attractive. It eliminates 7 possible failure modes (losing the wire connecting channels 2-8) which could result in loss of control. There's only 1 wire to get right, but also one wire to get wrong. Fortunately, the APM can handle complete loss of signal well, and if you set your failsafe to handle complete signal loss, all is good.
I was using this on my HK450GT FBL helicopter, along with a FrSky D8R-SP Rx. Again, as anybody who knows me is aware, I'm a big fan of FrSky radio equipment. It is affordable, and high performance offering true diversity on the Rx, easy to use CPPM (just requires a jumper on two pins), the Rx is very tolerant of low voltage (it'll keep going LONG after the APM would have shut down, down below 3V). And rebooting and reacquiring the Tx signal takes less than 1 second.
All of these features are superior to the big name brands, Futaba, Spektrum, etc.
And the telemetry features offered by FrSky is just the icing on the cake. RSSI (signal strength), Rx voltage, external voltage (flight battery), and even the option for a telemetry hub with all kinds of goodies This is somewhat negated by using an APM, but still, you can put one of these on your Tx and have all the info right at your fingertips.
Anyway, back to my little 450 heli with a HUGE Rx mounted on it, the D8R-SP.
I discovered that FrSky offer a new Rx, the D4Fr which is a small, yet full-range Rx. And while it only has 4 channels out, it can be configured to do 8-channel CPPM with the simple addition of a jumper! Brilliant, and there's no reason why any modern compact receiver shouldn't be able to do this. This Rx does not have as many telemetry features as it's big brother. It can only report RSSI and Rx Voltage, it does not have connectors for the telemetry hub or external voltage. But that's fine, as the APM can be used to report voltage anyway. So I ordered one and it arrived Friday.
I started working on setting it up, when the wheels started turning in my brain. The "case" is just a flimsy plastic thing, almost useless anyway. And I'm only going to have a single cable going from the Rx to the APM, and it's so small. And the APM2 has those stand-off pins onto which the daughterboard is soldered. And those pins are kinda long... And I've got this piece of empty prototype board sitting around...
Pretty soon I had de-cased the Rx and stripped all the input pins off the APM2.
I soldered the proto-board to the daughter board standoffs, and then glued the Rx board onto the proto-board. I soldered jumpers on both the Rx and APM2 in order to set both into CPPM mode. And then I soldered a jumper wire for the CPPM signal, power and ground. Done.
The complete unit is compact. There's no wires to get in the way of whatever vibration-damping mount you are using. There are no wires to come disconnected. I suspect this setup is at least an order of magnitude more reliable than a standard 8-channel APM2 setup.
Even better, it's compact, and easy to move between vehicles.
Comments
@Lyn is that a copper PCB???????? !!
Ravi, sorry, I don't have a good answer for that. In the future, I would hope to make channel mapping to functions a flexible thing, but it's not, and this is one place where the CPPM has a weakness.
i am palnning to use the same receiver. but the mode is on CH8 IN of the APM. my Futaba 7C is only 7 channel. in conventional wiring i connect the CH7 of Rx to CH8 IN of APM. how do i use Channel 7 of Tx as mode channel when using PPM IN form FrSky. i think i have to assign CH7 as mode Channel in APM code.
@R_Lefebvre: no problem, the actual way to do it is much easier than what is in the wiki. Full telemetry would be nice, although already the signal loss warning is really useful.
@Veikko, will try tomorrow - don't have the unit here. However, it really couldn't be simpler:
Connect channel two of the D4FR to channel one of the APM2 as per normal, with a 3 wire female-female servo connector.
Put a jumper on the signal pins of channel 2 and 3 of the APM2 to short those two signal pins together.
Put a jumper on the signal pins of channel 3 and 4 of the D4FR to short them together (images in the FrSky manual for the D4FR are clear and correct).
Sorry you had to go through that. I had identified the wiki was wrong a few weeks ago, but somebody said they would fix it. I'll see what I can do.
Yeah, I also noticed the D4FR got a bit warmish, I wonder what it would be like in the case!
I just ordered a D4R-II, and I'm going to do this again, then hopefully get full telemetry working. That would be awesome!
I got the FrSky D4FR to try setting this up. Neat, small cheap. Trivial to set CPPM on it - the instructions are clear, jumper on the signal pins of channels 3 and 4. CPPM out on channel 2. Easy.
So then I went to the APM2 and tried to set that to CPPM (aka PPM sum) in. Tried the wiki method. Solder blob connecting the pads marked ->ppm and <-ch5. Connected the FrSky CPPM signal to pin 5 on the APM2. Nothing. Back to the wiki. The posts there say there is a link between the ->ppm and <-ppm pads at the jumper on the APM2 and that needs to be cut. Sure enough there was, I cut it, still no joy. Then I tried to get it to work as it had before with standard inputs on all channels by de-soldering the jumpers on APM2 to return it to normal. No joy there either now. Dead APM2. Poked it for a bit with no response then gave up and went for a pint.
This morning I soldered a blob onto the pads between ->ppm and <-ppm and tried it as normal - back to life HOORAY.
Then I read R_Lefebvre's post above where he writes that it is the signal pins 2 and 3 that need to be jumpered then CPPM in on pin 1 of the APM2. The wiki is wrong.
Tried R_lefebvre's method - jumper on the APM2 input signal pins 2 and 3, then put a female-female servo connector between the channel 1 input of the APM2 and channel 2 of the FrSky D4FR (to take the signal and power the receiver), and..... success.... complete success as far as I can tell.
Four comments:
1. It is really annoying that the wiki is wrong.
2. Getting rid of all those wires is fantastic - the setup ends up very neat even with the wire connection.
3. If you are using it with arduplane make sure you reassign the 6-way mode switch setup - channel 5 really is channel 5 now, not channel 8.
4. The receiver gets quite hot - I guess its the telemetry transmitter. Hot enough to deglue itself from the packaging. Suggests power consumption might not be as low as it could be....
Looks like Hobby King now has the D4R-II
http://www.hobbyking.com/hobbyking/store/__24788__FrSky_D4R_II_4ch_...
No, CPPM basically means passing the entire PPM stream from the Rx direct to the APM.
Yes, you can use it to run an APM with a single wire, I have done that already without any of this de-casing business. Just take the Rx, plug single wire into the CPPM out on the Rx (ch 8 in the case of FrSky) and then plug it into Ch1 on the APM (1 or 2, they're the same). Done. My big heli is set up with this using an FrSky D8R-SP Rx.
If the APM1 has the updated firmware for the PPM. The update happened sometimes around the new year. I have one that came with the update, and one that I purchased in October that came without. It can be upgraded if you have an old one.
That does appear to be correct. Please let us all know if it works.
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