Getting PPM-stream data out of the receiver

Support related stuff, for various products
Daniel Wee
Site Admin
Posts: 2052
Joined: Wed 25 Feb 25 2009 8:00 pm

Getting PPM-stream data out of the receiver

Postby Daniel Wee » Mon 02 Nov 02 2009 1:06 pm

When a PPM-stream is needed from a radio, usually some kind of modification needs to be made. Most PPM receivers will have no problems with this since there will be the PPM-stream available somewhere in the receiver so what you basically need to do is to open the radio up and find the data stream and hook a wire up to it to, possibly, an unused servo channel (after cutting the trace for that channel first, of course).

For PCM radios, it's a bit more of a toss-up since some PCM radios do have the PPM data stream available on board although in some cases it may be limited (in channels - FAAST radios have a 5-channels PPM-stream available internally). In other cases the internal design may be a pure digital to servo implementation, in which case there will be no PPM stream available anywhere. For these, a channel multiplexer, active or passive would be required.

Some link(s) detailing such options follow:-
http://www.rcgroups.com/forums/showthread.php?t=1000132
http://www.rc-cam.com/forum/index.php?/ ... e__st__120
http://www.mftech.de/catalog/product_in ... ucts_id=44
http://store.diydrones.com/product_p/br-ppme.htm
http://jreise.de/PPM/R617FS.html
http://paparazzi.enac.fr/wiki/Other_Har ... _Receivers
http://www.mikrokopter.de/ucwiki/Katego ... %C3%A4nger
http://chebuzz.com/paparazzi/index.php? ... ucts_id=26
http://chebuzz.com/paparazzi/index.php? ... 0a57e1e7be
http://radrotary.com/OMMrxconverterRev2 ... rRev2.html
http://www.flashrc.com/mikrokopter/4444 ... serie.html
http://www.flashrc.com/flyboard/5470-se ... oudes.html
http://www.flashrc.com/flyboard/5929-se ... oudes.html

Daniel

Daniel Wee
Site Admin
Posts: 2052
Joined: Wed 25 Feb 25 2009 8:00 pm

Re: Getting PPM-stream data out of the receiver

Postby Daniel Wee » Wed 08 Sep 08 2010 1:42 am

The DOSD+ can basically fly your plane for you, but in order to do so, it would need to control your servos. However, at the same time, you want to retain manual control over your plane. In order to do this, for example, the DOSD+ will need to know your stick inputs and as such you will need to feed the receiver output to the DOSD+.

Now, you could use one wire for every channel that you want to tell the DOSD+ about. One way would be to connect one cable for the aileron output from the receiver to the DOSD+, another cable for the elevator, another for the throttle, another for the control, another for the rudder, and another for whatever functions you may need. As you can imagine, this scheme gets messy real fast with a large number of cables going from the receiver to the DOSD+. This was how we used to do things on the original DOSD, but with the DOSD+ we no longer do this because of the large number of cables that would be needed (and because I, as the designer, didn't like that old way.)

In order to circumvent this problem, we use a PPM data stream which contains all the channel information in one cable. In this way, you only ever need one cable from the receiver to the DOSD+ and the DOSD+ will know every input you make on your radio. This allows for the implementation of some very nifty features.

At this point, you'd be asking yourself - "Well, does my receiver have a PPM output stream? How do I get at it?" Receivers, can be thought of in three broad categories in this respect:-

1. Those that come with PPM output ready - such as the DragonLink and TLRS receivers, and also some more traditional receivers. You can further purchase some receivers that have been pre-modified for PPM output. If you are using one of these receivers, it would be a simple matter to just connect that one cable and you're ready to start using the advanced functions.

2. Receivers that do not have PPM ready but can be modified, usually by soldering a wire to the internal circuit board. Many Futaba receivers, for example, have a 5-channel PPM stream available internally. These modifications are usually documented to varying degrees and the resources for this is available on the DOSD+ support forums.

3. Receivers that simply do not have PPM available anywhere, such as some digital types. In this case, you would need to use an external multiplexer board which takes all the outputs of the receiver and combines them into one single PPM stream that can be used by the DOSD+. These multiplexers can also be purchased from various sites and the resources and links are available on the DOSD+ support forums as well. If you are handy with electronics, it would not be too hard to make your own multiplexer in some cases.

*The DOSD+ manual should contain more information about this subject.

I should add two things at this point. The first is that you DON'T need the PPM data stream for basic OSD functions. Even if you want to configure the DOSD+, you can simply use the excellent PC Commander software that Craig (Cralis) wrote, together with a USB-TTL-serial adapter (which costs less than USD6 on e-bay if you need to buy one). That should be enough to get you going and for the price, you'd have everything most of the other OSD's have and more. You will have home indicator, range indication, Voltage, Current, RSSI, GPS, IMU, HUD, altitude displays, along with audio variometer, and on and on. All of these you can have WITHOUT needing a PPM connection at all.

Where you will need the PPM data stream is when you want to use the advanced autopilot functions. This will enable features in your DOSD+ that compete with OSD's costing way more that what you paid for the DOSD+. As such, it would be worth the time, as you grow in your needs, to make the small effort to make a PPM data stream available to the DOSD+. Many have done this successfully and once you try it, you'll understand why it's not a big deal and the effort is more than paid off by the benefits gained. Having said that, do not feel like you need to jump into this aspect right away. The DOSD+ was designed as a very sophisticated instrument that will grow with you.

The second thing I wanted to add concerns this very sophistication of the DOSD+. Unlike many other OSD's, the DOSD+ is able to interface with many optional sensors. It can, for example, work with many different types of GPS modules, barometric altitude sensors, temperature sensors, magnetic compass, and so on. This is not your garden variety low-featured OSD you're dealing with. The features that it contains actually bring it into the low-end UAV market although it is being marketed primarily as an OSD. On top of that, new features are constantly being added (not just big fixes) so that your investment actually appreciates in value - that is, you are getting more and more for your money as time goes by. The feature list is getting so long that it becomes a bit hard to keep the documentation up to date (and even I forget how some of them work! sometimes).

Now, this kind of sophistication inevitably adds to the number of things you can do, but must learn about the OSD. Sure, you can use the basic stuff without ever going beyond the simple setup menus but you would be missing out on the really fun stuff. What I am trying to say is that, don't get put off by a lot of the technical discussion that sometimes go on in this forum because those are usually advanced stuff that the new DOSD+ user won't need to concern himself with. At the same time, it is worth making the effort to check out some of the features as they can greatly enhance your FPV experience, as well as provide a level of protection that you don't get with other OSD's.

Last of all, we like to think of DragonLabs (yes, NOT DragonLink - that's the name of the LRS product we make), not as a company out to make as much money as we can. We'd rather think of ourselves as providing a service to the FPV community while staying viable, and enabling technologies that allow you to push the limits of the FPV experience. There is a great group of people here that we like to think of as "family". Money isn't our bottom-line, having good fun is. In fact, we're not even a company - we're just a couple of guys like yourself who got caught up making some really nice toys for ourselves.

Daniel

Daniel Wee
Site Admin
Posts: 2052
Joined: Wed 25 Feb 25 2009 8:00 pm

Re: Getting PPM-stream data out of the receiver

Postby Daniel Wee » Wed 08 Sep 08 2010 1:58 am

There are two kinds of failsafes in operation. The first type is used with receivers that stop putting out PPM when they lose the signal and go into failsafe. Many PPM receivers are like this - when you turn off the transmitter, the receiver outputs nothing or just noise. When this happens, the DOSD+ will detect that PPM is lost and will activate failsafe mode according the failsafe parameters that have been set in the DOSD+ menu.

There is a second type of receiver which continues to put out PPM even when in failsafe mode. The DragonLink, TLRS, many PCM, and digital receivers (including the Spektrum and FAAST receivers) fall into this second category. When these receivers lose the signal, the will continue to put out a PPM signal, and in many cases it will put out the failsafe positions. In other words, the failsafe is stored and handled by the receiver. In this case, the DOSD+ never sees the PPM signal go missing and will not activate its own internal failsafe.

For this second type of receiver, you should program the CTRL channel on the receiver such that it will activate autopilot when it loses a signal and goes into failsafe. You should see that CH5 (or whichever the control channel is) goes into autopilot when you turn off your transmitter, if you have programmed the receiver failsafe correctly. This would turn put the plane into RTH mode. The correct settings for the receiver failsafe should be:-

Aileron, Elevator, Rudder - neutral
Control - AUTOPILOT
Throttle - the throttle you want to use during RTH (more complex if throttle control is used)
Flaps - neutral
Stabilization Gain control (if any) - set to where you want the gain to be for RTH

Daniel


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