AR6100 Servo Signals

[octane-link]

I am attempting to find some information about an AR6100 receiver before I go buy one, so hopefully someone knows.

I want to know how the signals are sent out of the pins, do they all go high at the same time, or is it set up similar to PPM where only one pin is high at any one time?

I am attempting to create a system to read the signals in and create a PPM signal to go to an autopilot built by someone else. That system only uses PPM signals, so I need to make it 2.4 GHz friendly.

[Fluffy Monster]

Looks like only the AR7000 & AR9000 have ServoSync, based upon if the others did have it, the Marketing people would have stuck it in a list of features. ServoSync does not work with module systems either.

[spinup]

I want to know how the signals are sent out of the pins, do they all go high at the same time, or is it set up similar to PPM where only one pin is high at any one time?I think I've read in these forums that the AR6100 does something in between those two choices. It sends the servo signals sequentially BUT they are slammed out one after another with no delay between each pulse. This makes it kinda difficult to separate the signals for a system that wants PPM.

EDIT: Actually, reading the first link below, it looks like there is a delay between pulses. Not the ~300us delay of a "normal" PPM signal, but still long enough to separate the pulses from a single wire. The OP in that thread still needed a converter to massage the pulses into a PPM format that the mixer would accept. But that converter did tie all the PWM outputs to a single wire.

I have a few threads bookmarked for this. This was the big one (on another forum):
http://www.rc-cam.com/forum/index.php?showtopic=2064

And at least one of these threads also discussed the problem:
http://www.rcgroups.com/forums/showthread.php?t=885325
http://www.rcgroups.com/forums/showthread.php?t=624290

Someone in these threads also mentioned this product as a possible solution:
http://www.mftech.de/catalog/product_info.php?cPath=23&products_id=44&osCsid=a0111a63e20a1cf35dafaf01e67fb852

Good luck. If you find out more, please post it here! :)

-Jeff

[AndyKunz]

I think I've read in these forums that the AR6100 does something in between those two choices. It sends the servo signals sequentially BUT they are slammed out one after another with no delay between each pulse. This makes it kinda difficult to separate the signals for a system that wants PPM.

That's exactly how they were done in the old days. The pulse train merely shifted the 1 across the shift register. What do you need a delay for? Interrupt-on-change works fine.

Andy

[cstratton]

I think the real question might be if there's enough delay that you could "OR" the servo outputs together to recreate a traditional PPM multiplexed stream or at least capture them with a single uC input?

[AndyKunz]

The PPM pulses are about 300us wide, with the rising edge determining the start of the shift. If you OR the servo outputs from an old receiver together, you should see a continuous high for the entire time, perhaps with a very narrow (a few ns) off-time as the shift occurs. It wouldn't be reliable.

So in short, the answer is NO. Not only that, but different brands and even different rx/tx combinations put the channels out in different orders. IOC is the reliable way to reconstruct the pulses.

Andy

[spinup]

Laughing...I should've defined "kinda difficult". :) Building a microcontroller-based adapter with your own ISR code for multiple-PWM to single-PPM conversion, I'd say that's "kinda difficult" for most RC hobbyists. ;)

Andy, that is interesting about the pulse timing in older rx's. So I guess it might be more difficult to do PWM-to-PPM conversion on an older rx. But...you can just grab the PPM signal before the shift register, so it isn't an issue, right?

I assume the Spektrum rx's output PWM pulses from a microcontroller, so there might be a longer (software-induced) delay between pulses. Nothing like 300us, but maybe 1us. Perhaps long enough to detect with single-pin IOC on the OR'd outputs.

But anyway, we don't know how the autopilot reads PPM (pulse timing, shift, channel order, etc). Chances are you'd need to buy or build a "kinda difficult" converter anyway. :)

-Jeff

[AndyKunz]

Oh. For me it was the easiest way to do it with a PIC.

It doesn't have to be an "older" receiver to grab the PPM stream. A little googling for the Hitec rx's here will pull up mods for the I think the Electron and 555 which will show you how to grab the data.

The Cypress parts that Spektrum uses are pretty configurable for generating things like pulses from the hardware directly, with no software interaction. I wouldn't be surprised if they had comparable performance to a shift register (it's not hard to do a shift register/timer combination in a PSOC). If I were designing their rx, that's exactly how I would have done it.

Andy

[spinup]

Right, it doesn't "have" to be an old receiver to easily grab the PPM stream. Probably most "new" 72mhz rx's use a simple shift register. Heck, I even read somewhere that you can snag PPM from inside some Futaba 2.4ghz rx.

My point was, you can't grab a PPM signal on a Spektrum rx, BUT because of the way that signal is processed, you might have a better chance of capturing all the PWM signals on a single pin.

This is just a guess. I know the PSOCs have some nifty general functional blocks, but can they really convert the DSM2 digital serial bit stream (with error correction) to PWM channels purely in hardware? Can you use the PWM blocks to generate pulses whose widths change on every frame? If not, the micro core on the PSOC is burning cycles for each channel. In that case, the delay between each channel could be 10x-100x the gate-level delay of a shift register. Long enough to detect reliably on a single pin.

In any case, it seems clear that you need a converter to get PPM from an AR6100. The only question I have is whether that converter needs 6 capture pins, or just 1.

-Jeff

[AndyKunz]

6. Just checked on my scope.

Andy

[spinup]

This is too funny. :) While you were checking, I was reading the thread I linked earlier:
http://www.rc-cam.com/forum/index.php?showtopic=2064

...and I had to edit my first post in this thread to clarify that there is a large enough delay between the pulses. Well, at least on the ar7000, according to that thread. Arthur posted the scope captures in that thread. He ended up building his converter with all the PWM outputs tied together.

There's another related thread in this forum:
http://www.rcgroups.com/forums/showthread.php?t=714299

It looks like this had some good info about the ar7000 internals, but that stuff was deleted last month. :( Ah well.

OK, so Andy, how long a delay did you find between the PWM outputs on the ar6100?

-Jeff

[AndyKunz]

Jeff,

I'll pick up my scope at lunch and post pix.

Andy

[spinup]

Sweet! Thanks Andy.

For comparison, post #30 in the rc-cam thread has the scope capture after Arthur tied all the PWM channels together:
http://www.rc-cam.com/forum/index.php?showtopic=2064
Unfortunately you have to register/login to see the pictures on that forum. :(

That capture is a coarse 2mS/DIV, so it is hard to tell exactly how long the delays are. The line widths for the rising and falling edges on the scope cover about 100us. I was guessing that since they appear at all, the delay is at least in the usec range. Maybe not. I'm very interested to see what you find at a higher resolution.

-Jeff

[AndyKunz]

I don't know what I was doing before, but I measured 60us between channels with an AR7000 (the only one I had handy). I know that they come out in a different sequence when you use different rx/tx combinations. That in itself might prevent making a general use of the data stream.

My WaveStar software is timing out talking on the RS-232 (different PC than usual). I'll get pix up when I resolve that.

Andy