Daisy Chain Hack Module to Receiver

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Rob Thomson
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Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Fri Sep 13, 2013 4:42 pm

Right.

This is very hypothetical.

I am in the process of building a small RSSI based FPV tracker system. Something along the lines of this:

http://www.rc-concepts.com/#!product/pr ... er-for-fpv

Now... with this particular build, I will be relaying my video signal with an additional transmitter. The idea being to cut out the need for my headset to be hooked up by cables to the moving antenna.

This got me thinking.

Can I do a similar thing with the RF signal for the frsky system, and simply mount a directional antenna on the same tracker.

My thoughts being.

1xFrSky Hack Module.
1 x 4ch RX in PPMSUM mode.

Theoretically.. I guess I could take bind my radio to the RX, then link the two together - effecticely relaying the PPM connection from RX to HACK Module and on to the model.

The theory is ok - in practice.. would it work?

Rob
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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Fri Sep 13, 2013 6:12 pm

Having second thoughts on this...

I may just put together a dedicated 9x for the fpv purpose - then make a custom module to feed the signal & telemetry through a slip ring to the moving platter.

This will probably be much safer - and retain telemetry/audio on my sky board :-)

Step 1... build the tracker. Step 2 Worry about this minor detail!
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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Mon Oct 07, 2013 7:10 pm

So... did a few tests on this subject today.

This is interesting - and actually works quite well!


Essentially, if you put an frsky rx into CPPM mode, and wire the signal pin, and the serial pins into the respecive pins on the frsky module... it simply relays the signal from:

transmitter => receiver => hack module => receiver

It 'just works'.

Now.. why would you want to do this?

In my case - I have a tracking antenna for fpv. My video signal is already relayed from this antenna over rf to my goggles. So adding the 2.4g repeater gives me two options.

1. wire free!
2. [the important one] I can use a high gain directional patch antenna that auto tracks with the video signal. Instant tracking long range antenna :-)

Rob
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Re: Daisy Chain Hack Module to Receiver

Post by ShowMaster » Mon Oct 07, 2013 8:06 pm

I've always wondered why FPV flyers don't put all the high powered UHF or other freq systems along with high gain antennas on a tripod remoted from the actual tx flying the plane?
Using cppm I would think you could eliminate most or all of the RFI, buzz, control issues associated with mounting a 2 watt power (or higher) module or amp on your tx. Not to mention that it all could be powered by a LARGE battery source now. Most serious FPV flyers I see bring a tripod with high gain antennas on the for video but still have all that "gingerbread" rf electronics on their poor tx.
Am I wrong in thinking that a Frsky tx link to a Frsky cppm rec would not only reduce clutter the tx but stop the tx from being overloaded rf and power source wise?

I have visions of a tripod, a suitcase full of required electronics at the tripod, a cppm mode rec and a car battery. At the tx I have visions of my tx talking to the cppm receiver far enough away from the tripod to keep its rf system out of the tx. I'd also have my goggles receiver and lipo power source in a small belt pack. I realize the video antenna may be an issue so maybe with your tracking idea I would downlink to the tripod on one band and rebroadcast it on another as to not conflict?
The idea is not have so much high power rf devices on the tx as well as total wireless freedom. All in a fast setup case.
I traveled with news gathering all over the world and we had flyaway cases of up/down link equipment. I could be live to New York 30 min after driving with the equipment.
I've avoided FPV because I lived RF and video for years and all it's issues. With the recent advancements in FPV I'm getting the bug.
I had to buy test equipment first and I'm good their now. Got freedom to use a calibrated power and spectrum analyzer if I want to drive 20 miles to the studios so I'm good there.
So what about my ideas? Did I miss something or think wrong?

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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Mon Oct 07, 2013 8:28 pm

I gave found one issue.

The serial goes down the wire OK.. But seems to get corrupted on route.

A bit like the two receivers are sending the same information?

Does this make sense?

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Re: Daisy Chain Hack Module to Receiver

Post by Kilrah » Mon Oct 07, 2013 8:36 pm

Getting many low power links to work together isn't necessarily easier than just one big fat one. You need different frequency bands for everything, which usually counts up to 4. Not many choices at this point, forcing you to use like 5.8 for airborne video, which some don't want because it's easily blocked by vegetation...

It's also annoying when you've got big antennas in a diversity setup on an antenna tracker to ensure a excellent link to the ground... only to get static/noise/dropouts from your local link.

Also note that chaining 2 FrSky links like Rob did will result in a good 100ms of latency, might not be comfiratble depending on the model.

I know several people who did that kind of setup but they all went back to the "normal" way for all those reasons.

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Re: Daisy Chain Hack Module to Receiver

Post by Kilrah » Mon Oct 07, 2013 8:38 pm

Rob Thomson wrote:I gave found one issue.

The serial goes down the wire OK.. But seems to get corrupted on route.
Are you trying to relay the telemetry down too? That won't work, as the data will be framed a second time by the 2nd link. The radio can't know about that (unless this is coded in of course).

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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Mon Oct 07, 2013 8:47 pm

Yup.

It sort of works... But goes wrong occasionally.

What would it take to code in the change?

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Re: Daisy Chain Hack Module to Receiver

Post by Kilrah » Mon Oct 07, 2013 8:59 pm

Basically instead of getting a data frame with RSSI and user data (hub), you've got a frame with RSSI/A1/A2 of the ground link, and in the user data is the whole frame of the air link (RSSI/A1/A2 + user data).

The radio will display the RSSI of the ground link (useless) and probably won't understand any of the rest.
You need to strip the outer wrapping.

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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Tue Oct 08, 2013 5:57 am

Right. Two ways forward from here.

- Some de-capsulate code on the tx.

or

- an updated firmware on a rx, that allows direct pass through of all information.

I personally think the best place for this is on the RX. It would open up lots of possibilities to use the RX/TX as a wireless modem for arduino/robotics etc.

Had a chat with Eva from Frsky this morning on the matter. She will be raising it in he next tech meeting. Maybe something will come of it!

If not.. time to code a protocol decapsulator!
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Re: Daisy Chain Hack Module to Receiver

Post by Kilrah » Tue Oct 08, 2013 7:36 am

I really doubt FrSky will do what would be a complete revision of the D8 protocol (can't send upstream data currently and the bandwidth wouldn't really make it possible) and break compatibility with all their gear! But we'll see...

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Re: Daisy Chain Hack Module to Receiver

Post by Kilrah » Tue Oct 08, 2013 9:44 am

After closer investigation chaining both and unframing in the radio will not work reliably.
The bandwidth used by chaining the 2 will exceed the serial port's resulting in loss of data.

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Re: Daisy Chain Hack Module to Receiver

Post by Rob Thomson » Tue Oct 08, 2013 5:23 pm

Had a chat with Mike on this earlier today.

Came to the same conclusion.

My next course of action will be an arduino module to translate key data from one to the other.

That way only what we really need would be sent.

So.. In my case, I would map the remote rssi to something like temperature. Its a bit messy, but would allow this critical message to get triggered by a custom alert.


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