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There isn't really any documentation about the 2 FrSky pages in ER9X's menus...

Lack of detailed user manuals is an ongoing issue with these kind of projects. So outside of looking at source code, finding how a software feature works is mostly experimentation and stick poking.

BTW, the D4FR is about to be replaced by the D4FRII which has the A2 connection and serial port available on a connector.

That is good to know.

So I now have configured a Locosys LS20038 with settings saved in ROM, and all is good. Simplified the sensor hub cable a bit, removing a ground wire to eliminate a ground loop.

Very nice. It would be cool if the GPS could be used without the sensor hub.

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Very nice. It would be cool if the GPS could be used without the sensor hub.

Yes! I agree about the other sensors needing the hub, but indeed the GPS would already be interesting on its own.

Hmm, what about a little 8-pin PIC between the GPS and receiver to fake the sensor hub frame?

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Weren't you the PIC wizard with NMEA parsing code already at hand? ;)

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You must have me confused with that younger guy that use to have some free time. :)

Given all the activity around these open source projects, it seems likely that someone has already created a DiY hub of some kind. For sure, it would be nice to find a published project to use a starting point for the GPS hack. So keep your eyes on alert.

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Kilrah, you are a monster. Because now I think we need to design our own FrSky sub-hub. Have you found any published projects that would eliminate re-inventing this wheel?

Here's my thoughts. The goal is a cheap DiY GPS for the FrSky that eliminates the big Hub box gadget.

1. Find the cheapest GPS module available that works well. Something from a popular China shop. Target price would be under $20. Of course generic modules would be supported, but it would be nice to identify a cheap GPS for others to buy. Any suggestions?

2. We need a PIC with a hardware UART to talk to the GPS. The hub interface UART can be a bit-bang'd so no need for two "real" UARTs. I suggest the PIC16F913 because I have them and they have the on-chip resources for job.

3. Include a 3-wire port for an Allegro 100A current sensor.

4. Include an attenuator for a 0-40V monitor.

5. Find the free time to design/test/debug.

Item 5 is the only big issue I can think of. The rest are easy-peasy, relatively speaking.

My first thought is that it might be more practical to just strip down the FrSky hub (remove case and cut off the unneeded connectors). Would this be a better way to go if I wanted something small? I don't have the hub box, so it is a mystery to me if there are easy opportunities to "repackage" it.

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Hehe :P

Have you found any published projects that would eliminate re-inventing this wheel?

I had a look yesterday and found someone on RCG who seemed motivated, but no news since... I've posted in the topic to ask, no answer yet. Maybe a little PM will be worth it in a couple of days if there's still no answer.

1. Find the cheapest GPS module available that works well. Something from a popular China shop. Target price would be under $20. Of course generic modules would be supported, but it would be nice to identify a cheap GPS for others to buy. Any suggestions?

If we want something good it would be nice to stick to something known - Maybe the GTPA010 / FGPMMOPA6B (used in most new OSD, and in the actual FrSky sensor)? Problem with these tiny modules is there's no battery backup. Or there's still the LS20038 that I used, I still have a bunch of them and they have the advantage of having that little space next to the antenna that could be used to hold a couple of components like a supercap and diode for backup... pretty cheap too from the manufacturer

2. We need a PIC with a hardware UART to talk to the GPS. The hub interface UART can be a bit-bang'd so no need for two "real" UARTs.

Or we could avoid talking to the GPS altogether, and use the single UART's TX to talk to the RX :)

I suggest the PIC16F913 because I have them and they have the on-chip resources for job.

3. Include a 3-wire port for an Allegro 100A current sensor.

4. Include an attenuator for a 0-40V monitor.

Hmm that looks big! The idea I had was really to use a little 8-pin part, hardwired in the GPS's cable with no other component. Maybe just stack a small battery, diode and the 8-pin SOIC PIC next to the LS20038's antenna. I think anything else would just be a lot of work for little advantage, as it would become as big (or bigger) as the sensor hub. Omitting the fact that we'd have something custom we can adapt for more useful sensors, or everything else we might like to do instead of having to stick with the FrSky set of sensors and protocol :)

My first thought is that it might be more practical to just strip down the FrSky hub (remove case and cut off the unneeded connectors). Would this be a better way to go if I wanted something small?

Yep it would as long as you has a use for more than just the GPS. Firstly, removing the case shaves off 5mm in both dimensions, leaving a 30x25mm PCB. Unpack the vario sensor, remove the long wires, stick it to the back of the hub with double sided tape, remove hub connector, solder tiny wires directly to the PCB, then do the same with the GPS, and accel sensor if you want it, and you have something that's already tiny... but still 3 times the size of the LS20038 alone ;)

I'll go with that first and take some pics.

Edited by Kilrah

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If we want something good it would be nice to stick to something known - Maybe the GTPA010 / FGPMMOPA6B (used in most new OSD, and in the actual FrSky sensor)? Problem with these tiny modules is there's no battery backup. Or there's still the LS20038 that I used

Anything cheap, readily available (to hobbyists), and reliable is fine. I searched around and the only low cost GPS I saw was the EM411 ($28 USD at DX), which is old technology and it doesn't have a SAW filter in it. There's some low cost modules for around $15-20 but they don't include an antenna. The likely candidates I saw were $45+. Quite a disappointment since I figured that China would be selling these things for pennies by now. But I'll try again and search out the FGPMMOPA6B.

Or we could avoid talking to the GPS altogether, and use the single UART's TX to talk to the RX

I was planning on having the PIC configure the GPS. But that task could be forced upon the user or be eliminated by limiting the GPS module choices.

The idea I had was really to use a little 8-pin part, hardwired in the GPS's cable with no other component.

Just a little PIC wrapped in heatshrink would be very cool. But the project gets a bit risky with a small part like the 8-pin PIC's. Debugging becomes a major hassle and memory size is often too limited. Plus it would be nice to be able to expand it later on. So that is why I leaned towards a bigger part. Which 8-Pin'er did you have in mind?

but still 3 times the size of the LS20038 alone ;)

I'll go with that first and take some pics.

Thanks. That will help me decide if a custom PIC solution is worth the trouble.

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If we want something good it would be nice to stick to something known - Maybe the GTPA010 / FGPMMOPA6B

I found that the FGPMMOPA6B GPS module is only $24 (shipped) from aliexpress.com. Other shops have it for about $40. It has a SAW filter too. So that's the one I would vote for. :)

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That looks good. Where did you get the BMP085 baro sensor?

Earlier today I submitted an order for some GPS modules at the aliexpress site. But they wouldn't accept PayPal, so I'll have to go elsewhere.

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That's the official FrSky sensor that I unpacked.

Good to know. I think I can DiY my own baro if that's all they have on their PCB.

Just curious, where does the ER9X display the baro altitude? Do you lose GPS altitude when the baro is installed?

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There's just the baro sensor and a 3.3V Vreg on the PCB. Doesn't even look like there's a level converter for the communication pins.

When you get to the telementry page on ER9X (by scrolling on the home page, or from r742 on directly with [LONG DOWN]), you can cycle between the basic screen, hub screen (with baro altitude), and GPS screen with

. These pages only appear if a hub is connected to the RX and ER9x sees hub-type frames coming in.

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These pages only appear if a hub is connected to the RX and ER9x sees hub-type frames coming in.

Thanks for the explanation.

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I finally got around to performing the FrSky failsafe setup and I'm not having any success. I hold the Tx sticks at the positions I want during Failsafe, press the little button on the Rx for 1-sec, and nothing happens. I'm suppose to hear a beep at the Tx. When I turn off the Tx the model does the default Failsafe "hold" instead of observing the failsafe pre-set position that I need. So I could use a bit of advice on how to make the failsafe feature work.

Edit/Update: I spent more time with it and after several attempts the Tx finally responded (with a beep) upon a Rx button push. Now the failsafe works. Then I tried again and it appears that setting the failsafe pre-sets is very intermittent. It ignores the button press 99% of the time.

Not sure what is going on. Might be a bad push switch on the Rx, so tomorrow when I have more time I'll try my other FrSky rx. But I won't be surprised if it turns out I am doing it wrong.

Edited by Mr.RC-Cam
Got it to work.

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Hmm I haven't tried setting failsafe yet.

I've however read there are 2 firmware "sets" (for TX and RX), the first one (normally stock) that sets failsafe like described in the manual with a press on the receiver's button, while the other set uses the TX's button. Of course the matching set bust be installed on both ends.

I'll have a try with my stuff...

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Hmm I just tried and had trouble at the start as well, however once I got it (looked like kind of a "burn-in"??? or it was me...) it seemed to work all the time on 5 receivers and 3 TX modules I have here on the table. The manual actually says "less than 1 second", and it seems all it takes is just a very short press.

Edited by Kilrah

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Thanks for checking. You must have the magic touch. Later today I will try again.

BTW, it is interesting that you mention there is another firmware version that involves pressing the Tx button instead of the Rx button. While I was struggling with getting it to work, I recall I did press the Tx button a couple times along the way -- maybe that is the reason I got it to work (without understanding why).

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Also, note that it seems only the "large" RXs (D8R***) do the beep on the TX. I have some D6FRs and D4FRs, and those rather blink the green LED on the RX twice.

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I have no explanation, but I did exactly the same button pressing today and everything worked perfectly. Go figure.

Also, note that it seems only the "large" RXs (D8R***) do the beep on the TX. I have some D6FRs and D4FRs, and those rather blink the green LED on the RX twice.

All the tests yesterday were with the 8-CH. Today I tried the 4-CH. They both work as you describe. Thanks for the help!

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Cool :)

Pretty much unrelated, but I checked my 3 modules on the SA. My DHT outputs ~20mW, my DJT about 35mW, and my DHT-U 65mW. That's some quite serious sample variation...

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My DHT outputs ~20mW, my DJT about 35mW, and my DHT-U 65mW. That's some quite serious sample variation...

That is a bit unusual. If you are measuring it from the SMA connector cable that each Tx module provided, maybe the cable itself is the problem. Especially if they are slightly different in length or the factory used different coax cable brands. Perhaps a good test would be to move the cable on the 65mW Tx to the 20mW Tx and measure again. However, if you measured at the U.FL connector then the issue is in the RF circuitry for sure.

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