buddy box wiring

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buddy box wiring

Postby mikem » Thu Oct 11, 2012 6:31 pm

I have some noobie questions about running two RC transmitters in trainer / buddy-box mode...

The transmitter I have on hand is a Futaba 7C FASST 2.4GHz rig. A second identical unit could plug in and work as a 'trainer' and would be ideal. Problem is, that's a lot of money spent for limited usefulness so I'd like to avoid it.

Plan B, considerably cheaper than a second 7C, would be getting a pair of more reasonably-priced transmitters. That option means swapping out receivers. Not a big problem but inelegant.

Getting one other brand of transmitter on hand would allow comparing the signals on the scope. There are no other RC people here to tap into unfortunately. (I suppose plan C is getting an order started with Hobby King since there is considerable lead time shipping here from HK... but which transmitter to test for compatibility?)

What I'm really wondering, does it makes sense to hack together a cable (and other electronics?) to match the Futaba box with some other transmitter? Are the trainer port signals anything alike between brands? If not, can the signals be massaged into a match (e.g., logic level shifter)? I can imagine that the signals are similar to I2C in that they need to be programmed into the system controllers. There is an interesting Wiki article on PPM which does not address trainer ports.

The 7C's 'trainer' port, the rectangular type, shows this pinout:
pin signal
1 n/c
2 GND
3 PPM OUT
4 Vcc, bat.+
5 Vcc (slave mode)
6 PPM IN

From Hobby King I got a trainer cable that mates with the 7C but has a round DIN plug (~12 mm round shell) on the other end. HK doesn't explain what the round plug mates with and there is no documentation. I assume it matches some other transmitter HK sells. Anyone know about that? I have another cable on order with the rect Futaba plugs on both ends (mostly so I can see how the signals are supposed to mate up).

Also I found a Futaba cable with a USB end to allow it run flight simulators, such as MSFS. The existence of such a cable seems to indicate that trainer port signals might have been standardized. Have been looking for such information online with no luck so far.

Any advice? I don't want to fry this box with a bad hookup.

I have a couple of friends with even less RC experience than me, if that is possible. I'd like to get them flying on a buddy box and maybe infect them with the RC bug.
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Re: buddy box wiring

Postby mikem » Fri Oct 12, 2012 1:04 am

I thought this was a simpler question I guess...

Here is a link that may be of interest:
http://users.belgacom.net/TX2TX/tx2tx/e ... 2txgb1.htm
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Re: buddy box wiring

Postby mikem » Fri Oct 12, 2012 7:53 pm

buddy box continued...

It looks like the fly in the ointment is the polarity of the PPM signal wrt the baseline. Futaba and Hitec pulse the signal below the baseline, for instance and others above.
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Re: buddy box wiring

Postby Bio Hazzard » Sat Oct 13, 2012 1:37 pm

If you figured something out I'd be interested, as I'm trying to teach my friends to fly my planes but its hard as it takes to long to swap the tx back and forth. Even just tapping into the trimpots from the sticks of a cheaper controller, scrapping the circuit board and using a homebrewed contraption to send out the ppm for the master tx to receive.
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Re: buddy box wiring

Postby BillGriffiths100 » Sun Oct 14, 2012 8:58 am

This is a viable alternative to buddy boxing leads:
http://www.hobbyking.com/hobbyking/stor ... ller_.html
You need another receiver but you can mix any versions of radio with this.
I made some myself hoping to be able to sell them but Hobbyking well undercut what I would have had to charge to make a profit.
DSCI0150.JPG
DSCI0150.JPG (128.27 KiB) Viewed 3214 times

Both work well we used the Hobbyking version in the club trainer and mine in my FPV Twinstar.
Last edited by BillGriffiths100 on Sat Mar 23, 2013 5:03 pm, edited 1 time in total.
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Re: buddy box wiring

Postby mikem » Sun Oct 14, 2012 1:46 pm

BillGriffiths100 wrote:This is a viable alternative to buddy boxing leads:
http://www.hobbyking.com/hobbyking/stor ... ller_.html

I had looked at that controller board earlier. A really a slick idea!

I don't see yet how to fit the extra parts into the little cockpit on my AXN: controller, a second receiver, and extra wiring. Probably making a set of short custom cables would help with bulk but would still put an extra 16-18 gm forward of CG.

I'd like to build a version of Bruce's FPV backpack. Maybe an AXN dual-receiver solution is to put the extra parts on top with that backpack. Will have to noodle on that a bit.

Your design for a buddy box dual receiver controller looks interesting. Any chance to get a look at the schematic you used?
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Re: buddy box wiring

Postby BillGriffiths100 » Sun Oct 14, 2012 3:06 pm

Your design for a buddy box dual receiver controller looks interesting. Any chance to get a look at the schematic you used?

Sorry I haven't got that to hand, (only Gerber files) but it's simply an Atmel controller running at 16 MHz, with minimal connections.
Up to six channels and a failsafe all in the software.
If one receiver fails it switches to the other if both fail then the failsafe cuts in but it continues to look for a signal from the two receivers.
It is a little bulky with all the leads but you can use one lead on it's side to pass three inputs that cuts down the number of connections on the input side.
You would be a bit pushed to get it all in the axn but the hobbyking version is smaller and there is plenty of room under the canopy so you could construct a tray to carry it all and move the battery back a few cm to compensate.
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Re: buddy box wiring

Postby mikem » Mon Oct 15, 2012 1:56 pm

BillGriffiths100 wrote:
Up to six channels and a failsafe all in the software.
If one receiver fails it switches to the other if both fail then the failsafe cuts in but it continues to look for a signal from the two receivers.


Does 'failsafe' mean simply throttling back to bring down the plane or is there a more complicated maneuver intended to regain radio contact? Just gaining altitude would help in many cases, I guess.

It would be comforting if the plane could head for home when trouble strikes. Bruce mentioned this 'Return to Launch' trick in one of his videos.

I can imagine several ways to setup a poor man's RTLS system using a microcontroller. I like the idea of continually taking magnetic bearings with a Parallax HMC5883L, say, and then making some kind of standard turn to a reciprocal course when necessary. I'm not sure how motor fields, etc., would affect the compass.

Integrating the output of a gyro, such as L3GD20, would accomplish the same thing without magnetic worries. Also would simplify rolling back to level after the turn. I played a bit with the ADXL335 accelerometer hooked to an Arduino, following some online tutorials. I'll have to think about how to use these two, gyro and acclerometer, together. Guess I have to order parts and try out some of this stuff....

A real RTLS would need GPS, of course. That would be interesting to try!

RC is a nearly ideal intersection of neat gadgets, isn't it?

-----------------------------------------
Compass Module 3-Axis HMC5883L
http://www.parallax.com/Store/Microcont ... roductName

L3GD20 (L3G4200 Upgrade) Triple-Axis Gyro Breakout Board
https://www.adafruit.com/products/1032

Arduino bootloader-programmed chip (Atmega328P)
https://www.adafruit.com/products/123

ADXL335 - 5V ready triple-axis accelerometer (+-3g analog out)
https://www.adafruit.com/products/163

Adafruit Ultimate GPS Breakout - 66 channel w/10 Hz updates - Version 3
https://www.adafruit.com/products/746

Wireless Buddy Box System 8CH (Dual RX Controller)
http://www.hobbyking.com/hobbyking/stor ... ller_.html
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Re: buddy box wiring

Postby BillGriffiths100 » Mon Oct 15, 2012 2:25 pm

The failsafe just takes the stick positions when you press its failsafe button.
I have a complete 3 axis giro / accelerometer autopilot with gps and way-points I bought, but I haven't shoehorned it all into my plane yet, I might have to build a new fuselage with more room.
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Re: buddy box wiring

Postby mikem » Mon Oct 15, 2012 4:44 pm

BillGriffiths100 wrote:The failsafe just takes the stick positions when you press its failsafe button.
I have a complete 3 axis giro / accelerometer autopilot with gps and way-points I bought, but I haven't shoehorned it all into my plane yet, I might have to build a new fuselage with more room.

I wouldn't have thought space a problem in Twinstars from pictures I've seen online.

FPV flying will require a buddy box if the pilot is required (at some point in the future) to have an observer ready to take over. Seems like it would be a natural fit to combine on one small pcb the dual receiver trick with an autopilot that can steer home if needed and maybe gather (and record?) OSD data as well.

I just ran across this lovely microcontroller breakout, a possible autopilot controller perhaps. 48 MHz, 10 PWM outputs, programs in C using an Arduino-modified IDE:
https://www.adafruit.com/products/1044
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