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Jul 01, 2013, 02:42 PM
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Converting a DM9 Spektrum module from JR to Futaba (Works!)


Hello all, I have a JR 8103 and a Futaba 9C. I also have a JR spektrum module for the 8103. Long story short, I am out of model memory on my 8103 and I want to transfer all of my 2.4ghz stuff to the 9C (expandable memory!).

Now, I have the JR spektrum module and I do not want to buy another one just for the Futaba radio, especially considering that I will no longer be using the 8103 for 2.4ghz any more. I know that those two brands have inverted PPM signals but would it be possible for me to use a NPN transistor to invert the Futaba signal and plug the spek module in? There has to be a way to shift and convert the PPM signal to look like a JR PPM signal.


I would like to try this before dumping another $120 on a module. I am also aware of people using the X1TXN chip with an Arduino acting as a PPM to Serial converter. I don't need to do all of that, since I already have the PPM to Serial converter from the JR/spek module.
Last edited by ziomatrixacs; Jul 03, 2013 at 01:08 AM.
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Jul 01, 2013, 11:04 PM
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I did some experiments. I hooked the 8103 and 9C up to my O scope to take some measurements of the PPM signal.

The 8103 seems to put out a PPM signal that is pretty close to battery voltage, around 10v.

The 9C is a little odd. I had to have a module plugged in or else the PPM was all jumbled and static-y. The voltage with the module plugged in was around either 2.5v or 5v. I kinda forgot.

So I hooked up a NPN transistor according to the schematics below in between the 9C and the JR Spektrum module. This inverted the PPM signal to be like JR's except the PPM voltage was a lot lower. Needless to say, it didn't work.

The orange light on the module did flicker for a split second. As an experiment, I connected only the + and - from the 8103 to the module and the orange light flickered the same way.

Since the module light flickered with just the + and - connected from an 8103 and I know that the PPM signal from the 9C is A LOT lower, I think I may need an Op-Amp to invert and boost the voltage. I know there is a way to do it, I just don't remember how. I would look into the trainer ports PPM stream but I need that for a head tracker.



It's late and I am just trying to save 120 bucks.
Jul 02, 2013, 04:22 AM
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Bruce Abbott's Avatar
You need a pullup on the input. The FP-TP-FM module has a 30k resistor going to +5V.

I think your 470 Ohm collector resistor is a bit low. With 2.5V input the transistor needs an hfe of at least 600 to pull the output down from +11V to ground, but the BC547C's typical hfe is less than 500 at 20mA. I suggest changing the resistor value to 4.7k Ohms.

Another possible issue is that the channel order is different between Futaba and JR. You can plug the servos into different channels, but failsafe will be on ailerons instead of throttle.
Jul 02, 2013, 09:01 AM
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Thank you for the input, do you think 10k would be enough for the pullup resister? I want to try it now but I have a bunch of work to do .


Wait- If I am able to convert the PPM signal to be compatible with the JR spektrum module, in theory it should work with a JR module also.
I would effectively have a Futaba radio that is compatible with JR receivers as long as the inputs of the JR module were satisfied. Actually, would I even need a JR module (and use a Futaba module with the shift inverted) since receivers just listen to the pulse train? I remember my old Hitec Eclipse could invert the signal and fly darn near any receiver.
Last edited by ziomatrixacs; Jul 02, 2013 at 09:09 AM.
Jul 02, 2013, 04:01 PM
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Bruce Abbott's Avatar
Quote:
Originally Posted by ziomatrixacs
Thank you for the input, do you think 10k would be enough for the pullup resister?
Yes.

Quote:
Actually, would I even need a JR module (and use a Futaba module with the shift inverted) since receivers just listen to the pulse train?
A crystal-based Futaba module with an inverter should work fine with JR receivers.
Jul 02, 2013, 06:25 PM
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You may have another couple difficulties:

The module for JR radios assumes the channels come in the correct order, TAER.

The module for Futaba radios assumes the channels come in the wrong order AERT, and then puts them into the expected order, TAER.

You might also find that the different expected frame rates cause problems for you.

Andy
Jul 02, 2013, 07:18 PM
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Yes, I am worried about the differences of the frame rate. I know there is a difference between the DM8 and DM9 as far as the max channels, I wonder if that has an effect on the frame rate window? I just got home and will try to wire up a test circuit with the advice given. I don't see how the order of the channels would make a big deal, as long as they are the correct shift it should be fine. I would just have to reorder a few channels on the receiver which isn't a big deal.

I wish I could reflash the firmware in the PPM to DSM2 Serial chip on the DM9 to accept Futaba frame rate if there are frame width issues. I could imagine the DM9 expecting a certain frame width inorder to convert it to serial. As a test, I can wire up the inverter and connect that to my JR crystal module and see if the 9C will transmit using the JR module.



It is worth a shot though especially if an opamp or NPN transistor can save me $120 for the real DM8 module. I have 25+ receivers on spek 2.4ghz, JR FM, JR PCM, and Futaba FM. To make matters worst, I have RX's on a lot of random channels so I have to have a synth module for both radios. I would really like to have breathing room and end this fragmentation. I do have the original Crystal modules, I am using those to play around with.
Jul 02, 2013, 08:30 PM
Registered User
Why not just order one of these from HK

OrangeRX DSMX/DSM2 2.4GHz Transmitter Module (Futaba Compatible) $29.00

Or better yet switch to the FrSky Futaba module and get the option of telemetry. About the same price and the FrSky receiver prices are much cheaper than Spectrum.
Jul 02, 2013, 08:56 PM
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Quote:
Originally Posted by Hugh P
Why not just order one of these from HK

OrangeRX DSMX/DSM2 2.4GHz Transmitter Module (Futaba Compatible) $29.00

Or better yet switch to the FrSky Futaba module and get the option of telemetry. About the same price and the FrSky receiver prices are much cheaper than Spectrum.
I know the Orange module is much cheaper and I have been eyeing it for a while actually. It's just that I have the DM9 module already and I would love to make it work with the Futaba radio. I really just want to learn more about the electronics and make something that works. If it hits that point, I will probably get the orange module instead of a DM8. I hate being a broke college student.

I have also considered the FrSky stuff, there is quite a bit of info on it. I own about 4-5 Spektrum receivers and I don't want to have to replace those too. 2.4ghz really does brand lock you a lot worst than Futaba and JR FM/PCM does. Atleast with 72mhz, you could buy autoshift receivers. If I went the FrSky route, I would have to sell all my spektrum gear (at a loss of course) or if I kept the spektrum stuff, I would need more airplanes for those receivers!
Jul 02, 2013, 09:58 PM
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Buy a Devo 10 or 7e, install www.deviationtx.com . Sell what you can and keep Rx's and no longer brand locked.
Jul 03, 2013, 02:10 AM
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I spent the whole night at it again. This time with success!

This time I made a cleaner wiring harness, cracked open some Forest Mims/Art of Electronics and did some more measurements and experiments and took the advice earlier in this thread.

Here is what I did (for those who want to skim). I built a circuit using a 2N3904 NPN transistor to invert the PPM signal from Futaba to JR, tricking the DM9 module that it is connected to a JR radio. In order to work, the signal being fed into the transistor needs to be pulled up with a 10k resistor. Only PPM, 10v, and ground needs to be connected to the module. It looks like a 1k resistor between ground and the RF good pin (center) can be added to turn on the RF LED on the front of the 9C. The circuit works, I can use a JR DM9 module with my Futaba 9C using $1 in components. In theory, if I supply 6v to a JR crystal module, I maybe able to use a Futaba radio to control JR receivers. It may be possible to feed the inverted signal into the Futaba module and control JR receivers but I heard about some modules flipping the shift in the RF stage.

Here is what I did (for others doing research/other newbies). (Warning, I am not an EE, some terminology may be off or flat out wrong)
First, I measured the 9C PPM output from the module with and without the 10k pull up resistor. (Connected between the 10v and PPM pins)
Without the resistor, the voltage was about 40.8mV, NOT the 2.5mV previously mentioned in this thread and looks very static-y. The signal appeared to be go from high to low. But once the 10k resistor is added, the signal stabilized at 10.2v and went to flipped shift. (Thanks Bruce!)
Next, I connected the 8103 to check out its signal. It was 10.2v and the opposite of the 9C. So the voltage levels are the same, but the signal is inverted.

I did a little research in my electronics books and there are lots of ways to invert a signal. I had NPN transistors on hand so I went that route. You can also use a NOT gate IC (Hex inverter?) or an inverting op amp maybe. The books showed an audio circuit that had a similar transistor inverter that looked like the one in my 2nd post of this thread. It looks like R1 and R2 set the gain of the output signal. I didn't want to change the ratio, I wanted it to be close to 1:1 as possible. So I used two 10k resistors. To be honest, they were the closest ones to me that I had two of. I would love to learn more out selecting the right resistor to match the transistor but my book was too limited.

So a 10k resistor pulls the signal up, and the npn transistor with a 1:1 resistor ratio inverts the shift.

Before frying a component, I put both the 9C with the inverter and the 8103 on my scope to measure the signals and voltages. They both looked almost the same! So the voltages were good, I went a head and wired it up and it worked!

The problem with the circuit earlier in the thread is that it didn't pull the voltage up. When I used a 5v source, it inverted the signal fine. It also inverted the 10v signal from the 8103 just fine. But using that circuit on the 9C resulted in a flat line with no visible signal. The signal must be boosted then flipped.


I got a lot more satisfaction by cracking open a few books to figure this out than googling the hell out of the subject. Hopefully this post helps others in the same situation.

On a side note, my next project will be a clonepac module and trying to integrate the DM9 into the 9C permanently. I can't leave my 8103 and 9C for another radio just yet for those suggesting I sell them. My 8103 is heavily modified. It has a PSP joystick integrated in the back as a 3rd gimbal (for airbrakes, canards, variable pitch on a plane, you name it). It also has the backlight mod and a few other special switches. The 9C also has a head tracker and 5 knobs that I need.



And of course, pictures for those interested in doing the same.
Jul 03, 2013, 05:58 AM
Who let the dogs out?
Phil_G's Avatar
I wonder, did you notice any channel timing differences when using the DM9?
Through the S/C revival project I've done a few conversions using DM8 and DM9 modules amongst many others, and I found that only with the DM9, and consistently, each channel is extended by exactly the width of the PPM pulse. Normally the PPM width isnt critical, channel width being edge to edge (rising to rising or v.v.). I discussed this with Andy Kunz who kindly checked with the Spektrum techs on my behalf, and the conclusion was that yes it does, but that they use much narrower than standard PPM pulses (140uS vs 350 ish) which will minimise the effect, the rest being accommodated within the Spektrum encoder. Once understood, its not a problem, I just wondered if you'd noticed it too.
Cheers
Phil
www.singlechannel.co.uk
Last edited by Phil_G; Jul 03, 2013 at 08:33 AM.
Jul 03, 2013, 08:57 AM
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I didn't seem to notice any issue with the timing. It seems everything is corrected between the RX and TX? I also came across this thread that mentions it :http://openrcforums.com/forum/viewto...t=2619&p=43482

I hooked up 8 servos the the AR9000 and used the 9C to control them with no issues. I centered the servos and plugged the module into 8103 and the servos did not move from their center point but naturally, the throttle and aileron channels were flipped. So centering works fine, the ranges appeared good, no ill effects were noticed.
Jul 08, 2013, 02:42 AM
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Did a little more work on the module today. I had "that gut feeling" to check the 8103 with the module installed. Sure enough, the PPM signal went from 10v to 5v with the module installed. So I added a 10k resistor between the ground and signal out of the adaptor. This acts like a voltage divider and took it down to a much more reasonable 5.2v. I may put a trimmer pot to fine tune it.

I am also looking into an Op Amp set up as an inverting amplifier to allow JR to Futaba module connections, or even a cross compatible trainer cord. (After rearranging the channel order of course) I even wonder if I could cut the PPM trace near the trainer port of a JR radio and add a PPM out/PPM in line similar to Futaba instead just PPM out. This would make it easier for some guys to use head trackers for FPV since they usually read the signal and modify it.
Jul 08, 2013, 03:54 AM
Registered User
I needed signal boosting and didn't have any opamps handy(ordered) but found this in the Iply thread


And worked very well indeed and for your needs Logic/inverting could be all done simply. I just used first n-fet i had handy HERE that i use on my 3amp ESC.
Maybe opamp would be better and I'm all ear's


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