Turnigy 9x -> frsky JR module. ppm impedance issue???

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Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby tjm » Tue Jun 21, 2011 6:31 pm

Hi All,

I've just started using a turnigy 9x (V2 hardware with er9x firmware) with frsky modules. First module and receiver used were 2 way and, although it range checked ok, I got bad jitter after a few minutes flight. I couldn't find anything wrong so changed it for a 1 way module, this time using the same 1 way receiver I have used for 6 months plus on another TX. 30 seconds into the flight I started getting lockouts! :shock:
Needless to say I landed quickly and am not using again until I can work out what is going on!

So, two modules, two receivers both had some sort of problem. So I am guessing it isn't the modules. Doubled checked all the connections and I can't find any problems. Also no sign of any jitter on the on screen readout of channel output values and, frustratingly, no problems seen in static checks. So I went on to look at the ppm signal to the module.

The signal is clean, as is the battery voltage to the module. However I noticed the ppm voltage was over 4 volts which rang alarm bells given previous discussions on the frsky ppm voltage and input impedance. A 4.3k resistor to either vbat or ground (without the module present) makes no difference leading me to believe the ppm signal is being driven low impedance with about 4.5pp signal.

So that begs the question: Is a low impedance drive of over 3.3V ok for the FRSKY JR module? Does it have input protection like the later hack modules? I have a feeling not as it will be assuming it is used in a JR transmitter and I think they have a high impedance ppm signal.

I think I might be clutching at straws here as I am sure plenty of people have used this TX with the FRSKY modules but if it isn't this then I am a bit stuck.....

Toby
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby mikeb » Tue Jun 21, 2011 10:11 pm

I'm using the FlySky version (bought from GC) of this Tx with a FrSky DJT module with no problems, even the telemetry is fine.
The PPM output to the module is direct from the processor. This is powered from +5volts, so the output will drive to 0V and 5V.
Did your Tx come with a Turnigy module and Rx? If so, perhaps it is worth testing the Tx with these.
There are reports of poor soldering on some of these units. It might be worth checking the boards in the Tx. I would particularly check the soldering of the pins that connect to the module. They will come under mechanical load when modules are plugged in and out, so any poor soldering on these is likely to show up. It might be a good idea to re-solder them anyway.
Presumably you have modified the Tx to get er9x loaded. If you have soldered wires to the main board it is probably worth checking in case a strand from one of the wires escaped soldering and is shorting to another signal. Er9x firmware is normally solid in producing the PPM signal.

Mike.
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby tjm » Wed Jun 22, 2011 7:15 am

Hi Mike,

Thanks, that confirmed what I thought with the ppm signal being low impedance 5V. Although my scope measured it at closer to 4V so it might be worth me checking the regulator to be sure it really is supplying 5V to the processor. If it is not that is a potential problem.
Yes, I've added the programming connector but I've double checked and I don't think there is anything wrong with my soldering ;)
I've also checked all the other solder joints and cannot find any problems. I also waggled various leads and prodded things with no affect on the ppm signal when viewed on the scope so I can't find anything wrong with the ppm.

Mine is the V2 so the original module had a hard wired antenna cable which I cut to remove. I could resolder it I guess but I've only ever seen this problem in the air and I am not keen on flying with this TX again until I have found something concrete to blame the fault on.

I think my next step will be to check the FRSKY module internally to see if it has protection on the ppm signal.

Toby
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby Akasa » Wed Jun 22, 2011 9:30 am

I've used the Frsky DHT and DJT with the Turnigy and FlySky versions of this radio with er9x and not had any problems. Have you tried a another transmitter with the same plane to make sure it is actually the transmitter that is causing the problem?
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby tjm » Wed Jun 22, 2011 10:55 am

hmm... I had a feeling I would get lots of responses saying people have used this combination just fine. Sort of implies I have some other fault I have not found yet. I'll keep digging.......

Yes, the plane (same RX) has worked fine for 9 months with a frsky hack module in a JR378 TX. So I am pretty sure there is nothing wrong that end.

Toby
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby Phil_G » Wed Jun 22, 2011 11:46 am

tjm wrote:I think my next step will be to check the FRSKY module internally to see if it has protection on the ppm signal.
I've not checked recently but the JR modules to date have had a transistor inverter in the ppm line so the 3.3v Frsky processor input limitation doesnt apply and 5v of ppm should be fine. I'd check how near the 'low' is to ground - maybe its near the upper limit for a '0'. Have a look at the width of the ppm pulses too, see if theres anything weird going on there. Expect somewhere between 200-400uS. The most common reason for an eratic link seems to be power - esp where people have been powering them from the trainer port etc - I'd check the module supply on load, see if the rail is dipping as the module transmits (remembering its a very tx low duty cycle). Try switching the module to low power to reduce its current draw, and see if the problem persists. If it does it might indicate a supply problem. Try a big-ish cap 100uF or more across the pos & neg supply to the module, not as a permanent fix, but just to help isolate a supply problem.
Cant think of anything else at the moment :)
Cheers
Phil
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby tjm » Wed Jun 22, 2011 12:40 pm

Thanks Phil,

taking your comments in turn....
1) if there is a transistor inverter then you are right, that cannot be my problem. It also makes sense as I would have though though there would be a rash or reports of failures if it wasn't protected. I thought I was clutching at straws......
2) I didn't check to thoroughly but on 2V per div I could not see any difference between signal low and ground on my scope so I think it is ok. I'll check properly tonight.
3) Pulse width I didn't check, other than to see the ppm varying with stick position. I'll check this tonight.
btw. can you confirm the the JR module i/f expects positive ppm? i.e. wide, varying pulses at 5V, fixed, narrower pulses at 0V?
4) Module supply looks ok. It appears to be Vbat (or very close) and I could not detect any noise with my 'scope. I'll check again though because I did see something odd happening at one point (almost looked like a low level ppm, AC coupled on Vbat) but I think that was a dodgy ground connection on my scope probe.

My biggest problem is I have not seen any problem at all on the bench so I am trying to find a cause without being able to see the problem. Frustrating!

Toby
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby mikeb » Wed Jun 22, 2011 2:04 pm

I believe the FrSky modules don't actually mind which way up the PPM stream is.

You might want to check the wiring to the power switch. According to the circuit I have, the switch is double pole. Both poles switch the battery power, one of them powers the processor, and the other powers the module. This is done so that when the switch is off, plugging the trainer jack plug in turns power on to the processor but not to the module. Dodgy wiring, or a dodgy switch, could cause the problem you have described.

BTW, what revision of er9x are you using, and have you done the telemetry mods to the Tx?

Mike.
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby tjm » Wed Jun 22, 2011 3:50 pm

Well, this is looking interesting.
The power to the module (inside the TX at least) looks absolutely fine, a clean 10V.
sharke is r344 (V1.4086erez, 15-4-2011), no telemetry mods (actually using a one way V8JT module atm)

what is interesting is.....
- without the module I get 4.8 (roughly, using my scope) ppm signal. Putting a 4k3 resistor to ground gives me about 0.05V less. So taking 1mA from the ppm doesn't affect it appreciably as I would expect if it is direct drive.
- Adding the module, and I get 4.2V p-p. So something is loading it a LOT more than 1mA which is not what I expected.
- Adding a 4k3 resistor in series with the ppm and I get 4.8V at the TX end (as you would expect) but 1V to 3.8V at the module end. Not at all what I expected.

So the module input appears to biased to mid rail. 1V through 4k3 isn't a great amount (0.23mA) so that doesn't account for the 4.2V p-p with the module installed.

I think I need to remove the top of the module and see what is actually getting to the module pcb.

Toby
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Re: Turnigy 9x -> frsky JR module. ppm impedance issue???

Postby mikeb » Wed Jun 22, 2011 4:39 pm

You could try the 4K3 resistor from the PPM output to +5v to check the processor is pulling the signal low properly. The processor has three ground pins, 22, 53 and 63. It is possible one of them is not soldered properly. If this is the case then some of the outputs won't drive to ground properly on load. You say with the module you have 4.2 V p-p. Is this from ground to 4.2 v, or somewhere in the middle?

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