Futaba PCM receiver servo pulse other than PPM? - RC Groups
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Feb 06, 2002, 10:21 AM
Electrics rule
risto's Avatar

Futaba PCM receiver servo pulse other than PPM?

I have been flying a long time my Logo20 electric helicopter with Futaba PPM receiver. Having no problems with it, but changed today to Futaba PCM (both are R138, one DF, the new DP) for security reasons.
Now for the problem - when moving my sticks very fast (basically moving the stick to one extreme and then letting the stick go - allowing the spring to center the stick) makes all my 3 Graupner/JR C4041 swash-plate servos approx. 25% of time when I do this to buzz very loudly. Well, buzzing wouldn't be a problem, but they wont move at all after a LARGE amount of stick deflection - meaning that the heli won't be smoothly controllable.

I tested the situation very thourougly and the conclusion is that the servos work fine and the receiver also (with other servos), but these two just not together. As the servos work just fine with the same type of receiver, but only PPM encoding (logically shouldn't be any difference in the servo side) I am wondering if the Futaba PCM receivers output a bit different pulse (different rate or not constant rate?) to servos?
Haven't put an oscilliscope to the output yet, but plan to do in near future to see what is going on.

So much for additional info more, that I am aware that some infrared airplane stabilizers (these automatic 4-LED things that are put under the airplane fuselage to "see" the horizont) won't all work with specifically Futaba PCM receivers.

Any comments or suggestions are greatly apprecieted.

(Just to add that this buzzing and "locking" won't appear any of my other servos - so as the last solution I can always change the servos, but won't do it still very gladly. Also the Graupner C4041 work just fine with Futaba PPM receiver, which indicates them to work correctly).

With many thanks,
Risto Koiva
Last edited by risto; Feb 06, 2002 at 10:26 AM.
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Feb 06, 2002, 03:58 PM
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Mr.RC-CAM's Avatar
My first guess is that the low voltage detect on the PCM Rx is doing its thing. Unlike PPM/FM Rx's, the voltage is checked during operation.

A struggling BEC (due to excess servo current or other issues) can cause trouble. Normally the Rx's failsafe settings determines what to do when low voltage is encountered. Your "buzzing" observation sounds odd, but sluggish servos are possible.

Here is a simple test. Remove the R/C stuff from the BEC (or whatever you use to power the R/C gear). Instead, power the Rx and servos off of a freshly charged 4.8V pack that is at least 1100mAH in size. If the problem goes away, then you know what to look at.

I am not sure what Graupner/JR C4041 servos are, but if they are digital or high torque type, you can bet that they are drawing a lot of current when you throw the sticks around.

The other thing that comes to mind is that the R/C signal is getting hit with EMI/RFI noise. This is a tough one to cure. Try new cable routing, look for metal-to-metal issues, etc.
Last edited by Mr.RC-CAM; Feb 06, 2002 at 04:04 PM.
Feb 06, 2002, 05:26 PM
Registered User
Futaba PCM receivers have quite a low voltage on the servo signal outputs. Most stuff works but occassionally you will come across a non-futaba part which will not. I had an ESC one time which absolutely refused to work with a futaba PCM Rx. I am not an electronics expert of any sort but since you mentioned a scope, try looking at the signal output on the white wire - I think that is where you will find the problem.
Feb 07, 2002, 05:36 AM
Electrics rule
risto's Avatar
Looked today with oscilloscope - this was no noise, no failsafe issue.

Chris was right, the output pulse to servo on Futaba PCM is low.
I tested with FP-R138DF (PPM) and FP-R138DP (PCM) receivers. With PPM model I get nice 5V TTL signal output level, with PCM, it's only half, around 2.5V with 4-cell Rx pack and around 3V with 5-cell Rx pack.

Other than this difference, the pulse update rate with PCM receiver is approx. 1.5 times faster than with PPM (but is also constant, like with PPM).

So it seems I have two choises:
a) build a simple signal amplifier for C4041's
b) change the servos

Thanks for your help,
Last edited by risto; Feb 07, 2002 at 05:46 AM.
Feb 07, 2002, 07:37 AM
Electrics rule
risto's Avatar
Take a look at the oscilloscope image:
Feb 07, 2002, 09:42 AM
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Mr.RC-CAM's Avatar
Gosh, no wonder you are having problems. This also explains some other "glitchy" futaba PCM stories I have heard.

I vote for a buffer (home built or store bought). I have had good luck using the CD4000 series parts.
Feb 07, 2002, 11:35 AM
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Mr.RC-CAM's Avatar

I forgot to mention one thing...

If you don't mind, please report your "low amplitude" findings on the RC-CAM forum. As you already know, the FMA Co-Pilot users have struggled with this problem. I believe that they would enjoy hearing what you found.
Feb 07, 2002, 11:45 AM
Registered User
Glad to help! What is amazing is that this problem has been around and known (at least by the radio techs and the occassional victim) for more than 10 years and nothing has been changed. I would think the quarter scale guys might have a buffer type arangement for sale, 2.5Volts has to be a problem with 4 feet of servo wire between the Rx and servo.
Feb 07, 2002, 01:30 PM
Electrics rule
risto's Avatar
just posted it in the rccam forum.

For fun I played with my PCM + C4041 system a bit more.
One more observation - the problem for Graupner/JR C4041 servos can be very easily seen if the Rx battery voltage is HIGH (using 5-cell Rx battery etc.) = lots of problems (locks and buzzing with Futaba PCM receiver). When using almost empty 4-cell battery, then there are no problems anymore with C4041's.

Well, seems that for simplicity I'll be getting Graupner/JR DS8041's for swash... (hoping them to work, as the mini DS361's I use in Logo10 work with Futaba PCM without problems).

Feb 12, 2002, 04:05 PM
Registered User

Possible cause of error

Hi Risto and all that have seen his thread.

Risto.....COuld it be that the 4041's have a low treshold TTL. Therefore, with the low PCM pulse amplitude and higher frame rate, they are being mixed up ? I would like to suggest trying to measure a JR or graupner output pulse train from the respective receivers and try to come to a conclusion to where is the fault....either the Futaba PCM TTL too low ( but other makes of servos don't show distress ) or the Graupner 4041 that has a low treshold of its input amp.

Personally I use Futaba, and........too bad, Today I returned a JR 3810 system that I was repairing for a friend. I could have taken pictures off the oscilloscope at different power supply levels....say from 3.5 - 6 volts.

That's all !!!!

Best Wishes from very sunny ... cloudless blue sky...no wind....Lovely day to take the heli for a spin....Malta, Europe.

Feb 13, 2002, 06:11 AM
Electrics rule
risto's Avatar
> Could it be that the 4041's have a low treshold TTL.
Yes George, you are probably right, because with my TTL-level PPM receiver they work just fine.

What I have heard that some Futaba PCM models do have normal TTL level output. I'm planning to get latest R149 model instead now, just waiting for Mr. Rother to check his R149 with the scope to see if it is better.

Feb 13, 2002, 11:09 AM
Registered User

Anyone has a Futaba FP-R129DP.

Hi Again,

Does anyone have a broken down Futaba FP-R129DP Pcm receiver that is doing nothing at all ?

I need an input board ( the RF part only )...the one connected to the antenna and has all the metal cans. It can be of any frequency....35, 40, 50, 72 Mhz. Do you have an old RX which you can get rid of after a crash.....for example or one which does not pass the range check test ?

I need it for testing purposes.

Drop me an e-mail if you can get hold of one.

Best Wishes
Feb 13, 2002, 05:06 PM
Registered User
As a matter of fact, I do! It doesn't work after a crash onto a paved road at highspeed many years ago but the is no obvious physical damage to that board.
Feb 14, 2002, 10:00 AM
Registered User
HI Chris True,

Thanks for letting me know and dropping a thread.
Please tell me what you want for the broken down RX and an idea of postage cost. I live in a lovely tiny island in the middle of the mediterranean sea....Malta, Europe.
Feb 14, 2002, 12:18 PM
Registered User
Well, it has been sitting in a drawer for about a decade - I think you can have it!

I'll see what the postage cost for a 3 ounce package is to Malta, I'd think it is only a few dollars. Assuming it isn't more than a few dollars I'll ship it out if you give me your address. You can send me the postage later or - if US currency is a problem maybe we'll just leave it as good intentions for the future.

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