yl5295

My Feedback: (1)

Can some one give me the details of the servo output from a multiplex IPD receiver. I have a device that works with every receiver on the market except multiplex. I assume the following:

Frame rate is about 40hz and positive PWM of 0.5-2.5 us and low the rest of the frame with about a 5V peak-to-peak voltage. There has to be something significantly different for my device not to work...

Can anyone send me a scope trace of it?

thanks

HarryC

My Feedback: (1)

The Multiplex IPD works with any tx sending PPM code, though in the USA you need to check it is the correct FM shift.

The clue to your problem is in the timing values that you quote. IPD, indeed any DSP PPM rx, looks for servo pulses that are "valid". If the pulse is "invalid" then an error is in the data and the failsafe mechanism is triggered. Your extreme pulses will trigger the failsafe because they are invalid. An IPD treats a signal shorter than 0.85 ms, or longer than 2.15ms, as an invalid pulse and therefore triggers the failsafe mechanism. If you are sending pulses as short as 0.5ms or as long as 2.5ms, you will be deep into invalid pulse and hence failsafe territory. Limit your pulses to the de facto standard that has arisen in the model radio industry of 0.9ms to 2.1 ms and the rx will behave as normal, if your FM shift is ok.

Harry

yl5295

My Feedback: (1)

Harry,
Thanks for the response but that is not the issue I am dealing with. I am trying to read the servo output from a multiplex Receiver being sent a signal from a multiplex transmitter. It's servo outputs does not appear to be the same as JR, Hitec, Kraft, Futaba, GWS, Expert, etc., etc.. I would expect a 1-2ms positive pulse about 40 times per second at somewhere between 3.3 and 5V. That does not appear to be what it is outputting? Does it expect a pull down or pull up on the servo output?

Bill

HarryC

My Feedback: (1)

Don't know but my IPDs work with every brand of servo. Are you getting any output at all? Are you using the synth or crystal version? Have you got a multiplex crystal in the rx, it will not work with anything else?

H.

yl5295

My Feedback: (1)

The receiver works fine with servos... I am just not able to read servo output correctly. I am thinking it needs a pull up or pulldown on the output now.

HarryC

My Feedback: (1)

A couple of years back someone was having problems with 2 hitec servos on a Y lead from an IPD, individually they worked but together they failed. A person on the forum with techie knowledge asked if the data pins were connected direct to the output from the processor, I opened one of mine and was able to see a resistor in the line from processor to the output pin. The techie person said this was good practice to buffer the processor but that other brands tend not to to save money, and the result would be to lower the high voltage of the data, which was why when 2 servos were Y leaded it dragged the voltage lower still and thos particular servos had a very high threshold so the square wave didn't register. Could that be related to the problem that you are having?

H

mglavin

My Feedback: (31)

Bill,

May I ask what kind of device is in question?

yl5295

My Feedback: (1)

Here is a thread on the device...

http://www.rcuniverse.com/forum/m_4211050/tm.htm

yl5295

My Feedback: (1)

I have someone working to test the series resistor to see if that is the issue. That is my first suspicion at this point... thanks for the input...

Gill

YL5295,

On 7/21/2002 BENGEN posted a problem about driving two Hitec 5645 servos on a Y harness. His problem turned out to be a faulty receiver. However I had a similar problem with the same servos being driven by a Multiplex 12 channel receiver. Troubleshooting this problem turned into a 4 page thread with considerable testing and many inputs. The problem was a servo load (because of two) on the Multiplex receiver which had isolating resistors on the outputs. The two servos raised the "low" voltage too high to trigger the servos.

Check out the thread. http://www.rcuniverse.com/forum/m_20...e_1key_/tm.htm

Its still in the archives, I just checked.

ALbert.S

Going back to my days at UNIVAC and doing TTL logic design The rules were that you could connect up to 10 circuits to a TTL output BUT they had to be in series with a terminating resistor at the end You were not allowed to have parrallel circuits since the signal went down one line bounced back and made all kinds of reflections and much trouble. It sounds like the output of the receiver in question could have a open collector output in which case you would not see a signal with a scope. Try putting a resistor from the output pin to the voltage(red wire) a 5k would work OK If you are going to connect more than one servo to a receiver output you will be creatting a parallel ciruit and you could have problems with signal levels and servo operation use the Y harness that have IC in them as this gives each servo it's own output I have never tried this but putting a resistor from the signal pin to the red wire would probably help also

yl5295

My Feedback: (1)

That is actually what I instructed one of my prototype testers to do but I told him 10K... I think it is one of the following things:

1) Open Collector
2) series resistor too strong to drive 24" wires with only capicitive load (pull up should fix this also - he is trying shorter wires before adding pull-up).
3) frame/pusle timing which I doubt

So we are thnking the exact same thing. Reflections are not really a problem at 40hz so if was a reflection problem it might have a little gitter as it would see false edges.

ALbert.S

As I recall we had little inline packages of termination resistors and they were 1K but that was for TTL logic The chip is probably Cmos so 10K should work OK

yl5295

My Feedback: (1)

I think if you are talking about 1K you are talking about when you were at UNIVAC and not the multiplex receiver. I would expect the series resistor on a multiplex receiver to be between 5-100 ohms (my guess is 11, 22, or 33 ohms) for current limit and noise rejection. Any more than that and it is going to turn into a voltage divider and the RC time constant becomes a problem. Series termination is still a valid thing to do though at the other end and will quite down the wire.

No insult intended because your basic premises are exactly on target but everything is mosfets now and much faster. Loading is more capacitive than resistive. I just don't know the drive strenght and slew rates on the receiver outputs. - Otherwise I would spice it...

Gill

YL5295,

The series resistors in the Multiplex receivers are:

7 & 9 channels, 1 Kohm
12 channel, 3.3 Kohm.

See post 64, page 3 of the referenced thread, above. The information came from Ernie Pritchard, Multiplex service. This thread is long and easy to miss data.

The servo device must be a combination input (for programming) and output (for servo operation). That would explain why I measured the very high ohms on the servo leads.

FrankE3535

Hi everyone with a Multiplex RC system.

Here is a story with a happy ending. I am using a MPx 3030 Tx, a MPx 7 ch IPD Rx and two mini MPx throttle servos in my Canadair 415T Water Bomber and trying to get Bill Wilkes "Engine Synchonizer" circuit to work properly was an exercise where we both learned something. The following is a testament to the expertise and tenacity of Bill Wike to understand and remedy the problem. When I purchased one of his BETA units for my water bomber. I was of the opinion that it would work. After doing a benchtop setup, I found it would not operate the two engine servos. After doing a number of tests and phone calls back and forth to Bill, he found the problem and modified a chip to provide the proper Tx frame rate to occur and allow the servos to operate correctly. His patience and knowledge on my behalf is a shining example how the world should operate and I have every confidence that he will help me again if needed and others in the same way.

To date due to weather and time constraints, I have not flown my plane but having test run it three times in my backyard it has performed to my expectations and I am satisfied it will give its same wonderful synchronized song over the lake as it did in my backyard. Check with Bill for your own system, you'll be glad you did!