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PCM glitch counter

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Old 08-18-2003, 07:13 PM
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Strykaas
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Default PCM glitch counter

Hi,

Can u tell me what a PCM glitch counter is ? How does it work ? Does it require any extra receiver channel ? Where to buy one ?

Thanks.
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Old 08-18-2003, 09:10 PM
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Default PCM glitch counter

I looked into a glitch counter once. The feedback I received resulted in dropping the idea.....
If you are having hits... the glitch counter will tell you how many but not where or why etc etc......
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Old 08-18-2003, 09:31 PM
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dickj
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Default GLitch counter

I purchased a glitch counter from WWW.EMSJOMAR.com and it plugs into a spare channel on youre reciever. It will tell you how many frames are missed and other data. I use it to run the engine, then count missed frames, taxi down runway and check missed frames, then if all is well I take off and count ten minute flight. Now many think this is a waste of money and time, howevr my frined had 270 plus missed frames just running his engine. By moving the antenna wire routing we were able to elimate the missed frames. I have found the antenna and its routing to be source of many problems. I recommend it, our club has one and others have used mine.

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Old 08-18-2003, 09:45 PM
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HarryC
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Default Re: GLitch counter

Originally posted by dickj
I purchased a glitch counter from WWW.EMSJOMAR.com and it plugs into a spare channel on youre reciever. It will tell you how many frames are missed and other data
That won't work with PCM though, will it? The data sent to servos is standard PPM data, even from PCM Rx. If data is faulty it can only be recognised because it is too short or too long for the PPM pulse, or because it stops being sent to the servo. But the job of a PCM Rx failsafe is to ensure that the data sent to the servo is always within accceptable frame length and is never missing, if necessary it sends good data from memory to replace missing data. So a glitch counter plugged in to a PCM Rx servo socket will never be allowed to see glitches - that's the whole point of failsafe.

Harry
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Old 08-18-2003, 10:29 PM
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Default PCM glitch counter

How does it work then, since it seems to be working with DickJ ?
Does this system detect a very sudden order change, like a switch to "fail safe" mode, or something else ? AFAIK, no smooth transition is ensured when Rx switches to "Fail Safe" mode.
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Old 08-19-2003, 04:10 PM
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Default PCM glitch counter

The EMS/JOMAR glitch counter known as "The Snitch" counts both gltiches and missed frames. It is used as a tool to check installation or installation changes to see if one wire layout gets more or less glitches or missed frames. It will work with PPM or PCM as the PCM will pass a some glitches through the receiver before its own microprocessor says I've seen enough and actually switches to the failsafe mode. Every flight usuually has some glitches or missed frames in either PPM or PCM mode. Obviously the lower the number the better. At several hundred glitches one starts to notice the airplane twitch in the air. Less than a hundred are usuually not noticeable.
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Old 08-19-2003, 06:26 PM
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Default pcm

Originally posted by mschwing
The EMS/JOMAR glitch counter known as "The Snitch" counts both gltiches and missed frames. It is used as a tool to check installation or installation changes to see if one wire layout gets more or less glitches or missed frames. It will work with PPM or PCM as the PCM will pass a some glitches through the receiver before its own microprocessor says I've seen enough and actually switches to the failsafe mode. Every flight usuually has some glitches or missed frames in either PPM or PCM mode. Obviously the lower the number the better. At several hundred glitches one starts to notice the airplane twitch in the air. Less than a hundred are usuually not noticeable.
It was my understanding that if PCM has bad data it will supply the servo with the last good data until it goes in failsafe. If it gets good data before the failsafe is triggered then it will resume reception. How does the glitch counter know that this dummy data is not good data? It seems to me the glitch counter will give a false reading when used with the PCM receiver
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Old 08-19-2003, 08:06 PM
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Default PCM glitch counter

It was my understanding that if PCM has bad data it will supply the servo with the last good data until it goes in failsafe.
Yes, that is the way modern PCM Rx's operate. To use a typical "glitch" counter with a PCM Rx, you merely plug it into an unused channel. Then, program the channel to revert to the opposite direction upon failsafe. Using this trick, the Glitch counter operates as a "failsafe counter." That is as useful as it is expected to get.

I have never used Jomar's Snitch, so if it somehow can detect realtime glitches from the PCM Rx then I would certainly like to know what/how it does such a trick. I never say never ...
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Old 08-19-2003, 08:45 PM
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Default PCM glitch counter

A PCM radio does not go to failsafe on the first glitch or missed frame it sees. They vary somewhat by brand but a certain amount of glitches can pass through to the servos before the receiver goes to failsafe. Any glitch counter should see and count those glitches. Once in failsafe mode, there are no glitches or missed frames and the glitch countr will not detect or count anything. It is before switching to failsafe that we are most concerned about, not during failsafe.
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Old 08-19-2003, 09:16 PM
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Default PCM glitch counter

I swear by the EMS detector.

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Old 08-19-2003, 09:38 PM
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Default PCM glitch counter

Sounds like a nifty system!
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Old 08-19-2003, 10:24 PM
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Default PCM glitch counter

I thought the point of PCM is that a glitch is impossible. Even if the incoming PCM data is faulty or missing entirely the Rx still has to create a valid PPM signal for the servo output and it simply can not generate a PPM signal that is outside the accepted limits. Since the Rx has to convert the incoming signal then if it detects a fault, and it will check every frame, it has to do something to generate a PPM servo output. That something is to use the last good data from memory, which is the hold mode before full failsafe. If the incoming data is a number greater than 512/1024, or is corrupt within that limit, the Rx simply can not pass it to the servo, it absolutely has to send a valid signal by using the stored memory. If a PPM servo signal is within the allowed time limits, you can not tell if it is "wrong" since no position gives you the ability to predict the position of the next frame - if you switch retracts it goes from min to max or max to min length in one frame.

So how do you get glitches, i.e. invalid time length signals on the servo output of a PCM Rx when the Rx simply can't generate invalid signals?

H
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Old 08-19-2003, 10:28 PM
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Default PCM

Originally posted by mschwing
A PCM radio does not go to failsafe on the first glitch or missed frame it sees. They vary somewhat by brand but a certain amount of glitches can pass through to the servos before the receiver goes to failsafe. Any glitch counter should see and count those glitches. Once in failsafe mode, there are no glitches or missed frames and the glitch countr will not detect or count anything. It is before switching to failsafe that we are most concerned about, not during failsafe.
It was my understanding that the PCM receiver would use a checksum to determine that a frame is bad then supply the servo with the last good data until the data becomes good again or it goes into fail safe. That would mean the glitch counter would only see one bad pulse unless the receiver was going in or out of range. At any rate it seems to me that the glitch counter would miss a lot of the corrupted pulses.
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Old 08-19-2003, 10:55 PM
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Default PCM

Originally posted by dickj
I swear by the EMS detector.

dickj
Have you used it on a PCM receiver?
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Old 08-20-2003, 06:48 AM
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Default Re: PCM

Originally posted by dirtybird
That would mean the glitch counter would only see one bad pulse unless the receiver was going in or out of range.
It won't even see one. When the PCM Rx gets one bad frame, then it does as you said, and passes the last good data to the servo instead. So the servo only ever sees good data, and it has nothing to tell it if that data came direct from the Tx or if it is hold or failsafe setting data from the Rx memory. All it ever gets is a valid ppm time signal. So I would like to know how a glitch counter works on a PCM output.

H
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Old 08-20-2003, 03:36 PM
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Default Re: Re: PCM

Originally posted by HarryC
It won't even see one. When the PCM Rx gets one bad frame, then it does as you said, and passes the last good data to the servo instead. So the servo only ever sees good data, and it has nothing to tell it if that data came direct from the Tx or if it is hold or failsafe setting data from the Rx memory. All it ever gets is a valid ppm time signal. So I would like to know how a glitch counter works on a PCM output.

H
It appears to me that you are correct. I sure would like to hear from someone that has tried it on a PCM receiver.
If not, when I return to AZ in Oct I will try it. I have access to a glitch counter there
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Old 08-20-2003, 03:58 PM
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Default PCM glitch counter

Long ago I played with a BC-6 flight monitor. It could not detect realtime glitches on our PCM system. Instead, it had to be set up as I described in my post above. It counted failsafe hits.

The BC-6 Instructions: http://members.rogers.com/meneer/ynt...stallation.htm

When I had a scope on my old PCM Rx and I could not detect glitches, no matter how hard I tried. However, I was only looking at pulse widths. I did not carefully observe the pulse rate. I think that might be where the magic could happen.

If the period between good frames is slightly different than while in the corrupted frame mode, then a glitch detector could be designed to work. Even a couple microseconds difference would matter. Perhaps the Jomar unit capitalizes on this?

If I still had a PCM Rx I would perform some measurements. But I sold my PCM stuff awhile ago. So Harry, can you measure this for us? Keep in mind that microsecond difference may be in order.

It would be interesting to get some real data from measuring the outputs of the various PCM Rx brands. Their exact behavior all depends on the software design.
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Old 08-20-2003, 08:10 PM
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Default PCM glitch counter

tabqa PCM but I had no bad frames, it did however register the pulse code as stated in the literature. The literature that I got with it states for PCM.
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Old 08-20-2003, 08:22 PM
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Default The JetCat ECU

has a detector like you are asking about. It really counts failsafes, in our case we use the throttle channel for a failsafe indicator.

What varies by manufacturer is the number of frames that must be bad before the RX goes into failsafe. Before it goes into failsafe, it just holds the last position.

I have never heard of any PCM radio that passes glithces through, because if they did, as HarryC said that means they must be decoding bad packets and sending them on through?
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Old 08-20-2003, 10:26 PM
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Default PCM

It would be nice if the manufactor provided a pulse out that we could count when it is sending out dummy data.
In fact I think they should be required to. It would make a PCM receiver safer to use.
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Old 08-20-2003, 10:44 PM
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Default PCM glitch counter

It would be nice if the manufactor provided a pulse out that we could count when it is sending out dummy data.
The latest FMA DSP based PPM Rx has an error counter built in. They offer a Rx-PC interface too. Kind of like Multiplex's PPM error correction, but on steriods.
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Old 08-21-2003, 03:24 PM
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Default pulse output

Originally posted by mr.rc-cam
The latest FMA DSP based PPM Rx has an error counter built in. They offer a Rx-PC interface too. Kind of like Multiplex's PPM error correction, but on steriods.
Thats great! FMA should be commended. Now if only the PCM makers would follow their lead. PCM is where its really needed.
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