"Well, I was letting my buddy fly the plane. He lined it for a pass and as it came toward us he yelled 'I got nothin!' so I grabbed the box and he was right. Nothing. Absolutely nothing. It's the worst kind of crash you can possibly have because you just sit there and watch the plane go in. I had no control whatsoever. There was nothing I could do. I hate PCM!"
Why the PCM? Sounds like when one of those locks up you are screwed, plain and simple!
Any comments?
This topic has been covered many, many times, and a search through RCU or other newsgroups will get you a whole load of opinions, old wives tales, urban legands and some facts all mixed together. e.g. try:
http://www.rcuniverse.com/showthread...813&forumid=27
The fact that many people totally misunderstand how PCM works unfortunately does not in any way lessen their enthusiasm for spreading myths.
Without going into huge details (coz you can find that elsewhere), here is the core:
With plain PPM (lets ignore IPD for now), the receiver positions the servos according to whatever incoming signal it receives. If that signal is "clean", then the aircraft does what you told it to. If the signal is an amalgalm of the signal from your TX and some other source (i.e interference) then the receiver does not know that the signal is now garbage, and continues to position the servos accordingly. This is what causes the "glitching" that you frequently see when a PPM system receives interference.
With PCM, the signal is encoded in such a way as to carry not only the information stating where each servo should be positioned, but also a checksum which acts as a fairly good (but not 100% accurate) way of double-checking the integrity of the signal. The receiver receives the "frame" (one cycle's worth of data), extracts the servo position information, and (using the same algorithm as the TX does) calculates a checksum based on that information. Then it compares its newly calculated checksum with the one it received in the incoming frame. If they match, then the receiver positions the servos accordingly. If they do NOT match, then the receiver checks to see how many other invalid frames
immediately preceded this one. If that number is large enough to match a pre-set parameter (typically matching 1/4, 1/2 or 1 second depending on what you set in the TX) then the RX will "go into failsafe" - i.e if you have programmed in some failsafe settings other than hold, the servos will be moved to those failsafe settings; if the number of immediately preceding invalid frames has not yet reached that limit, the receiver simply holds the servos in their last set position.
Now, here's the important part... as soon as the PCM receiver receives a valid frame, it responds to it immediately, and resets its count of how many consecutive invalid frames it received. In other words, no matter how many garbage frames there are, as soon as there is a single good frame, it is acted upon.
In other words, both PPM and PCM will act exactly the same if a valid signal exists. There is a lot of misunderstanding about what the failsafe "delay" setting means. A lot of people seem to think that this is a "lockout period" - meaning that the failsafe will stay engaged for at least that long before the RX will allow a good signal to be acted on. I have no idea where this idea originated, but its nonsense. As stated above, the RX will act IMMEDIATELY on a valid frame.
Consequentlky, the idea that "flying through the interference" is more likely with PPM than PCM (as suggested by your buddy's comments) simply does not hold up.
Both systems will respond to even a single valid signal; PPM will glitch with even one or two frame's worth of bad data, whereas PCM will filter those out and stay in the last valid position; with continuous bad data, PPM will continue to glitch, and PCM will activate failsafe.
I used to fly just PPM. Then I got shot down, and I watched as my aircraft came screaming vertically out of the sky at full throttle, and dug a huge hole. I thought about what would have happened if the aircraft had pointed itself towards the pits instead of the weeds, and immediately went out and bought a PCM system so that I could program in at least a throttle-kill. That way it will come down with less enegry, and less energy means less killing power. (The failsafe isn't there to save the aircraft - its there to minimize the chance of killing someone with an out of control aircraft).
Also make sure that you choose PCM for the right reasons. I've heard of several people choosing to use PCM in their big gassers because they have so much noise from their ignition system that PPM is unusable. This simply means that you are using PCM to mask the problem, and the problem is still there, just waiting to bite you. Shield the ignition, plugs, boots etc if necessary, instead of just hiding the problem.
Last but not least - using PCM without properly setting your failsafe is irresponsible. My wife was hit by an aircraft that had its failsafe left as the default "hold", and which happened to be at full throttle when failsafe activated. If the pilot had correctly set his aircraft to idle / kill on failsafe, then she would have had several months less of pain to endure. Similarly, the death of 11 year old Adam Kirby in Britain a few years ago was ruled to be at least partly caused by the choice of "throttle hold" in the failsafe settings of the pattern plane that hit him. If you are using PCM, or any other failsafe method, please think seriously about the consequences, and choose your settings carefully.
As for IPD - I'll leave that to someone else, as I don't have any real-world experience of it.
Gordonn