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Switching the Tx and Rx on and off

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Old 04-14-2018, 01:53 PM
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Stickslammer
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Default Switching the Tx and Rx on and off

Since getting back into R/C airplanes and buying a 2.4 GHZ transmitter and receivers, I carried over the practice of switching the airplane on first and off last. I was given this advice from the HS owner who warranted the 72Mhz Futaba many years ago that I had bought there. He said this would prevent any possibility of the Rx "spiking" the Tx if switched on after the Tx.
He said that may have been what happened in this case. After getting the replacement, I never had any more trouble. I always made sure to turn on the Tx last and off first.
I just read a post in RCU FAQ about a plane going full throttle and injuring the pilot after the Tx was switched off first. Has turning off / unplugging the plane first always been the proper procedure?

p.s Accident also posted in Humor sub forum. Not very funny, though, IMHO.

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Old 04-14-2018, 02:30 PM
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What you suggest is opposite of what I was taught and what is logical, especially with new radios. Transmitter always on first and off last! There are technical reasons but I don't have time to type the list at this time.
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Old 04-15-2018, 08:50 AM
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I referred to both my old 72Mhz manual and the newer 2Ghz one and they both state what you did. The 72Mhz manual states "the engine may go full throttle unexpectedly and injury."
The 2.4Ghz didn`t elaborate. I`m glad to have not learned the hard way as the other member of this forum did.
I guess the danger is that switching the Tx first off could cause a spike that would drive the throttle servo to one directional limit?
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Old 04-15-2018, 11:30 AM
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Originally Posted by Stickslammer View Post
I referred to both my old 72Mhz manual and the newer 2Ghz one and they both state what you did. The 72Mhz manual states "the engine may go full throttle unexpectedly and injury."
The 2.4Ghz didn`t elaborate. I`m glad to have not learned the hard way as the other member of this forum did.
I guess the danger is that switching the Tx first off could cause a spike that would drive the throttle servo to one directional limit?
You need to read the section of your manual explaining the "fail safe" .

In a nutshell , when you bind your RX to your TX (Bind = initial pairing of RX to a specific TX model memory) the RX stores the stick positions as it's default in case of signal loss . When you bind , you have the controls in their neutral positions and the throttle is closed and those positions are stored . So wait a minute , if the RX stored the closed throttle position as it's default , how could the throttle ever go wide open if the RX / TX connection were to be lost ? What happens is this ; MANY times after the initial bind it's found that some servos need their directions of travel reversed , in our case let's say the throttle is operating opposite to it's stick movement , no problem we go into the TX's menus and reverse the throttle servo and now the throttle responds correctly to the stick . BUT WAIT ! During the initial binding we've actually stored the opposite direction , the direction that is now full throttle instead of no throttle , as our RX's failsafe position ! Danger Will Robinson !! Anytime that model looses contact with it's TX it's going full throttle !!

The simple solution ? ANY TIME you make ANY changes in the TX's menus that affect how the airplane responds to the TX , you MUST do a rebind of the TX/RX pair to set the correct travel directions as your failsafe positions .

Here's to hoping I explained that well enough to instill the "bind , bind , and rebind" mantra whenever anything more than the model's name is changed in the TX's menus ....

Last edited by init4fun; 04-15-2018 at 11:33 AM.
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Old 04-15-2018, 12:02 PM
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Here's the history.

Back when 72 was en vouge, high power electrics were few and far between, the required batteries weighed more than the airplanes. Glow motors didn't start spontaneously (or sometimes at all but thats a different story)

So what we would do is turn on the rx first, to see if someone was using a tx w/o a pin, then turn on the transmitter if the servos were quiet.

With 2.4, TX on FIRST, and off LAST.

And as someone pointed out, always rebind when setup is done so the low throttle position is memorized
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Old 04-15-2018, 02:47 PM
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All that bind and rebind sort of thing is completely unnecessary when using Futaba and some other brands of equipment. The receiver remembers the failsafe settings as instructed by the transmitter during setup. Spektrum and some JR equipment does require the rebind where Futaba is link once and forget.

Even during the 72 MHz years, if the transmitter wasn't on first, electrical or random RF noise or even poor (but common) receiver decoder design could cause one or more servos to run to extreme when the receiver was turned. By turning the transmitter on first, proper decoder operation was assured and the servos and associated linkages and surfaces were happier and w/o damage.
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Old 04-15-2018, 02:51 PM
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Got it. Thanks Guys. I`m setting up the flight controls on my current build project and my spektrum DX6 warns me to re-bind after reversal, which I did and it`s all set. Isn`t all this new technology amazing? 7 models added now. 243 left to go.
I guess if the smoke ever gets let out of it I can use one of my old Futabas that still work fine.
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