When Futaba radios are connected in a master-student setup with a trainer cord, the student radio powers up automatically as soon as the cord is connected.  At this time, the student radio is not transmitting, it is just sending the signals across the cord to the other radio to be transmitted.

The question is, which battery does the student radio draw power from ?  From its own battery, or the master radio's battery ?

Dont know about futaba, but in such cases, atleast in my RDS8K, the trainer cord carries +8V and ground. considering that, I assume its powered by the master battery.

To best decide, remove the student battery and try.

Right, it came from the master.  That is why these things work.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTFE1

Now the next question comes.  What happens when you connect a simulator via USB ?  Where does the power come in that case ?

Some Futaba radios have battery +ve connected to one of the trainer port pins. This can be shorted to Vin pin to power on the baseband section alone of the radio. Not all Futaba radios (for example the 6EXA) have the battery out connected to the trainer port.

It is not just Futaba, right ?  JR/Spektrum also powers up when plugged in as student. How is the power handled there ?

I'm haven't used them much so there could be exceptions to this. JR and hitec uses two pin trainer cable. So there is no possibility of getting power from the master. The trainer socket has a switch that turns on the slave radio from its own battery when plugged in.

The reason I started this thread was that it was a surprise at the field yesterday when someone showed that the student radio was indeed powered up from the master, when both radios were Futaba  9Cs.  I was under the impression all this time that it was always taken from the student itself.

So it looks like, if we consider all brands, most of the time it is taken from the student itself, except for some cases with Futaba.  Even in those Futaba models, it is based on how the PINs are shorted. In the case of sims, the power is taken from the student radio itself, and in the case of trainer mode, it is taken from the master.

have a look at this site !

http://users.belgacom.net/TX2TX/tx2tx/english/tx2txgb7.htm

1.  It does not show the most common Futaba configuration which is "square to square".

2.  It does confirm that with Futaba, there are two possible configurations (at least in many cases). It is possible to either draw power from master OR use slave battery itself.

Quote1.  It does not show the most common Futaba configuration which is "square to square".

anwar those are all the possible ways one can use different radios in master/slave mode kinda more like DIY compatible cables n stuff, see the like for some info on square connector

http://users.belgacom.net/TX2TX/tx2tx/english/tx2txgb3.htm

Sure.

What I was after was that various people were reporting that they had to manually turn on their Futaba radios while running their sims, and others did not have to. Looks like this is expected behavior depending on a combination of the radio model and how the wires on the square connector are setup.

one more link

http://www.mftech.de/buchsen_en.htm