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1) Principle
2) Lead & PPM-inverter
3) Connections
4) Futaba
5) Graupner
6) Conclusions
7) Examples

Contact me :
(auch in D)
(ook in NL)

(<500 kB)


The person that send me an E-mail on the 25th of september 2008 with a solution
for connecting a Graupner to an EVO-12, please contact me again; your E-mail was accidently deleted by my anti-spam filter

Okay, if you think you can handle the described problems in the previous chapter, take a coffee-break before attacking this chapter.

For the making of the leads, unfortunately I can only talk about what I know. Up to the readers to send me more information in order to complete this website.



In order to connect 2 transmitters to each other for buddy box-purpose, one needs at least a PPM-signal of the pupil-TX that consists of :
   1. the GROUND (negative pole) of the battery, and
   2. the signal itself (OUT).

The tension (voltage) between these two contacts can vary according to the brand and type of transmitter, as example some figures hereunder :
-MC10    0.6 V
-MC 14    2.4 V on one connector (fig.5-05), 9V on the HF-printboard (fig.5-06)
-FC18    4.3 V
-MPX Europa Sprint    7.2 V
-Graupner 314    8.2 V

This PPM-signal from the pupil is guided through the lead to the master's transmitter (see fig.2-01) :

THE Ground to the ground,

the signal OUT of the pupil to the IN of the master

This is the big principle to remember for the making of a buddy box lead

Exception for Multiplex : these transmitters do not possess the IN-connection : between recent transmitter of the same brand, the connection OUT from the pupil goes to the OUT of the master (see further)

In the buddy box lead one can eventually include the positive pole of the master's battery connected to the pupil's positive signal located after the on/off switch (+V SWITCHED) and not between the battery and said switch of the pupil's-TX.

Attention danger n°1 : most of the transmitters are operated with a 9.6V battery (8 cells), some with 8.4V (7 cells) or even with 7.2V (6 cells). There could thus eventually exist a danger of over-voltage if you connect a master-TX 9.6 V with a slave-TX 8.4V. The reverse can also create concern : a slave-TX operated with 9.6V can perhaps not be operated with 8.4V.

Attention danger n°2 : never use only the pupil's battery for operating both transmitters ; if by misfortune the lead is disconnected or defective, then the master-TX is not powered anymore. At this stage you can take your garbage back out of the car to pick up some balsa spread on the fields.

In any case and as mentioned before, I dissuade everybody to use a lead that gives battery-power from master to pupil transmitter (note that Futaba/Robbe brought on the market in 2000 a buddy box system and lead that does not include the battery power anymore).

The lead itself : always use a shielded lead (HIFI-cable), of a length between 2 and 2.5 m, with a minimum of 4 wires inside (a matter of diameter and strength). The GROUND is soldered on the shield. For the PPM-signal (OUT - IN) I have the habit to solder it to 2 wires twisted together. The 2 not used wires are cut in such a way as to create no obstruction. Personally, I fill the connectors with epoxy glue or filler with zap in order to avoid any rupture of the wire at the edge of the soldering. See fig. 2-02.

Shielded lead, 4 wires :
1 = insulated jacket
2 = shield : to be soldered to the Ground contact of both plugs
3 = 2 twisted wires :
    - to be soldered on the pupil's side to the OUT-signal of the plug
    - to be soldered on the master's side to the IN-signal of the plug


If the PPM-modulation of the pupil's transmitter is not the same as the master's transmitter (see chap. 1 : positive versus negative modulation), then the pupil's PPM-signal must be inverted.
On this website www.flyheli.de/flugsim/simrx.htm and www.rc-sim.de, I found a PPM-signal inverter. When adding a zener diode (modification kindly proposed by J.F.Pion, visit his site of electronic projects for RC : http://jean.francois.pion.free.fr/), the output of the PPM-signal is limited to maximum 4.7 Volt.
I personally tested this inverter (see fig. 2-03) with 2 different transmitters and it worked well (tested with 5 volt and 10 Volt).

(this inverter finally made my FC18 work with FMS on Win98 through the printer-port, does NOT work on Win 2K or XP)


If you really want to isolate both transmitters, 2 possibilities exist :
1) The expensive solution from Graupner by optical cable, but I don't know how to adapt this system to other brands.
2) A lead that isolates the 2 transmitters by means of an opto-coupler.
A Dutch hobbyist made a buddy lead with optocoupler to connect a Futaba to a Multiplex transmitter. The cost does not exceed 25 Euro and it works wonderfully well. Visit his website here : http://www.delemarre.ciscon.nl/.
Do not ask me how to adapt his cable to fit other brands, my knowledge is just too limited.