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

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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


Being assigned as a volunteer in my club for instructing RC-flying of model airplanes and a buddy lead being compulsory for training, I was very soon obstructed by two major problems :
1) the pupil thinks the buddy box system (and the buddy lead) far too expensive
2) the pupil is owner of a transmitter from a different brand than the instructor

I started my experiments (and not the research, my knowledge of electronics being much too limited) with my radio FUTABA FC18 V3 Plus interconnected to other Futaba radios. After a while other brands where tested with my radio.

I will not get too much into electronic details about the PPM signal graph and such, I will restrict myself to the practical side of how to make a buddy box and lead and how to make it compatible between brands.


It's simple : the low frequency PPM-signal (pulse position modulation) of the slave transmitter (pupil-TX), before being transformed in the High Frequency amplifier (HF), comes trough the buddy lead to the instructor's transmitter (master-TX). The instructor either uses the signal of the pupil-TX (with the help of a spring-activated switch), either the signal of his own TX to fly the model. This means that the pupil-TX must not send out any HF-signal, and therefore one shall remove either the crystal, either the removable HF-module (if present). The antenna of the pupil is thus not necessary and must not be pulled out. The master-TX is using the appropriate crystal and sends out on the frequency of the said model (see figure 1-01).

Exception to this system : all old MULTIPLEX transmitters ,i.e. the radios manufactured before the MC-serie (MC 3010,3030 and 4000) and before the Picoline and Cockpit serie. Also the old Futaba light-brown transmitters from 1976-1980, the Mac-Gregor radios, probably the Kraft, etc…These older radios where interconnected by a lead that only handed-over the master's TX battery voltage. Either the instructor was transmitting, either the instructor switched off his radio and automatically the pupil-TX was switched on. Both radios were thus equipped with a crystal, a HF-module (if removable) and a pulled-out antenna in order to transmit both. These older Multiplex radios are however equipped with a PPM-outlet, so that they also can be used as pupil-TX only with the newer type of Multiplex transmitters (or other brands).

Among the more récent transmitters, one can distinguish 3 types of master-transmitters :

type 1 : the master transmitter accepts the PPM-signal from the slave and « manipulates » this signal by adding the same mixings and settings as on the original signal of the master-transmitter, with the possibility to hand-over in a selective way some or all channels to the slave (f.i.. only channel 1 and 2 handed-over to the slave). This type of radio permits more possibilities : the slave radio must not be a programmable radio, not even for a V-tail or for helicopter, a simple basic 4-channel radio is perfect enough.
It is sometimes possible to correct the slave without taking over the control from the slave.
type 2 : the master radio has the possibility to hand-over in a selective way some or all channels to the slave.
It is sometimes possible to correct the slave without taking over the control from the slave. The slave transmitter must thus be able to control on her own the flying model (inclusive the necessary mixings needed).
type 3 : the master radio hands-over the commands to the slave in an absolute way (all channels). A sort of all-or-nothing. The slave transmitter must thus be able to control on her own the flying model (inclusive the necessary mixings needed).

type 1 : Multiplex 3010, 3030, Evo, 4000, Graupner MC22, MC24, Futaba FF8/8U, FF9/9C, Futaba/Robbe FC18V3.1 and V3.2
type 2 : Futaba/Robbe FC18V3.0, …Futaba 9ZAP-9ZHP, Graupner ? ? , JR PCM9X
type 3 : Futaba/Robbe FC18V1 & V2, FC16, F16, FX14 & FX18, Futaba Skysport 4/6, Futaba FF6/T6XAS/6EXA, Graupner MC10, MC12, MC14, MC15, MC16, MC17, Hitec Flash4/5 and Eclipse 7, Sanwa/Airtronics VG400, VG600, VG6000, RD6000Super & Sport, RD8000.
More info on any other radio is always welcome to complete the list ! ! ! ! ! Contact-me ! ! !

1.1 THE PPM-SIGNAL (and compatibility)

The low frequency PPM-signal is transformed into high frequency, which will be send by the transmitter. The receiver will receive this high frequency, will transform it in a PPM-signal, the latter being transformed in understandable signal for the servos.
The PPM-signal is a signal with as many modulations as there are channels on the transmitter, but I still didn't understand very well.
Look here for details :
   -http://home.nordnet.fr/~fthobois/ , see theory section

What I think to have understood and what is important for the compatibility question, is the sign of the modulation of the PPM-signal. Some transmitters are modulating the PPM-signal under the carrier, and then we talk about negative modulated PPM-signal. Other transmitters are modulating the PPM-signal above the carrier, then we talk about positive modulated PPM-signal (see fig. 1-02).

A transmitter with a positive PPM modulation will in theory not be compatible with a transmitter working with a negative PPM-modulation. In practice, my FC18V3Plus seems to understand sometimes an opposite PPM-signal.
According to my limited researches, it seems that :
- JR and Sanwa/Airtronics are working with a positive PPM-modulation
- Futaba, Hitec and Multiplex are working with a negative PPM-modulation
- Graupner sometimes positive, sometimes negative modulation.
These positive and negative PPM-modulation have nothing to do with the positive and negative SHIFT system which applies to the United-States and Canada. Futaba and Hitec are working in negative shift, the other brands in positive. This makes the positive shift receivers incompatible with négative shift transmitters and vice-versa. I repeat that this problem only exists in the United-States and Canada and not in the rest of the world. So this positive and negative shift does not apply to the coding of the signal, but to the HF-part of the signal.

For those interested in a free downloadable oscilloscope for visualizing or searching the PPM-signal, download it from the website of the Moscow State University :


Indeed, with this device you can search the PPM-signal on an old transmitter not fitted with a buddy connection, in order to use it as with a flight simulator or as a slave transmitter. There is also a French scope, but I find it less practical : www.winoscillo.com.
Downloading and installing is fast and a piece of cake, and you only have to connect the PPM-signal on the line-in connection of your soundcard (the ground of the transmitter to the ground of the line-in connector, the PPM-signal tot the signal-contact of your line-in connector). Attention : signals above 20kHz cannot be measured. Moreover, never try to measure a signal of more than 15 Volt, this to avoid your soundcard to be smoked-up.


In fig. 1-03 you see a measure taken by the Russian scope. Not bad, isn't it?


If there is an incompatibility problem due to inversed PPM-signals, you only need 4 electronic components to inverse the PPM-signal of the pupil in order to make it compatible. See chapter 2 for details about the PPM-signal-inverter and chapter 5 fig.5-10 for an example.


1) a lead, which, above of passing the PPM-signal, also gives battery power of the master-TX to the pupil-TX, the pupil will not switch on his transmitter, because powered by the master-TX.

2) a lead that will only transfer the PPM-signal from pupil to master ; the pupil will switch on his TX.

Advantage of the second solution :
a) no risks of short-circuit due to a defective lead : a short-circuit between the power-wires coming from the batteries of the master-TX would probably result in a destroyed master-TX and a destroyed model.
b) the making of such a lead is much more simple (only 2 wires to be connected)

Disadvantage of the second solution :
in certain cases the lead is not reversible, i.e. one plug must exclusively be connected to the master-TX, the other plug must be connected to the pupil-TX. In order to distinguish one plug from the other, I fit a piece of red heat shrinking hose on the pupil's end of the lead.


The pupil-TX-must normally be set in the PPM-mode and not in PCM, even if the receiver of the model is of PCM-type. In this case, of course, the master-TX will be set in PCM-mode.
Just know that there are some exceptions, where both transmitters must be in PCM-mode with a PCM receiver. Some exceptions (non-limiting) of which I learned of :
slave --> master
FC18 --> FC18
any TX --> FF6/6X
FC18 --> FF7/7UAP
FC28 --> FF7/7UAP
9ZHP --> FF7/7UAP
FF7/7UAP --> FC28
FC18 --> FC28
9ZHP --> FC28
Do not try to connect different brands of transmitters to each other when both in PCM-mode : all brands have their own PCM-code. That's also the reason why PCM-receiver of different brands can't be mixed.


If the pupil wants to fly in a different way than the master (f.i. one with throttle left, the other one with throttle right), it is perfectly possible with the buddy box. Both transmitters shall just be programmed in the appropriate stick-mode.


The neutral impulse of the servos said "UNI" is an impulse of 1.5 msec. Some older Robbe transmitters were working with 1.3 msec. And unfortunately the Multiplex Profi MC3000 (all versions) and the 4000 are using a servo neutral of 1.6 msec.
The result of connecting 2 radios with a different neutral impulse of 1.5 and 1.6 msec, is that the neutral of the servos are offset of several degrees (normally 8 to 10°) compared to the basic settings of the master transmitter. And if the trim-portion cannot be set electronically, the offset-difference, I think, is just too important to be corrected on the slave transmitter by setting the trim-levers in the corners. If the slave transmitter is of a non-programmable type, you only need to loose a little the stick-resistors and to turn them slightly in the right direction in order to obtain the same servo-neutral as with the Multiplex.
Attention with the Multiplex Cockpit MM and the Pico : both have "UNI"-neutral by default. With the Cockpit MM, it is possible to change the neutral from UNI to Multiplex-neutral from within the program (chose MN or MR in the servo menu of the part taking care of the normal/inverse servo setting. In France, it is possible to re-program for free the Pico in Multiplex-neutral. Multiplex is then taking re-programming and return-costs for his account.
(info from Hubscher, Multiplex Dealer France, who is even running a question-answer newsgroup. See here : www.hes-online.net)


Most of the receivers are compatible with a transmitter of another brand, which means that the HF-signal and probably the low frequent signal (PPM) are of the same type (but sometimes inverted).

Unfortunately, nearly each brand of transmitter utilises her own assignment of channels. And it is here that the problems of compatibility could become unsolvable for programmable radios which are limited in their programs for assignment of channels (and unfortunately most of them are).

As the learning process is logically done with a simple 3 or 4 axis model with only one servo for both ailerons, one will need the 4 basic channels controlled by the two sticks on the transmitter.

Hereunder you'll find a table that shows the assignment of channels on the receiver-outlets :







channel 1






channel 2






channel 3






channel 4






To understand the compatibility-problem, here's an example :

Suppose : a teacher with a Futaba FC18 and a pupil with a Graupner MC10, the plane flying well with the teacher's transmitter :

The connection of the PPM-signal between the two transmitters doesn't create any problem. However the chaos is nearly total. If the pupil moves his throttle-stick (channel 1 Graupner), the signal comes through the Futaba master-TX and is send to the receiver, and it is the channel 1 Futaba which is moving, i.e. the ailerons will move! If the pupil moves his elevator-stick (channel 3 Graupner), on the plane it will be the throttle-servo which will move, etc...

Solutions :

1) the pupil has a non-programmable transmitter, fitted with interchangeable connectors :
No problems : he must only interchange the connectors connecting the potentiometers of the sticks to the printboard, this in order to obtain the same assignment of channels as the transmitter of the teacher.
(Graupner FM314, FM414 and FM4014, Multiplex Europa sprint, Multiplex Pico-Line, Robbe Terra Top FMSS/PCMS,Futaba/Robbe F14 & F16...)

2) the pupil has a non-programmable transmitter not fitted with interchangeable connectors :
In this case the transformation is more radical. The wires from the potentiometers of one or more sticks must be unsoldered and soldered to other potentiometers in such a way as to obtain the same channel-assignment as the teacher's TX. Slight problem if the pupil-TX is fitted with dip-switches for servo reverse : the numbering of these dip-switches will not correspond anymore to the same number of the channels (Robbe CM-Rex, Robbe Terra Top FM...).

Interchanging the wires of the potentiometers is not a problem if the trim is mechanical on the same potentiometer. If the trim is mechanical but on a separate potentiometer (f.i. Robbe CM-Rex), one must also interchange the wires of these trim-potentiometers (which I did on the CM REX ; channel 3 and 4 have been interchanged as to become compatible with Futaba).

3) the pupil has a programmable transmitter :
Two solutions -far from ideal- are possible in this case :

a) changes in the pupil's transmitter : If the transmitter has interchangeable connectors connecting the potentiometers of the sticks to the printboard, (Multiplex Cockpit), one shall only have to change the place of the connectors in order to change the assignment of channels. In absence of said connectors, unsolder the wires of the potentiometers of the sticks and interchange them, eventually also the wires of the trim-potentiometers if present. WARNING if the trims are digital (Cockpit, Hitec Flash 5,...): probably these are directly connected to a printboard (without wires), so most probably not possible to change, which means that some trims could not correspond anymore to the sticks they normally would affect. For buddy boxing it's not too serious, but the factory channel-assignment of this transmitter should definitely be re-installed from the moment the pupil can fly on his own.

The problems and limitations which result of these changes are numerous concerning the programming.

Take for example the Graupner MC10, programmable basic radio : suppose that you interchanged the wires of channel 1,2 and 3 in order to make it compatible with Futaba. The MC10 has a dual rate for the ailerons (Graupner channel 2) and a dual rate for the elevator (Graupner channel 3). But as you tampered with the radio, channel 2 has now become the elevator and channel 3 the throttle. You now obtain a dual rate on the throttle (totally unuseful) and one on the elevator, but no dual rate for the ailerons. Also problems to inverse the channels in the program : if f.i. you activate in the program "channel 1 reverse", it will not reverse the throttle as scheduled in the handbook, but it will affect the ailerons.

b) changes in the master's transmitter : if the teacher possesses a non-programmable radio (which would rather be astonishing), it is possible to change the assignment of channels as described earlier, in order to make both radios compatible. Same for a programmable radio, but I doubt a teacher would tamper on his own radio.

But if the master possesses a programmable radio, it is sometimes possible to change the assignment of channels by programming. If the program gives you only the choice between stick mode 1 or 2 (sometimes 1,2,3 or 4), it is not possible like for the following transmitters :
Futaba : FX 14, FC 16, FC 18, FX 18, FF 6, FF 8, ...?
Multiplex : Cockpit
Graupner : MC10, MC12, MC14, MC 15, ...?
Hitec : Flash 5, Eclipse 7
Sanwa : ??
JR : ??

With the Multiplex 3010, 3030, Royal EVO and 4000 (probably the most complete radios on the market, but unfortunately grumbled by those who prefer a programming all made up in factory with a lot of restrictions as a consequence) ,there is no problem at all : every single channel of the transmitter can be assigned to whatever channel on the receiver, and all the mixings being free, there is not any restriction or limitation or problem in this sense neither.

With the Futaba FC 18 V3Plus or the FC 28, menu 21 permits the assignment -with certain limitations- of the channels as to become compatible to another transmitter.

As example : the master with a Futaba FC18 V3Plus and the teacher with a Graupner MC10.

If the master flies with the throttle right and wants to give lessons to this pupil, then menu 21 shall be programmed as follows :
menu 21    1 2 3 4 5 6 7 8 (numbers of the channels of the sticks of the TX)
                2 1 3 4 5 6 7 8 (outlet receiver)

This resolves a problem for the pupil and unfortunately creates problems for the master, the program facilities being limited on the FC18V3Plus : this radio stays obstinate and persists to pretend a.o. that the servo linked to channel 1 of the receiver is used for the ailerons. Menu 15 of this radio (dual rate/exponential ailerons) will be effective on outlet n°1 of the receiver, which is for Futaba the outlet ailerons. Alas, in this configuration this receiver's outlet is activated by stick n°2 which is used as elevator-stick. To obtain a dual rate and exponential on the ailerons, one must use menu 16 which is explicitly named "D/R elevator" ...and the D/R for the elevator shall be programmed with menu 15 (D/R ailerons). Pre-programmed mixings like delta mixing (elevons) f.i. cannot be used anymore...So be careful as to avoid confusions...and hoping that my explanations are not too confused.

4) the electronics comes to save you :
It is possible with the means of an electronic circuit to transform the assignment of channels as to make Futaba compatible with Graupner/JR or vice-versa. For this one must by in Great Britain 2 programmed integrated circuits and build a sort of interface. Visit the site www.welwyn.demon.co.uk/lead/lead.htm. This article was also published in a popular RC-magazine of Great-Britain.

5) For those who can't find any solution to their problem, there is a solution, but more expensive :
a device to be installed into the plane together with 2 receivers.
a) a French or Belgian company from which I forgot the name made such a system. This firm regularly made some advertising in French RC-magazines until the year 2000. Who help me to find this company again??

b) JAMARA WIRELESS TRAINER : (ref. Jamara 080003) Price : +/- 65 Euro www.jamara.de/testberichte/wireless/wireless-1.jpg
It's a 2-channel buddy box system that must be installed aboard the model and requires 2 receivers : one from the instructor, one from the pupil, which allows us to use 2 different units. Thus no buddy lead to be connected between the instructor's transmitter and the pupil's transmitter. Alas only 2 channels to be handed over to the pupil...... However it is possible to connect a second system like this in serie with the first one, in order to obtain 4 channels for buddy box purpose. With this system, no more problems of channel assignment : you only need to connect the right outlet of the receivers to the embarked buddy box system.
According to German users, it works wonderfully well, all brands mixed.

6) And then you have the self-made wireless system :
Take a Graupner C16 receiver, equipped with a crystal of the same frequency and band as the one fitted in the transmitter of the pupil. The pupil will transmit his steering-commands to this receiver. Inside the C16, you will find an IC 4017. The PPM-signal is to be found on connection 14, the Ground on connection 8 and the +5V on connection 16. By feeding the receiver with a 4.8V-pack and by connecting the PPM-signal of that receiver to the transmitter of the teacher, you are now the proud owner of a wireless buddy-box system. The PPM-signal of the C16 receiver seems to be a negative modulated signal and needs perhaps a signal-inverter like described in fig. 2-03 as to be Graupner/JR/Sanwa-compatible. It is probably possible to do the same with other receivers, but I have no other information. You would just have to find the PPM-signal of that receiver with the downloadable scope. The info I have comes from www.flyheli.de/flugsim/simrx.htm. The idea to transform a wireless PC-interface for flight simulators into a wireless buddy box system has been given and tested by a German on a German newsgroup.
Thus the slave must transmit in another frequency than the master, and may transmit in another band (f.i. one in 35Mhz, the other in 72Mhz). But unfortunately, the biggest problem of compatibility is not solved with this system : the possible incompatibility due to the different assignment of channels still remains.

7) Another wireless system, but a commercial product : ACT-T3S (35 or 40/41 Mhz)
ACT-Europe, a German based company, is manufacturing a wireless system of the same principle as the self-made wireless system, but with a specific scanner-receiver. See here : www.acteurope.de/html/t3s-system.html
So no crystal needed, the scanner will be set on the pupil's frequency. The price is around 65 Euro + 15 Euro for the cable between the scanner-receiver and the master's transmitter. This is probably an ideal tool for a club. But…
But unfortunately, the biggest problem of compatibility is not solved with this system : the possible incompatibility due to the different assignment of channels still remains. To quote the website of ACT : "it must be assured that both transmitters work with the same channel assignment (aileron in slave as well as master transmitter on channel 1, etc.)"
German quote : "es muss nur sichergestellt werden, dass beide Sender die gleiche Steuergeber- Reihenfolge haben (querruder sowohl im Lehrer- als auch im Schüler-Sender auf Kanal 1, usw.)" unquote.

8) And for those who dislike the buddy box system :
join the club of "ANTI-BUDDY BOX LEAGE"