| 1) Principle |
| 2) Lead & PPM-inverter |
| 3) Connections |
| 4) Futaba |
| 5) Graupner |
| 6) Conclusions |
| 7) Examples |
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| DOWNLOAD
THE SITE UNDER WORD-FORMAT (<500 kB) |
The person that send me an E-mail on the 25th of september 2008 with a solution RC-FLYING TRAINING WITH BUDDY LEAD AND COMPATIBILITY PROBLEMS BETWEEN BRANDS OF TRANSMITTERS 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 : 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. 1. THE BUDDY BOX WORKING PRINCIPLE : 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. type 1 :
Multiplex 3010, 3030, Evo, 4000, Graupner MC22, MC24, Futaba FF8/8U,
FF9/9C, Futaba/Robbe FC18V3.1 and V3.2 1.1 THE PPM-SIGNAL (and compatibility) THE PPM-SIGNAL :
 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. 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. CLICK HERE TO SEE HOW TO CONNECT AND USE THE SCOPE. In fig. 1-03 you see a measure taken by the Russian scope. Not bad, isn't it?  PPM COMPATIBILITY : 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. 1.2 TWO TYPES OF BUDDY LEADS EXISTS 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 :
Disadvantage of the second solution :
1.3 PPM-PCM 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. 1.4 STICK MODE 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. 1.5 NEUTRAL IMPULSE OF SERVOS 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. 1.6 ASSIGNMENT OF CHANNELS 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 :
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 :
2) the pupil has a
non-programmable transmitter not fitted with interchangeable connectors
: 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 : 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 : 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 : 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 : 5) For those
who can't find any solution to their problem, there is a solution, but
more expensive : 6) And then
you have the self-made wireless system : 7) Another wireless system, but
a commercial product : ACT-T3S (35 or 40/41 Mhz) 8) And for
those who dislike the buddy box system : |
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