PPM in R/C Transmitter

The electronics in the R/C transmitter convert the voltage of the axis resistors into a binary signal. This is forwarded to the buddy box and at the RF-part. In the RF-part a modulation of the signal happens on a bearer frequency of 35 MHz to a spread of 10kHz, that corresponds to minimum channel distance of different crystals. The receiver in the model demodulates the signal, it dismantles it into its 8 channel information and sends these to the appropriate servo. Only in the servo, the PPM-Signal is decoded and the servo arm moves. Consequently, the entire transfer trek can be built up without costly electronics or processors since the PPM-Signal is not changed during communication.

PPM format:

A complete PPM-Frame has a length of 22.5 ms. It exists of an overlong start information and 8 channel information. A stop information with 0.3 ms follows on every channel impulse. The length of a channel impulse ranges from 0.7 ms to 1.7 ms and corresponds to the stick position. Consequently, the illustration shows a transmitter with full amplitude right in channel 1, channel 2 on full amplitude left, channel 3 on middle,.. the start impulse supplements the 8 channel impulses, so that the total length of 22.5 ms emerges.

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://www.chez.com/silicium31/les_docs.htm
- http://www.aerodesign.de/peter/2000/PCM/PCM_PPM_eng.html
- http://www.mp.ttu.ee/risto/rc/electronics/
- http://www.skystreakers.org/Articles/HowRadiosWork.htm
- http://www.omegaco.demon.co.uk/mechtml/fmectech.htm#anchor160947
- http://home.nordnet.fr/~fthobois/ , see theory section

However, some transmitters are modulating the PPM-signal under the carrier - negative modulated PPM-signal. Other are modulating the PPM-signal above the carrier - positive modulated PPM-signal (see fig. 1-02).
Positive Modulated PPM:
JR and Sanwa/Airtonics
Negative Modulated PPM:
Futaba, Hitec and Multiplex

Check the PPM signal :

For visualizing or searching the PPM-signal, a cheap Sound Card Oscilloscope can be used. You can find this topic in my Web or a free version downloadable Moscow State University http://polly.phys.msu.su/~zeld/oscill.html or a less practical french scope www.winoscillo.com. Installation is easy and you only have to connect the PPM-signal onto the line-in connection of your soundcard (the ground of the transmitter to the ground of the line-in connector, the PPM-signal to 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.


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

R/C Buddy box plugs/ pin assignment:

For running a R/C transmitter with the R/C Interface the transmitter has to be equipped with a buddy box, that makes the PPM-Signal accessible from outside. Unfortunately, each manufacturer uses its own type of buddy box connector:

Multiplex

Futaba

Robbe

Graupner/ JR

all

 

DIN female plug 5-pin 45?(180?, mating face

GND: 3

Signal: 4

short-circuit 3 with 5

(1: Bat+, 2: Bat+, 5: NC)

(DIN female plug 7-pin: additional pins 6, 7 for copying, 5-pin male plug also fits the 7-pin female plug)

appropriate male plug for female plug

 

Hitec

Fokus 4, Eclipse 7, Flash 5, ..

DIN female plug 6-pin 60? mating face

GND: 0 = shielding

Signal: 4

short-circuit 3 with5

( 1: Bat+, 2: Signal in, 5: Bat+, 6 : GND)

Jamara

Compa x4+1 Trainer

DIN female plug 6-pin 60? mating face

GND: 0 = shielding

Signal: 4

short-circuit 3 with 5

( 1: Bat+, 2: Signal in, 5: Bat+, 6 : GND)

transmitter of model "Calisto"

4 pin female mini DIN plug (without blue contacts)

GND: 9

Signal: 3

1+2 trough for plastic pin

4 pin male mini DIN connector

Awesome R/C Product by Model Engines

Transmitter E8 of model Twister CP

4 pin female mini DIN plug, default colors of the cord

GND: 5

PPM Signal: 1

 

9ZAP/9ZHP, FF8/8UAP, FF7/7UAP, FC16, FC18, FC28, ..

DIN female plug 6-pin 60? mating face

GND: 0 = shielding

Signal: 4

short-circuit 3 with 5

( 1: Bat+, 2: Signal in, 5: Bat+, 6 : +5V )

3PJ, ..

female plug for 3.5 mm mono jack

GND: 1

Signal: 2

3.5 mm mono jack connector

FX15, FX18, ..

female plug for 2.5 mm mono jack

GND: 1

Signal: 2

2.5 mm mono jack connector

4C, 7C, 9C, FF 4, FF 7, FF 9, T6EXA, Skysport T4YF

female plug, square 6-pin, mating face

GND: 2

Signal: 1

short-circuit 5 with 6

(3: NC, 4: Signal in, 5: Bat in, 6: Bat+)

Walkera WK-701

4 pin female mini DIN plug, default colors of the cord

GND: 3

WK-701 PCM Signal: 2

Walkera 4-channel

4 pin female mini DIN plug, default colors of the cord

GND: 3

short-circuit 3 with 4

UART PCM Signal: 2

connect 2 with 1 temporarily to activate the signal output

or

PPM-Signal: connector for crystal oscillator with crystal unplugged

GND: 3 or charging connector

Walkera 22 E (35, 36, 39)

4 pin female mini DIN plug, default colors of the cord

GND: 3

PPM Signal: 2

 

CM-Rex, Terra Top FMSS/PCMS, ..

 

DIN female plug 5-pin 45?(180?, mating face

GND: 2

Signal: 1

(3: Bat+, 4: Signal in, 5: Bat+)

 

 

Sanwa

RD8000, RD6000, VG400, VG600, Radiant, Vanguard PPM, ..

 DIN female plug 5-pin 45?(180?, mating face

GND: 2

Signal: 3

(4: Bat+, 5: Signal in)

Stylus, M 11, ..

4 pin female mini DIN plug (without blue contacts)

GND: shielding

Signal: 7

1+2 trough for plastic pin

4 pin male mini DIN connector

M Zechs, ..

female plug for 3.5 mm stereo jack connector

Signal: 1

GND: 2

NC: 3

3.5 mm st. jack connector

Kyosho

Caliber M24 transmitter

female plug, square 6-pin, mating face

GND: 2

Signal: 1

short-circuit 5 with 6

(3: NC, 4: Signal in, 5: Bat in, 6: Bat+)

FM 314, mc-10, mc-12, mc-14, mc-15, mc-16, mc-16/20, mc-18, mc-20, mc-22, mc-24, FM4014, .. .

female plug for 3.5 mm stereo jack connector

Signal: 1

GND: 2

NC: 3

3.5 mm st. jack connector

MX-12, MX-22

American RF part and buddy box: X-347, X-387, X-388, X-756, X-3810, ..

female plug for 3.5 mm mono jack

GND: 1

Signal: 2

3.5 mm mono jack connector

Micro 47G

4 pin female mini DIN plug (without blue contacts)

GND: shielding

Signal: 7

1+2 trough for plastic pin

4 pin male mini DIN connector

Lexors

Nova 4, Nova 6

female plug for 3.5 mm stereo jack connector

Signal: 2

GND: 1

NC: 3

3.5 mm st. jack connector

 

All female plugs viewed from mating face/ outside = view on solder side of the corresponding male plugs; blue bridges are used for deactivation of the RF-part. The RF-part should be switched off in pupil/master mode because no disturbing antenna is plugged into the transmitter and, therefore, the generated energy is not emitted, that can lead to heating and destruction of the RF-part.

Your transmitter is not equipped with a buddy box..

.. is not intented to be used in pupil/master mode, but it should be connected to the PC. That is possible anyway, if the transmitter uses PPM. With following steps, one can find out this and can upgrade the transmitter:

- Choose microphone as input for your sound card. Test the setting with a real microphone.
- Solder 2 wires at a 3.5 mm stereo jack connector (to contact 1 and 3, see figure of Graupner connector).
- Plug the cord into the soundcard, open the case of your transmitter and switch it on. Touch the ends of the cord with contacts inside the transmitter. If you hear a buzzing noise that changes while moving the sticks, you have found the signal. Commonly, GND is the negative pole of the battery and PPM signal one of the wires leading from the main board to the RF part.
- Drill a small hole in the R/C transmitter casing and attach e.g. a 3,5 mm female stereo jack plug at the casing and connect it with GND and PPM signal.

more detailed information about buddy boxes: http://users.belgacom.net/TX2TX/tx2tx/english/tx2txgb1.htm

Upgrade Example Graupner FM 314

A low priced, used FM314 is optimally suitable for flight simulators and for buddy box operation. Unfortunately, the manufacturer doesn't offer any body boxes, however, it can be upgraded easily with the same parts as with the mc15. There is the FM314 in 2 different versions, that don't differ externally however.

v1: used signals are 2 GND and 1 PPM OUT on the transmitter's circuit board

v2: used signals are 1 GND and 4 (or 5) PPM OUT on the transmitter's circuit board, right beside the connections for the axis resistors

The signals are connected though the same two resistors with the female stereo jack plug: If the male jack connector is plugged in, its top is connected with PPM-Signal and the middle with GND.


This is FM314 v2.
On the circuit board right beside the connectors for the axis resistors is the 5-pin row. GND is the most upper pen, PPM-Signal of the 4th. GND is connected through a 100 ohm resistor with the middle contact and the PPM-Signal through 120 ohms with the rear contact of the 3.5 mm stereo jack plug.

Master / Slave Transmitter Connection:

A trainer lead that will only transfer the PPM-signal from pupil to master transmitter. A simple 2-wired lead is used. However, in certain cases the lead is not reversible, i.e. one plug must exclusively be connected to the master, the other plug must be connected to the pupil Transmitter. Besides,

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. Just know that there are some exceptions, where both transmitters must be in PCM-mode with a PCM receiver. Some exceptions (non-limiting) :
slave --> master
FC18 --> FC18 ,, 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.

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.
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.

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. Below is a table that shows the assignment of channels on the receiver-outlets :

receiver

Futaba/Hitec

Graupner/JR

Multiplex

Sanwa

Robbe

channel 1

ailerons

throttle

ailerons

elevator

ailerons

channel 2

elevator

ailerons

elevator

ailerons

elevator

channel 3

throttle

elevator

rudder

throttle

rudder

channel 4

rudder

rudder

throttle

rudder

throttle