Thursday, 2 March 2017

Radio facts


Pulse Position Modulation (PPM)

https://forum.arduino.cc/index.php?PHPSESSID=sfo5l66mk99p6acvaifims7jr0&topic=349193.msg2409572#msg2409572
  • Transmitter hardware encodes all input channel positions into a one analog signal. 
  • A signal peak per channel, peak width corresponds to input position
  • Radio stage of transmitter converts PPM into an over the air signal
  • Receiver decodes over the air signal into PPM and splits the signal into individual PWM signals for each servo 

Futaba S.BUS 

  • Rather than PPM, input channel positions are encoded into a 10 bit number
  • All channels are packed into a packet with start and stop bits 
  • Packet is sent UART style to the radio stage
  • S.Bus receivers either decode the packet into per channel PWM signals for servos or send the packet directly to an S.BUS bus.
  • S.BUS servos connected to an S.BUS bus are programmed with the channel number they need to listen to.

Pulse Width Modulation (PWM)

https://en.wikipedia.org/wiki/Servo_control
  • A PWM signal (channel) per servo
  • "Duty cycle" of signal determines servo position

JR transmitter modules.

  • A plastic box that plugs into the back of a transmitter.
  • In the box is the radio stage of the transmitter which converts PPM into an over the air signal 
  • It is, in essence, a physical enclosure, connector and signal standard
  • 5pin header poking out the back of a transmitter
    • Pin 1 PPM output, open drain.
    • Pin 2 6V
    • Pin 3 BAT+
    • Pin 4 GND
    • Pin 5 ANT
  • Transmit only (transmitter body to module).
  • ANT not used on any 2.4GHz modules (not good enough RF performance i expect)

Training ports

  • Functionality
    1. Instructor and Trainee transmitters are connected via a cable (or wireless)
    2. Trainee transmitter sends it's inputs to the instructors transmitter via the cable (instead of over the air)
    3. Instructor transmitter transmits to receiver over the air
    4. Instructor holds a switch and trainee transmitter's inputs are now sent via the instructors transmitter instead of their own.
    5. Instructor lets go of switch and is now back in control.
  • JR Training port
    • The PPM signals that would be encoded into an over the air format is just sent down an 3.5mm audio cable
    • "Trainer" switch just swaps between the trainee and instructors PPM signal 
  • Futaba training port ?

 RC Failsafe

  • A preset state is applied to channels if transmitter signal is lost
  • A per channel PWM duty cycle is stored in some receivers (e.g FrSky).  Other cheaper ones just set throttle to zero.


2.4 GHz RC TX/RX protocols

https://www.rcgroups.com/forums/showthread.php?2301242-The-Beginners-Guide-to-RC-Protocols
http://www.rchelicopterfun.com/RC-Spread-Spectrum.html
  • A number of protocol players (not exhaustive)
    • Spektrum DSM2/DSMX
      • Variants
        • DSM2 is not spread spectrum, only dual channels
        • DMSX is spread spectrum frequency hopping
        • Products are usually compatible with DSM2 or both.
      • One way TX to RX.
      • "Bind N Fly" (BNF) models from e-flite, horizon hobby and others have DSM2/DSMX receivers built in.
    • FlySky (Turnigy radio products are re branded FlySky products)
    •  FrSky (also company name)
      • 4 product variants
        • V 8 channel simplex only (transmitter to receiver)
        • D 8 channel duplex (a.k.a telemetry) Receiver to Transmitter signal is not PPM
        • X 16 channel duplex (Backwards compatible with D,V) Receiver to transmitter signal is not PPM and also not the same as a D variant
        • L long range receiver only compatible only with X transmitters
    • Walkera and Walker-S (a.k.a DEVO)?
    • Various toy protocols.
  • Various JR modules available for each protocol
    • Banggood four in one (DSM2, FlySky, FrSky, DEVO)
    • DSM2/DSMX
      • Spektrum branded
      • OrangeRx (Hobbyking orange in colour)
      • Orangerx (black in colour)
      • Banggood
      • DIY (rip out tx hardware from spektrum product and make a module)
    • FlySky
      • Turnigy 9X v2 module (comes with 9X) ADHFS
      • FlySky FS-RM0002 (Banggood)
      • There are no AFHDS2A modules on the market yet
    • FrSky
      • DJT D variant transmitter module
      • XJT X variant transmitter module

Turnigy 9X series transmitters.

  • The Turnigy 9X is a rebranded FlySky FS-TH9X.
  • The Turnigy 9X is the cheapest, hacker friendly transmitter available.   The 9XR Pro is the same (approx.) price but without a transmitter module and needs a special Turnigy battery.
  • 9X includes a module, but 9XR and 9XR pro need a JR compatible transmitter module to talk to a receiver.  All are available from hobbyking and there are ways to build your own
  • People often modded the 9X.  9XR and 9XR pro incorporated common mods. http://fpvfrenzy.com/turnigy-9x-mods-and-upgrades/
  • The 9XR pro is further improved on the 9XR with a 32bit micro instead of an old 8-bit one. SD card reader and a USB port.
  • 9XR pro has futaba and JR training ports
  • er9x firmware comes on all 9x series, which is an open source transmitter software that also works on some other receivers.
  • open-tx is another open source firmware but later versions do not work with 9X.
  • Some extra money for Taranis X9D is allegedly worth it, because it comes with an internal FrSky XJT transmitter, accepts JR transmitter modules in addition to it's internal transmitter, is better quality, and you will eventually buy one anyway.  https://www.reddit.com/r/Multicopter/comments/4533eg/taranis_x9d_plus_vs_turnigy_9xr_pro/
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