Protocol decoder:sbus_futaba

From sigrok
Jump to navigation Jump to search
Pd-sbus futaba-message.png
Name Futaba SBUS (Serial bus)
Description Serial bus for hobby remote control by Futaba
Status supported
License GPLv2+
Source code decoders/sbus_futaba
Input uart
Output sbus_futaba
Options min/max value for proportional channels

The sbus_futaba protocol decoder can decode SBUS ("Serial bus") communication as it is seen in hobby remote control by the Futaba vendor. Which is not to be confused with the SPARC computer bus system which is also referred to as S-Bus.

The protocol runs on top of UART communication. Typical parameters are 100kbps, 8e2 frame format, inverted (high voltage level is logic low).

SBUS messages consist of 25 UART bytes. A message provides 16 proportional channels of 11 bits each, 2 digital channels of 1 bit each, and two flags (frame lost, failsafe). Applications may communicate values between 192 and 1792 on the wire, and map these to values 1000 to 2000 for their firmware purposes. Some implementations happen to use different value ranges.

SBUS messages take 3ms to communicate. Typical repeat intervals are: 14ms (analog mode), 7ms (high speed mode).

A frame has the following layout:

  • Data is always sent with the LSB first.
  • Inter-frame gaps allow to synchronize, data content is not sufficient for synchronization (the same fixed patterns can also occur in other data fields).
  • Header byte, always 0x0f.
  • 16 channels with 11 bits each, dense packing. Results in 22 bytes length.
  • Flags byte. Upper nibble always 0. Lower nibble carries: failsafe, frame lost, channel 18, channel 17.
  • Footer byte, always 0x00.

The sigrok decoder was developed based on exernal documentation (see #References below) and synthetic data. Real world captures have yet to become available.

Example interactive use

These images illustrate the interactive use in the GUI application. The data used in this setup is synthetic.

TODO Add illustrations of error conditions: Short messages. Excess data after complete messages. Invalid header and footer values. UART frame errors. Value range violations (when the decoder grows support for them, and when captures become available).

Example command line use

TODO This section is work in progress. Finish these notes. Provide the command's output. When captures become available.

 $ INPUT="-i file.bin -I protocoldata:protocol=uart:bitrate=100000:frameformat=8e2"
 $ P_UART="uart:baudrate=100000:parity=even"
 $ sigrok-cli $INPUT -P ${P_UART},sbus_futaba -A sbus_futaba


  • the RPG Quadrotor Control wiki has a very good description of the SBUS protocol, concise and complete (except for idle between messages), easy to grasp
  • this blog mentions inter-message idle phases and message repeat intervals, refers to proportional and digital channels, references the Futaba S-BUS controlled by mbed article by Uwe Gartmann
  • the vendor page(?) kept failing to load when this wiki page was written