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ysoldak / HeadTracker

Zero configuration, auto-calibrating wireless DIY Head Tracker, for Nano 33 BLE and XIAO BLE Sense boards
The Unlicense
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SBUS output signal #18

Open MrMap opened 2 years ago

MrMap commented 2 years ago

Regarding HT output formats: Presently there is BT and PPM; SBUS could be an additional format. SBUS would be useful for all Radiomaster & Jumper users that do not have BT and are OK with a wire connection between HT and radio (servo cable type). They could use SBUS wire connection between HT and radio serial port 1/2 (used as SBUS trainer port) - and even power the HT via the very same wire (supply HT power from radio serial port).

Two benefits over PPM: Digital serial protocol - no jitter. HT power can be supplied without need of USB.

Here is some info on the SBUS serial protocol: https://github.com/uzh-rpg/rpg_quadrotor_control/wiki/SBUS-Protocol

Nice to know: For test purposes this gadget, found in many RC pilots toolbox, can display the values transmitted on all the SBUS channels: https://www.isdt.co/down/pdf/BG-8S_en.pdf

ysoldak commented 1 year ago

Notes harvested from internets

The SBUS protocol uses an inverted serial logic with a baud rate of 100000, 8 data bits, even parity, and 2 stop bits. The SBUS packet is 25 bytes long consisting of:

       Byte[0]: SBUS header, 0x0F
       Byte[1 -22]: 16 servo channels, 11 bits each
       Byte[23]
       Bit 0: channel 17 (0x01)
       Bit 1: channel 18 (0x02)
       Bit 2: frame lost (0x04)
       Bit 3: failsafe activated (0x08)
       Byte[24]: SBUS footer

Note that lost frame is indicated when a frame is lost between the transmitter and receiver. Failsafe activation typically requires that many frames are lost in a row and indicates that the receiver has moved into failsafe mode. Packets are sent approximately every 10 ms or 20 ms, depending on the system configuration.

Note on CH17 and CH18: Channel 17 and channel 18 are digital on/off channels. These are not universally available on all SBUS receivers and servos.

FrSky receivers will output a range of 172 - 1811 with channels set to a range of -100% to +100%. Using extended limits of -150% to +150% outputs a range of 0 to 2047, which is the maximum range acheivable with 11 bits of data.

Because SBUS is a digital bus format, it is an excellent means of receiving pilot commands from a transmitter and an SBUS capable receiver. If SBUS servos are used in the aircraft, SBUS is also an excellent means of sending actuator commands - servo commands can often be sent with lower latency and, by only using a single pin to command up to 16 servos, additional microcontroller pins are freed for other uses.