moteus-protocol

Low-level CAN-FD protocol types for moteus brushless motor controllers.

This crate encodes and decodes the CAN-FD frames used to communicate with moteus controllers. It performs no I/O of its own: you bring the CAN-FD transport, and this crate builds the frames you send and parses the frames you receive.

It is no_std compatible and requires no allocator, so it is usable on embedded systems as well as in standard environments.

Most applications should use the higher-level moteus crate instead, which builds on this one to add blocking and async controllers, transport implementations (fdcanusb, SocketCAN), and device discovery. Reach for moteus-protocol directly when you are on an embedded target or have your own CAN-FD transport.

Encoding a Command

Commands use a builder pattern, and serialize into a CanFdFrame:

use moteus_protocol::{calculate_arbitration_id, CanFdFrame};
use moteus_protocol::command::{PositionCommand, PositionFormat};

// Address servo ID 1 from source ID 0, requesting a reply.
let mut frame = CanFdFrame::new();
frame.arbitration_id = calculate_arbitration_id(0, 1, 0, true);

let cmd = PositionCommand::new()
    .position(0.5)   // revolutions
    .velocity(1.0);  // revolutions / s
cmd.serialize(&mut frame, &PositionFormat::default());

// frame.data and frame.size now contain the encoded command, ready
// to hand to any CAN-FD transport.

Requesting Telemetry

A query describes which registers the controller should report, and at what resolution. It can be appended to the same frame as a command, or sent on its own:

use moteus_protocol::{CanFdFrame, Resolution};
use moteus_protocol::query::QueryFormat;

let mut frame = CanFdFrame::new();

let mut query = QueryFormat::default();
query.position = Resolution::Float;  // full precision
query.velocity = Resolution::Float;

let expected_reply_size = query.serialize(&mut frame);

Parsing a Reply

use moteus_protocol::{CanFdFrame, Mode};
use moteus_protocol::query::QueryResult;

// A reply frame as received from the transport.  This one reports the
// mode register as an int8 and the position register as a float.
let mut reply = CanFdFrame::new();
reply.data[..9].copy_from_slice(&[
    0x21, 0x00, 0x0A, // reply int8, register 0x000: Mode = 10 (position)
    0x2D, 0x01, // reply f32, register 0x001: Position
    0x00, 0x00, 0x00, 0x3F, // 0.5f32, little endian
]);
reply.size = 9;

let result = QueryResult::parse(&reply);
assert_eq!(result.mode, Mode::Position);
assert_eq!(result.position, 0.5);

Key Types

CanFdFrame: a raw CAN-FD frame (arbitration ID, payload, flags), independent of any particular transport.
command: builder-style command types such as PositionCommand, CurrentCommand, VFOCCommand, StayWithinCommand, StopCommand, and BrakeCommand, with matching *Format resolution descriptions.
query::QueryFormat / query::QueryResult: telemetry requests and replies.
Register, Mode, Resolution: the moteus register map, operating modes, and wire resolutions.
WriteCanData, WriteCombiner, parse_frame: multiplex primitives for reading and writing arbitrary registers.
calculate_arbitration_id / parse_arbitration_id: CAN ID routing helpers.

Building with Bazel

When building from the moteus repository: