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.
• fdcanusb: an allocation-free codec for the fdcanusb text line protocol, which moteus also speaks directly over a TTL UART. Encode can send lines into a caller-provided buffer, parse rcv/OK/ERR lines, and compute the CRC-8 checksums used over UART.
• diagnostic: payload framing for the tunneled diagnostic stream, including the flow-controlled poll variant for lossy transports.

UART Hosts

To command a moteus over a TTL UART from an embedded host (see the UART integration reference), encode each CAN frame as an fdcanusb line and parse the response lines:

use moteus_protocol::fdcanusb::{
    encode_can_send, parse_line, strip_checksum, EncodeOptions, Line,
    MAX_LINE_LENGTH,
};
use moteus_protocol::query::QueryResult;
use moteus_protocol::{CanFdFrame, Mode};

// Fill a frame as in "Encoding a Command" above.
let frame = CanFdFrame::new();
let mut line = [0u8; MAX_LINE_LENGTH];
let len = encode_can_send(
    &frame,
    &EncodeOptions { disable_brs: false, checksum: true },
    &mut line,
).unwrap();
let command = &line[..len]; // write this to the UART
assert!(command.starts_with(b"can send"));

// ... then for each line received from the UART, validate and strip
// the checksum, then classify the line:
let (content, _had_checksum, valid) = strip_checksum(b"OK *BD");
assert!(valid);
assert!(matches!(parse_line(content), Line::Ok)); // command acknowledged

// `rcv` lines carry a CAN frame from the device, parsed like any
// other reply — this one reports the mode and position registers:
match parse_line(b"rcv 0100 21000A2D010000003F") {
    Line::Rcv(reply) => {
        let result = QueryResult::parse(&reply);
        assert_eq!(result.mode, Mode::Position);
        assert_eq!(result.position, 0.5);
    }
    Line::Ok | Line::Err(_) | Line::Other(_) => panic!("expected a reply frame"),
}

The retry, timeout, and checksum-escalation policy is up to the host; the moteus crate's transports implement one for std environments.

Building with Bazel

When building from the moteus repository:

tools/bazel build //lib/rust/moteus-protocol