CAN Protocol

Controller Area Network (CAN) is a robust, differential serial bus designed for real-time embedded systems. It is the standard communication layer for Robstride and most MIT-derived QDD actuators.

Why CAN?

• Multi-drop — up to 127 nodes share a single twisted pair, no hub needed
• Deterministic — priority-based arbitration gives hard latency bounds
• Differential signaling — immune to ground-loop noise in motor cabinets
• Error detection — hardware CRC, bit stuffing, and ACK checks
• Speed — 1 Mbit/s gives a ~1 kHz control loop with eight motors on the bus

Physical Layer

CAN uses two wires: CAN-H (dominant high, ~3.5 V) and CAN-L (dominant low, ~1.5 V). The differential voltage is ≈2 V for a dominant (logic 0) bit and 0 V for a recessive (logic 1) bit.

        dominant bit        recessive bit
CAN-H:  ────────────┐       ┌─────────────
                    │       │
CAN-L:  ────────────┘       └─────────────
        3.5 V / 1.5 V       2.5 V / 2.5 V

Termination

Both ends of the bus must have 120 Ω termination resistors across CAN-H and CAN-L. Without termination you will see reflections and intermittent errors.

Host interface          Node 1          Node N
   [120Ω]────────────────────────────────[120Ω]
    CAN-H ═══════════════════════════════ CAN-H
    CAN-L ═══════════════════════════════ CAN-L

CAN 2.0B Frame Format

Robstride uses the extended frame (29-bit identifier):

┌──────┬────────┬─────┬──────┬────────────────────┬─────┐
│ SOF  │  ID    │ RTR │ DLC  │   Data (0–8 bytes) │ CRC │
│ 1 b  │ 29 b   │ 1 b │ 4 b  │   0–64 bits        │15+1 │
└──────┴────────┴─────┴──────┴────────────────────┴─────┘

The Robstride protocol encodes motor ID and command type inside the 29-bit extended ID field, following the MIT Mini Cheetah motor driver convention.

MIT Motor Protocol (Robstride)

Extended ID layout

Bits 28–24: Command type
Bits 23–8:  Parameter (depends on command)
Bits 7–0:   Motor CAN ID

Command types

MIT Control frame (Cmd 0x03)

This is the main motion command. The 8-byte payload encodes desired position, velocity, stiffness (Kp), damping (Kd), and feedforward torque:

Byte  0–1: position   (uint16, mapped from –π…+π rad)
Byte  2–3: velocity   (uint16, mapped from –30…+30 rad/s)
Byte     4: Kp        (uint8,  mapped from 0…500 N·m/rad)
Byte     5: Kd        (uint8,  mapped from 0…5 N·m·s/rad)
Byte  6–7: torque_ff  (uint16, mapped from –18…+18 N·m)

All values are linearly mapped to the wire format using:

raw = (value − min) / (max − min) × (2^bits − 1)

Interface Cards

To connect a Linux host to the CAN bus you need a CAN interface:

All of these appear as can0 / can1 under SocketCAN on Linux.