USB to CAN

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Protocol Overview

Mode	Switch	Use Case	USB TX (USB->CAN)	USB RX (CAN->USB)
Transparent (CAN extended frame)	AT+ET	CAN extended frames	Header 2 bytes (0x45 0x54) + CANID ext 4 bytes + data 8 bytes + tail 2 bytes (0x0D 0x0A)	Header 2 bytes (0x45 0x54) + CANID ext 4 bytes + data 8 bytes + tail 2 bytes (0x0D 0x0A)
Transparent (CAN standard frame, firmware V4.1+)	AT+ET	CAN standard frames	Header 2 bytes (0x53 0x54) + fixed 2 bytes (0x00 0x00) + CANID std 2 bytes + data 8 bytes + tail 2 bytes (0x0D 0x0A)	Header 2 bytes (0x53 0x54) + fixed 2 bytes (0x00 0x00) + CANID std 2 bytes + data 8 bytes + tail 2 bytes (0x0D 0x0A)
Advanced mode	AT+AT	Standard + extended, variable payload length	Header 2 bytes (0x41 0x54) + frame ID 4 bytes + DLC 1 byte + data up to 8 bytes + tail 2 bytes (0x0D 0x0A)	Header 2 bytes (0x41 0x54) + frame ID 4 bytes + DLC 1 byte + data up to 8 bytes + tail 2 bytes (0x0D 0x0A)

• Query firmware version: AT+VER.

• Default mode after power-on is transparent mode.

• Switch to transparent mode: AT+ET.

• Switch to advanced mode: AT+AT.

Example Code

Python example: test_usb2can.py

Advanced Mode Example

Send AT+AT before transmitting data.

Header (2 bytes)	Frame ID (4 bytes)	DLC (1 byte)	Data (max 8 bytes)	Tail (2 bytes)
0x41 0x54	0x00 0x00 0x00 0x00	0x08	0x48 0x45 0x4C 0x4F 0x01 0x02 0x03 0x04	0x0D 0x0A

Frame ID is 4 bytes (32-bit):

Bit31-Bit21	Bit20-Bit3	Bit2	Bit1	Bit0
Standard CAN ID or first 11 bits of extended ID	Last 18 bits of extended ID	1=extended, 0=standard	1=remote, 0=data	Fixed 0

# USB to CAN test tool
pip3 install pyserial
python3 test_usb2can.py

# CAN standard frame example
# Send AT+AT first
std_can_id = 0x142
rtr_bit = 0
frame_id_val = (std_can_id << 21) | (rtr_bit << 1) | (0 << 2)
frame_id_bytes = frame_id_val.to_bytes(4, "big")
dlc = 8
data_bytes = bytes([0x9C, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
payload = b"\x41\x54" + frame_id_bytes + bytes([dlc]) + data_bytes[:dlc] + b"\x0D\x0A"

# Parse return (standard frame)
frame_id_val = int.from_bytes(frame_id_bytes, "big")
ide_bit = (frame_id_val >> 2) & 0x01
rtr_bit = (frame_id_val >> 1) & 0x01
std_can_id = (frame_id_val >> 21) & 0x7FF

# CAN extended frame example
ext_can_id = 0x0000FD01
rtr_bit = 0
frame_id_val = (ext_can_id << 3) | (rtr_bit << 1) | (1 << 2)
frame_id_bytes = frame_id_val.to_bytes(4, "big")
dlc = 8
data_bytes = bytes([0x00] * 8)
payload = b"\x41\x54" + frame_id_bytes + bytes([dlc]) + data_bytes[:dlc] + b"\x0D\x0A"

# Parse return (extended frame)
frame_id_val = int.from_bytes(frame_id_bytes, "big")
ide_bit = (frame_id_val >> 2) & 0x01
rtr_bit = (frame_id_val >> 1) & 0x01
ext_can_id = (frame_id_val >> 3) & 0x1FFFFFFF