# SPI to RS485

- The following protocol applies to firmware V4.1 or later
- Send the AT+VER serial command to check the firmware version; if no version is returned or it shows V33, please update to the <a href="https://gitcode.com/sanpo/robot/tree/v4/firmware/sanpo_spine" target="_blank">latest firmware</a>
- The <a href="https://sanporobot.com/sanpo-studio" target="_blank">SANPO Motor Debug Tool</a> can generate test code quickly and supports CAN/RS485 motors

## Protocol Overview

| Use Case | Wiring | TX Protocol (SPI->RS485) | RX Protocol (RS485->SPI) |
| --- | --- | --- | --- |
| RS485 motors (fixed 23-byte frames) | CS1 controls RS485-1/RS485-2; CS2 controls RS485-3/RS485-4 | Transparent, fixed length 23 bytes. Last 3 bytes reserved: last is CRC, second last is valid data length, third last is Channel. | Transparent, fixed length 23 bytes. Last 3 bytes reserved: last is CRC, second last is valid data length, third last is Channel. |

- The first two bytes of RS485 frames must not be the reserved headers 0x45 0x54, 0x41 0x54, or 0x53 0x54

### Frame Examples

TX frame

| RS485 data passthrough (pad to 20 bytes with 0x00) | Channel | Data length | CRC (1 byte) |
| --- | --- | --- | --- |
| 0x22 0x33 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x00 0x00 | 0x00 | 0x17 | 0x0F |

RX frame

| RS485 feedback passthrough | Channel | Data length | CRC (1 byte) |
| --- | --- | --- | --- |
| 0x22 0x33 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x54 0x00 0x53 0x00 0x00 0x00 0x00 | 0x00 | 0x16 | 0x0F |

### Code Snippet (SPI to RS485, Python)

Full example: <a href="https://gitcode.com/sanpo/robot/blob/v4/demo/spi_demo/unitree" target="_blank">spi_unitree_GO_M8010_demo_v4.py</a>

```python
# TX: motor command data (<=20 bytes, pad with 0x00) + Channel + Data length + CRC
def build_spi_rs485_frame(cmd: bytes, channel: int) -> bytes:
    data_bytes = bytearray(cmd[:20])
    data_len = len(data_bytes)
    if data_len < 20:
        data_bytes.extend([0x00] * (20 - data_len))
    payload = bytes(data_bytes) + bytes([channel & 0xFF, data_len])
    crc = crc8_calculator.calculate(payload)
    return payload + bytes([crc])  # fixed length 23 bytes


tx = build_spi_rs485_frame(cmd, channel=0x00)
rx = spi.xfer2(list(tx))  # fixed length 23 bytes

# RX parse: CRC check + read valid data length
data_part = rx[:22]
crc_ok = crc8_calculator.calculate(data_part) == rx[22]
data_len = rx[21]
motor_data = rx[:data_len]
```

## Examples

- SPI to RS485 example (Python): <a href="https://gitcode.com/sanpo/robot/blob/v4/demo/spi_demo/unitree/" target="_blank">Download</a>  

- SPI uses fixed 23-byte frames by default