# SANPO SPI to RS485

SANPO SPI to RS485 lets a Jetson, Raspberry Pi, or another host controller operate RS485 motors and devices periodically. The host places a raw device command in a fixed-length `RT` message and reads feedback during the current or a later SPI transfer.

## SPI Settings and Length

| Item | SHORT | LONG |
| --- | ---: | ---: |
| Total transfer | 23 bytes | 73 bytes |
| Message area | 22 bytes | 72 bytes |
| RS485 data area | 18 bytes | 68 bytes |
| Final byte | CRC-8 | CRC-8 |

Both modes use SPI Mode 0 and MSB First. CRC-8 uses polynomial `0x07`, initial value `0x00`, and no final XOR.

Factory default is SHORT. For more than 18 bytes of RS485 data, send over USB serial:

```text
AT+SPIMODE=LONG
```

After receiving `OK`, wait at least 10 ms and begin 73-byte transfers. The setting is saved to Flash.

## Select an RS485 Channel

| SPI chip select | Channel | Target |
| --- | --- | --- |
| First MCU | `0x01` | RS485-1 |
| First MCU | `0x02` | RS485-2 |
| Second MCU | `0x03` | RS485-3 |
| Second MCU | `0x04` | RS485-4 |
| Either chip select | `0x00` | Both RS485 interfaces on the current MCU |

Internally, `1/3` select the first local interface and `2/4` select the second. Replies use local Channel `1/2`.

## SHORT Message

Transmit and receive use the same layout:

| Byte position | Content |
| --- | --- |
| 0 to 1 | Fixed `52 54` (`RT`) |
| 2 | Channel |
| 3 | RS485 data length `0` to `18` |
| 4 to 21 | 18-byte RS485 data area; fill unused bytes with `00` |
| 22 | CRC-8 over the first 22 bytes |

For a 17-byte Unitree command on Channel 2, the message area before CRC is:

```text
52 54 02 11 FE EE 11 00 00 00 00 00 00 00 00 00 00 00 00 18 7C 00
```

Append CRC-8 over these 22 bytes.

## LONG Message

| Byte position | Content |
| --- | --- |
| 0 to 1 | Fixed `52 54` (`RT`) |
| 2 | Channel |
| 3 | RS485 data length `0` to `68` |
| 4 to 71 | 68-byte RS485 data area; fill unused bytes with `00` |
| 72 | CRC-8 over the first 72 bytes |

The CRC algorithm is shown in [SANPO SPI to CAN](spi_can).

## Read RS485 Feedback

The board places received RS485 data into the same `RT` layout:

- Byte 2 identifies the first or second local RS485 interface.
- Byte 3 is the length of this feedback block.
- Data begins at byte 4.
- Unused data bytes are `00`.

SPI is full duplex. Data read while sending a command may be older feedback or all `00`. If the target feedback is not available, continue sending all-zero frames of the current length.

One RS485 receive event is not guaranteed to contain exactly one complete device-protocol frame. Use Channel to separate buses, then reassemble data by the device protocol's header, length, and checksum.

## Unitree GO-M8010 Example

Example: [SPI to RS485 Unitree motor](https://gitcode.com/sanpo/robot/blob/unified-main/products/spine/v6/demo/rs485/spi2rs485_unitree_GM8010_demo_v6.py)

```bash
sudo python3 spi2rs485_unitree_GM8010_demo_v6.py \
  --spibus 0 \
  --cs 0 \
  --motors 1 \
  --channel 1
```

Use the current script help for exact options. For the first run, reduce motion amplitude and prepare an immediate power disconnect.

## Troubleshooting

| Symptom | Action |
| --- | --- |
| Received data is all `00` | No feedback is queued; wait briefly and send another empty frame of the current length |
| Reply Channel is 1/2 | Replies use local numbering; this is normal |
| A 17-byte command works in SHORT but a longer one fails | SHORT supports at most 18 bytes; switch to LONG |
| CRC fails | Check Mode 0, MSB First, fixed length, SPI clock, and chip-select timing |
| Device does not reply | Check chip select, Channel, RS485 settings, device ID, checksum, A/B wiring, and power |
| Feedback is split into blocks | Reassemble it by the device protocol; do not treat one SPI read as exactly one motor reply |