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:

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:

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.

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

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