SANPO SPI to RS485

SANPO SPI to RS485 lets a Jetson, Raspberry Pi, or another host controller operate RS485 motors and devices in real time. The host sends raw RS485 commands inside fixed 23-byte SPI frames and reads device feedback during later transfers.

Use V47 or later firmware.

SPI Connection and Settings

Item	Setting
SPI mode	Mode 0
Bit order	MSB First
Transfer length	Exactly 23 bytes
Initial test clock	Start at 2 MHz
Recommended frame gap	At least 100 microseconds
Check	Byte 23 is CRC-8 over the first 22 bytes

Board chip select CS1 controls RS485-1 and RS485-2; CS2 controls RS485-3 and RS485-4. On a Linux host, software CS0 and CS1 normally connect to the board connectors marked CS1 and CS2.

Factory RS485 settings are 4 Mbps, one stop bit, no parity, and eight data bits. To save other settings, use AT+SETRS485 over USB as described in RS485 Raw Passthrough. For a temporary SPI-side change, use spi_set_baudrate.py. Temporary SPI-side settings are lost after a power cycle.

Example: configure the first RS485 channel selected by Linux SPI0 CS0 for 4 Mbps, 8-N-1:

sudo python3 spi_set_baudrate.py --spibus 0 --cs 0 --mode rs485 --rs485num 1 --baudrate 4000000 --stop-bits 1 --parity 0 --data-bits 8

Verify the result with an actual device command. The setting is not written to Flash.

Select an RS485 Channel

Board chip select	Channel	Target
CS1	`0x01`	RS485-1
CS1	`0x02`	RS485-2
CS2	`0x03`	RS485-3
CS2	`0x04`	RS485-4
CS1 or CS2	`0x00`	Both RS485 channels controlled by the selected MCU

23-Byte Transmit Frame

Byte position	Content
0 to 19	Raw RS485 data; pad to 20 bytes with `00`
20	Channel
21	Raw-data length `1` to `20`; use `0` for an empty polling frame
22	CRC-8 over the first 22 bytes

CRC-8 uses polynomial 0x07, initial value 0x00, and no final XOR. See SANPO SPI to CAN for sample code.

Send a 17-byte Unitree motor command on Channel 2:

FE EE 11 00 00 00 00 00 00 00 00 00 00 00 00 18 7C 00 00 00 02 11 67

Bytes 17 to 19 are padding, 02 is the Channel, 11 is hexadecimal for 17 bytes, and 67 is CRC-8.

Read RS485 Feedback

SPI is full duplex. Data returned while sending a command may be an older cached reply or all 00. To wait for feedback, continue sending all-zero 23-byte polling frames.

Reply format:

Byte position	Content
0 to 19	RS485 device feedback; pad to 20 bytes with `00`
20	Source Channel: `0x01` for the first channel or `0x02` for the second
21	Actual feedback length `0` to `20`
22	CRC-8 over the first 22 bytes

The reply Channel is local to the selected MCU. Feedback from RS485-3 and RS485-4 through CS2 therefore also reports 0x01 and 0x02. The host should verify CRC before reading Channel and length.

The Channel field is valid starting with V47. Earlier firmware always returned 00. Existing applications that ignore byte 20 remain compatible; new applications can use it to identify the source RS485 channel.

Examples

Limitations

A raw RS485 command can contain at most 20 bytes.
Raw data must not begin with 41 54, 45 54, or 53 54. Contact support if the device protocol uses one of these prefixes.
Every transfer must contain all 23 bytes, including padding and CRC.

Troubleshooting

Symptom	Action
Received data is all `00`	No feedback is queued; wait briefly and send another empty frame
CRC fails	Check Mode 0, MSB First, 23-byte transfers, and then lower the SPI clock or increase the frame gap
RS485 device does not reply	Check chip select, Channel, raw-data length, RS485 settings, command checksum, A/B wiring, and power
Command is sent on both RS485 channels	Channel is `0x00`; select a specific channel
Occasional loss during continuous high-rate traffic	Start at 2 MHz with a 100 microsecond frame gap and increase the rate only after stable testing