SANPO USB to RS485

SANPO USB to RS485 lets a PC select one of RS485-1/2/3/4. The host places a raw device command inside an RT wrapper. The board only selects the interface and forwards the data; it does not change the motor protocol.

To send unwrapped data without a Channel field, use RS485 Raw Passthrough. Raw passthrough requires at least 3 ms of idle time between commands; use the RT wrapper on this page for continuous high-rate real-time control.

Connection and Communication Settings

A complete board normally exposes two USB serial ports. One accesses RS485-1/2, and the other accesses RS485-3/4. For first use, connect one device to identify the USB-to-interface mapping.

Factory defaults are 4 Mbps, one stop bit, no parity, and eight data bits. Send AT+SETRS485? before use and change the settings through Communication Settings when necessary. The bitrate selected in a USB serial application does not change the RS485 bus.

Select an RS485 Channel

Current USB port	Channel	Target
Port for RS485-1/2	`0x01`	RS485-1
Port for RS485-1/2	`0x02`	RS485-2
Port for RS485-3/4	`0x03`	RS485-3
Port for RS485-3/4	`0x04`	RS485-4
Either USB port	`0x00`	Both RS485 interfaces on the current MCU

For transmission, 1/3 select the first local interface and 2/4 select the second. Replies use local Channel: 0x01 for the first and 0x02 for the second.

RT Frame Format

Transmit and receive use the same wrapper:

Field	Length	Description
Header	2 bytes	Fixed `52 54` (`RT`)
Channel	1 byte	RS485 interface number
Data length	1 byte	Raw RS485 data length, up to 64 bytes
Data	0 to 64 bytes	Raw command or feedback defined by the device manufacturer
Tail	2 bytes	Fixed `0D 0A`

When transmitting to RS485, the board removes RT, Channel, length, and tail and sends only the raw Data field. When a device replies, the board adds an RT wrapper and sets Channel to the first or second local interface.

Unitree GO-M8010 Example

Raw Unitree command:

FE EE 11 00 00 00 00 00 00 00 00 00 00 00 00 18 7C

Wrap the 17 bytes for Channel 2:

52 54 02 11 FE EE 11 00 00 00 00 00 00 00 00 00 00 00 00 18 7C 0D 0A

02 is the target and 11 is hexadecimal for 17. The RS485 bus still carries only the 17 raw bytes beginning with FE EE.

A motor reply has a form similar to:

52 54 02 10 FD EE ...16 bytes of motor feedback... 0D 0A

Example: USB to RS485 Unitree motor

Windows:

python usb2rs485_unitree_demo_v6.py --port COM9 --motors 1 --channel 2

Linux:

sudo python3 usb2rs485_unitree_demo_v6.py --port /dev/ttyACM0 --motors 1 --channel 2

Reply Handling

One RS485 receive event represents one continuous block and is not guaranteed to contain exactly one complete motor-protocol frame. Channel identifies the source interface, but the host must still reassemble complete feedback using the motor protocol’s header, length, and checksum.

USB is also a byte stream. One read may return part of an RT frame or several frames. Find 52 54, read the length field, and wait for the complete 6 + data-length bytes.

Troubleshooting

Symptom	Action
Command is sent on both RS485 interfaces	Channel is `0x00`; select a specific channel
No feedback	Check USB mapping, Channel, RS485 settings, device ID, command checksum, A/B wiring, and power
Reply Channel shows 1/2 instead of 3/4	Replies use local numbering; this is normal
Reply does not begin with `52 54`	RAW replies are enabled; send `AT+RS485RAW=0`
Need to send raw RS485 data directly	Enable RAW replies as described in RS485 Raw Passthrough
Reply is split into several blocks	Reassemble the `RT` wrapper by its length, then parse Data according to the device protocol