System Workbench for STM32

I though lwip is for TCP/IP connections only, not modbus RTU.

When you found a RS485 inferface for your board you can have a look at the modbus software implementation.

Modbus RTU is a really simple protocol. I wrote my own modbus stack at work. Have a look at

http://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
and
http://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

Modbus is well documented. 😊
I does take some time though to get the timing right.