interface between touch screen and BL4S100 by Modbus (rs-232)

My touch panel is quite easy to connect to PLC, but I need to use the board BL4S100 as PLC. I thought to use Modbus protocol and, in particular, rs-232 (avaible on my display). But Im new with this tech and I have some problems to type a proper C-code. Somebody can help me?
Thay you all

Hello, Check the below forum post.