RS232 MODBUS RTU

Hello Fellows, my name is Humphrey. I am completely new into this
I am using a JackRabbit BL1800 and I want to make a serial RS232 comunication over MODBUS RTU.

I have no idea how to use any of this libraries and specially the one for MODBUS RTU

Please can any kind soul, could send an example? Or where i can read about this?

Thanks in advance
Humphrey