NodemCU Modbus RTU

I have planned to connect my Device in above format. I will send modbus request from nodemcu & read the value and display on blynk app;
How to proceed to execute it,
i have attached some example like this but they are using as salve , But i want to use nodemcu as master and my device as slave . Send request and deliver the response .
The slave is working on modbus RTU format.

Modbus example