From my experience it is absolutely needed to have an Arduino Mega or equivalent (needs to have at least 2 serial ports), you won't be able to read the Modbus if Serial is also used by USB Serial ...

This example demonstrates how to use the IS4310 Modbus RTU Slave chip with the Arduino platform. The sketch creates a Modbus actuator using the IS4310. The Arduino reads Modbus Holding Registers 1, 2, ...

Abstract: This paper presents the design and implementation of a multipurpose networked control system using Arduino and Modbus RTU over RS-485 communication. The system enables robust and scalable ...