Which statement best describes Modbus RTU usage in field devices?

• A. It uses a fiber optic backbone to connect devices.
• B. It uses serial communication, suitable for field devices, while Modbus TCP uses Ethernet.
• C. It is only used for wireless devices.
• D. It requires a specialized gateway for all connections.

Answer: (B)

Modbus RTU is a serial communication method designed for field devices, while Modbus TCP runs over Ethernet. In practice, RTU uses a binary framing over serial links such as RS-485 or RS-232, which makes it simple, robust, and well-suited for short to moderate distances and multiple devices on a single bus. This is why RTU is commonly used in field devices like sensors and controllers that sit on physical serial networks. Modbus TCP, on the other hand, encapsulates the Modbus data within TCP over Ethernet, enabling networked communication over standard IT infrastructure.

That’s why this statement is correct: it highlights the serial nature of Modbus RTU for field deployments, contrasted with the Ethernet-based Modbus TCP. The other options don’t fit because Modbus RTU isn’t defined by a fiber optic backbone, isn’t restricted to wireless devices, and isn’t inherently requiring a gateway for every connection—gateways are optional bridges between networks, not a universal requirement.