What is Modbus RTU and when would you use Modbus TCP in a DDC system?

• A. Modbus RTU is a wireless protocol; Modbus TCP is a wired Ethernet protocol.
• B. Modbus RTU is a protocol for Ethernet; Modbus TCP is for serial devices.
• C. Modbus RTU is for USB-based devices; Modbus TCP is for Bluetooth devices.
• D. Modbus RTU is serial communication; Modbus TCP uses Ethernet. Use RTU for serial field devices, TCP for Ethernet networks.

Answer: (D)

Modbus comes in two common transport options: RTU over serial and TCP over Ethernet. RTU uses a binary framed protocol on serial links (like RS-485 or RS-232), which makes it simple and robust for short-distance, field-device connections. Modbus TCP wraps the same Modbus messaging inside TCP/IP, so it runs over Ethernet networks, allowing easier integration with IT systems, more devices, and longer reach through switches and routers.

Use RTU when you’re connecting serial field devices directly to a controller, with short cable runs and a straightforward, low-bandwidth setup. Use TCP when you have an Ethernet network in the building or campus, want IP addressing and remote access, and need to span larger distances or connect many devices across different network segments.

In short, RTU is serial communication for field devices; TCP uses Ethernet networks, and you choose RTU for serial field devices and TCP for Ethernet networks.