How does Modbus RTU differ from Modbus TCP in building automation?

Modbus RTU operates over RS-485 serial connections with master-slave topology, limits cable runs to approximately 1,200 meters, supports 32 devices per segment (without repeaters), and uses a compact binary frame format at baud rates typically 9,600–115,200. Modbus TCP encapsulates the same Modbus data model inside TCP/IP ethernet frames, allowing integration with existing IT networks, providing better speed (10/100/1000 Mbps), and removing the distance and device-count limitations of RS-485. In building automation, Modbus RTU remains dominant at the field device level (VFDs, power meters, small PLCs) while Modbus TCP is gaining ground as more devices ship with embedded Ethernet ports. Neither protocol provides built-in security (authentication or encryption), so when using Modbus TCP on shared IT networks, VLAN segmentation or dedicated control networks are essential — a significant limitation compared to BACnet/SC which provides native TLS security.