MODBUS-RTU

Modbus RS-485 serial binary CRC-16.

Definition

Modbus RTU frame: Slave Address 1 byte (1-247) + Function Code 1 byte + Data N bytes + CRC-16 2 bytes. Typical baud rates: 9600, 19200, 38400, 115200. RS-485 multi-drop max 32 devices per segment (limit without repeater). Limitations vs TCP: no concurrency (master/slave half-duplex), limited bandwidth, limited distances (~1200m RS-485). Legacy adoption: Schneider Twido automates, Modicon TSX, HVAC equipment, AC drive motors.

Origin

Modbus protocol published by Modicon 1979 ; Modbus RTU specification formalised in Modbus Application Protocol Specification V1.1b3 published 2012.

Example in context

An ABB ACS580 VFD motor receives start/stop commands via Modbus RTU RS-485 ; Allen Bradley CompactLogix master controller polls 8 VFDs in daisy chain.

Related terms

• Modbus TCP — Ethernet counterpart.