What Is Modbus? The Industrial Communication Protocol Explained

Quick Answer

Modbus is an industrial communication protocol developed by Modicon in 1979 that uses a master/slave request-response model. Its simplicity, openness, and universal manufacturer support made it the most widely deployed protocol in industrial automation — used in PLCs, RTUs, flow computers, drives, meters, and sensors worldwide.

History

Modicon (now Schneider Electric) created Modbus in 1979 to connect PLCs to field instruments. Unlike proprietary protocols, Modbus was published as an open specification, allowing any manufacturer to implement it without licensing fees. This openness drove universal adoption.

Variants

• Modbus RTU — Serial (RS-485/RS-232), binary encoding, CRC-16. Most common for field devices.
• Modbus ASCII — Serial, ASCII encoding, LRC. Legacy/debugging use.
• Modbus TCP — Ethernet, TCP port 502. Same data model, no CRC (TCP handles it).

Data Model

Four register tables: coils (1-bit R/W), discrete inputs (1-bit RO), input registers (16-bit RO), and holding registers (16-bit R/W). This model is identical across all variants.

Limitations

• No native security (no authentication, encryption)
• Polling only (slave cannot initiate communication)
• 16-bit register width (32-bit values require two registers with byte order ambiguity)
• No built-in timestamping

Modbus vs Other Protocols

DNP3 adds unsolicited responses, event-driven reporting, and timestamps — preferred for pipeline SCADA. BACnet is purpose-built for building automation with scheduling and alarm objects. OPC-UA provides an information model with built-in security for enterprise integration. Modbus remains the universal baseline protocol supported by all platforms.