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Understanding Modbus Protocol: Key Features and Applications in Automation

Modbus is an open data communication protocol developed in 1979 by Modicon, designed for supervisory control and data acquisition in industrial settings. It’s flexible, well-structured, and supported by various SCADA and HMI software. Modbus operates over RS-232/485 and Ethernet (TCP/IP) with two transmission modes: ASCII and RTU. The protocol defines a query-response cycle between Modbus clients (masters) and devices (slaves), allowing for seamless communication of digital and analog data inputs/outputs. This overview highlights its architecture, message structure, and key applications.

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Understanding Modbus Protocol: Key Features and Applications in Automation

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  1. Supervisory Control & Data Acquisition Communication Technology Modbus Protocol

  2. What is Modbus ? • An open data communication protocol developed in 1979 • Published by Modicon • Open structure • Flexible • Widely known • Supplied by many SCADA and HMI software • Have OPC Servers • 2 serial transmission modes: • ASCII  10 bits • RTU (Binary)  11 bits • Communication interface • RS-232/485 • Ethernet (TCP/IP)

  3. Application structure (general) Modbus Client (Master) SCADA HMI Internet RS-232/485 Modbus Device (Slave) Modbus Device (Slave)

  4. Query-Response Cycle Query Station Number Function Code Station Number Data Bytes (Flexible) Function Code Data Bytes (Flexible) Error Check Error Check Response

  5. Hardware classification • Station Device: 0 ~ 255 • Digital input • 1xxxx: 4 digits for hexadecimal address (0000 ~ FFFF) • 1xxxxx: 5 digital for decimal address (0 ~ 65535) • Digital output • 0xxxx: 4 digits for hexadecimal address (0000 ~ FFFF) • 0xxxxx: 5 digital for decimal address (0 ~ 65535) • Analog input • 4xxxx: 4 digits for hexadecimal address (0000 ~ FFFF) • 4xxxxx: 5 digital for decimal address (0 ~ 65535) • Analog output • 3xxxx: 4 digits for hexadecimal address (0000 ~ FFFF) • 3xxxxx: 5 digital for decimal address (0 ~ 65535)

  6. 1 Start Bit 7 Data Bits 1 Parity Bit (Even/Odd) 1 Stop Bit 1 Start Bit 7 Data Bits 2 Stop Bit 1 Start Bit 8 Data Bits 1 Parity Bit (Even/Odd) 1 Stop Bit 1 Start Bit 8 Data Bits 2 Stop Bit Two serial transmission modes • ASCII Mode • Data system ASCII character, ‘0’~’9’,’A’~’F’ • Bits per data unit • Error Check Field Longitudinal Redundancy Check (LRC) • RTU Mode • Data system 8-bit Binary, 00~FF • Bits per data unit • Error Check Field Cyclical Redundancy Check (CRC)

  7. Start Station Number Function Code Data Error Check End 1 Char 2 Chars 2 Chars n Chars 2 Chars 2 Chars : LRC CR,LF Start Station Number Function Code Data Error Check End 3.5 Char 8 Bits 8 Bits n x 8 Bits 2 x 8 Bits 3.5 Chars Silence CRC Silence Prefixed Data Station Number Function Code Data 6 x 8 Bits Modbus Message Packet • ASCII Mode • RTU Mode • Modbus Plus network Byte 0, 1: transaction ID – usually 0 Byte 2, 3: protocol ID = 0 Byte 4, 5: number of bytes following

  8. Read/Write Modes • Single Read/Write • Multiple Read/Write

  9. Basic application #1 Modbus/TCP Clients (Masters) Modbus/TCP command protocol Internet Default Firmware Modbus/TCP Slave Device

  10. Modbus/TCP Clients (Masters) Serial Device Clients (Masters) Internet Modbus/TCP Slave Device Default Firmware Serial Remote Device Basic application #2 Modbus/TCP command protocol Command protocol depend on serial devices Virtual Comm Technique

  11. Modbus SDK

  12. Modbus Embedded Controller Modbus Client (Master) Modbus protocol User-defined protocol SCADA HMI Modbus Embedded Controller Check Modbus Request Update I/O RS-485 Run User Process . . . RS-232 PLC

  13. Modbus/TCP Clients (Masters) General TCP/IP Client (Masters) Serial Device Clients (Masters) Modbus/TCP Slave Internet User-defined TCP/IP Slave Special Firmware modified by users Modbus/RTU Master Serial Device Master Advanced application

  14. Modbus Resources http://www.modicon.com Web page of original Modbus protocol inventor, MODICON Inc. http://www.modbg A community for Modbus users http://www.winsite.com/bin/Info?3088 ActiveX freeware to communicate with equipments via Modbus(RTU/ASCII) or Modbus/TCP http://www.pmcrae.freeserve.co.uk/readregu.html Example C code for Linux Modbus/RTU communication

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