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Modbus Essential

Modbus Essential. Prepared by: Ray Chen Date: 11-09-2012. Training Essential. Serial Communication TCP/IP Networking. Modbus Essential. What Modbus is How Modbus Works One Request and One Response Modbus RTU/ASCII vs Modbus TCP What a Modbus Gateway is. What Modbus is.

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Modbus Essential

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  1. Modbus Essential Prepared by: Ray Chen Date: 11-09-2012

  2. Training Essential Serial Communication TCP/IP Networking Confidential

  3. Modbus Essential • What Modbus is • How Modbus Works • One Request and One Response • Modbus RTU/ASCII vs Modbus TCP • What a Modbus Gateway is Confidential

  4. What Modbus is Confidential

  5. What Modbus is • Fieldbus, IEC 61158 • CAN (Controller Area Network) • PROFIBUS (Process Field Bus) • Modbus • … Confidential

  6. Modbus History • Developed by Modicon in 1979 to exchange information between products on the factory floor. • Modbus Organization, Inc. • http://www.modbus.org Confidential

  7. Where You can Find Modbus Confidential

  8. How Modbus Works Confidential

  9. Request Response Modbus Communication Confidential

  10. Physical Media of Modbus Modbus RTU/ASCII devices Modbus TCP devices Confidential

  11. Modbus Variations Confidential

  12. Request Modbus RTU/ASCII Message Frame ASCII Message Frame Maximum size of ASCII Message Frame is 513 Bytes RTU Message Frame >= 3.5 char Maximum size of RTU Message Frame is 256 Bytes >= 3.5 char Confidential

  13. Function Code Codes are listed in decimal Confidential

  14. Modbus Data Model Confidential

  15. Request Response Normal Request & Response Modbus RTU Request Modbus RTU Response Confidential

  16. Request Response Abnormal Request & Response Modbus RTU Request Modbus RTU Response Confidential

  17. Exception Code Confidential

  18. Exception Code (cont.) Confidential

  19. Request Modbus TCP Message Frame Modbus TCP Frame 7-byte MBAP header Confidential

  20. Request Response Normal Request & Response Modbus TCP Request Modbus TCP Response Confidential

  21. Request Response Abnormal Request & Response Modbus TCP Request Modbus TCP Response Confidential

  22. Mixed Modbus Requirement • Modbus Gateway • Modbus TCP devices to work with Modbus RTU/ASCII devices Confidential

  23. Modbus Gateway Solutions ① Master Slave ①RTU to RTU ② TCP to TCP ③ RTU to TCP ④ TCP to RTU ④ ③ ② Confidential

  24. How a Modbus Gateway Works Modbus RTU Frame Modbus TCP Frame Confidential

  25. You should Remember • One Request & One Response • Modbus Gateway communicates between • Modbus RTU/ASCII • Modbus TCP Confidential

  26. Appendix Confidential

  27. Modbus Plus (Modbus+ or MB+) 7 6 5 4 3 2 1 Application Modbus TCP Modbus Modbus+ Presentation Session Transport TCP Network IP 802.4 Token passing Master - Slave Link Ethernet (CSMA/CD 802.3) Physical RS232/422/485 Confidential Physical layer works with HDLC (High-level Data Link Control), which is different from standard RS-232, RS-422, and RS-485.

  28. Modbus Message Frame – Example of Request and Normal Response Master Query, ASCII and RTU Framing Slave Response, ASCII and RTU Framing Confidential

  29. Slave Address Field • Address • 0: Broadcast • 1 – 247: Slave address • 248 – 255: Reserved Confidential

  30. Data Field • Data • Request • Start address • Length • Response • Length • Data • Address starts at 0x0000 Confidential

  31. OSI vs Modbus Confidential

  32. State Machine – Modbus Master Confidential

  33. State Machine – Modbus Slave Confidential

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