Interview questions on Modbus

21. How does the data transfer occur in TCP/IP?

The complete Modbus TCP/IP Application Data Unit that is discussed above is embedded into the data field of a standard TCP frame. This TCP frame is sent via TCP to port 502, which is specifically reserved for Modbus applications.

Modbus data transfer occur in TCP/IP

22. Through which port is the Modbus data transmitter?

Modbus TCP/IP clients and servers listen and receive Modbus data via port 502

23. What is the difference between Modbus ASCII and Modbus RTU?

  • Modbus RTU mode uses binary coding and CRC error-checking.
  • Modbus ASCII messages are less efficient and use less effective LRC error checking.
  • ASCII mode uses ASCII characters to begin and end messages whereas RTU uses time gaps (3.5 character times) of silence for framing.
  • The two modes are incompatible so a device configured for ASCII mode cannot communicate with one using RTU.
  • Modbus ASCII messages require twice as many bytes to transmit the same content as a Modbus RTU message.

24. What is the maximum Modbus RTU communication?

  • RS – 232: Maximum distance of 15 meters.
  • RS – 485 & RS – 422: Maximum distance of 1200 meters.
  • TCP/IP – Can be used at any range with proper network design.

25. RS 232 and RS 485 connections.

RS 232 Pinout
RS 232
RS 485 Pinout
RS 485

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