What are the difference between Modbus RTU and TCP(Modbus RTU vs TCP)

What is MODBUS?

  • MODBUS is a serial communication protocol.
  • Published by Modicon (now Schneider Electric).

Reasons for the use of MODBUS in the industrial Environment?

  • Developed with industrial application in mind.
  • Openly published and royalty free.
  • Easy to deploy and maintain.
  • Moves raw bits or words without placing many restriction on vendors.
  • It enables communication among many devices connected the same network

Protocol Version

  • MODBUS RTU.
  • MODBUS ASCII.
  • MODBUS TCP/IP.
  • MODBUS OVER UDP.
  • MODBUS+.
  • MODBUS PEMEX
  • ENRON MODBUS

Modbus RTU VS TCP

  • Modbus RTU is an open,serial(RS-232 or RS-485 ) protocol derived from the master slave architecture.
  • It is a widely accepted protocol due to its ease of use and reliability.
  • MODBUS RTU is widely used within Building Management system(BMS) and industrial automation system.
  • MODBUS RTU messages are a simple 16-bit CRC(cyclic -redundant Checksum).
  • The simplicity of these messages is to ensure reliability .

MODBUS is considered an application layer messaging protocol, providing master slave communication between devices connected together buses or networks.

The function codes of MODBUS are elements of MODBUS Request/reply PDUs(protocol data unit) Data Object Properties

  • MODBUS RTU packets are only intended to send data; they do not have the capability to send parameters such as point name,resolution, units, etc.
  • If the ability to send such parameters is needed, one should investigate a BACnet, EtherNeVIP, or other modern protocols.

Modbus RTU Address Requirment

  • Standard RTU node addresses are 1-254, with being reserved for broadcast messages and write only.
  • ‘O’ address is rarely used due to the fact there is no confirmation that the message was properly received at this slave node.
  • If physical layer is RS-232 as only one node can be implemented anyway
  • RS-485 limits the number of nodes to 32.

Modbus TCP VS RTU

  • MODBUS TCP/IP is a variant of the MODBUS family of simple, vendor – neutral communication protocols intended for supervision and control of automation equipment.
  • It covers the use of MODBUS messaging in an ‘Intranet’ or ‘internet’ environment using the TCP/IP protocols.
  • The MODBUS TCP/IP is being published as a automation standard.
  • In MODBUS, data transaction are traditionally stateless, making them highly resistant.
  • TCP/IP allows a very large number of concurrent connections

Data Encoding

MODBUS uses a ‘big-endian’ representation for addresses and data items. This means that when a larger numerical quantity larger than a single byte is transmitted, the most significant byte is sent first.

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