MODBUS is a serial communication protocol used for industrial automation. There are different versions, including MODBUS RTU which uses serial communication over RS-232 or RS-485, and MODBUS TCP/IP which uses TCP/IP for communication over an intranet or internet. MODBUS RTU uses a master-slave architecture and simple messages with CRC checksums to ensure reliability over serial communication. MODBUS TCP/IP allows for more connections than MODBUS RTU and uses the TCP/IP network protocol stack. Both versions are open, royalty-free protocols that enable communication between industrial devices.

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Difference between
MODBUS RTU &
MODBUS TCP

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 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 to the
same network.

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Protocol Version
 MODBUS RTU.
 MODBUS ASCII.
 MODBUS TCP/IP.
 MODBUS OVER UDP.
 MODBUS+.
 MODBUS PEMEX
 ENRON MODBUS

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MODBUS RTU
 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.

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•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).

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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, EtherNet/IP, or other modern protocols.
MODBUS RTU ADDRESS REQUIREMENT
 Standard RTU node addresses are 1-254, with 0 being reserved for
broadcast messages and write only.
 ‘0’ 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.

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 RS-485 limits the number of nodes to 32.

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MODBUS TCP/IP
 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.

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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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