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Modbus Protocol Overview

The document discusses Modbus, including its history, physical layers, and strengths and weaknesses. It was created in 1979 by Modicon to allow PLCs to communicate with programming tools. Today it is used for master/client communications between intelligent devices. It can operate over RS232, RS485 and other media. The document describes the Modbus serial link and its use of master/slave access. It also discusses Modbus Plus, Ethernet TCP/IP, and the RS485 physical layer specifications. The link layer uses a master/slave transmission method with up to 120 data words. Modbus ASCII and RTU formats are covered.

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80 views7 pages

Modbus Protocol Overview

The document discusses Modbus, including its history, physical layers, and strengths and weaknesses. It was created in 1979 by Modicon to allow PLCs to communicate with programming tools. Today it is used for master/client communications between intelligent devices. It can operate over RS232, RS485 and other media. The document describes the Modbus serial link and its use of master/slave access. It also discusses Modbus Plus, Ethernet TCP/IP, and the RS485 physical layer specifications. The link layer uses a master/slave transmission method with up to 120 data words. Modbus ASCII and RTU formats are covered.

Uploaded by

Nguyễn Duy Khôi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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5/19/2014

Chapter 4: Modbus
Modbus History
Modbus and the ISO model
Physical layer
Link layer
Application layer
Profiles
Strengths -Weaknesses

4.1 History

The MODBUS protocol is a message handling structure created


by MODICONin 1979 to connect PLCs to programming tools.

Today, this protocol is mainly used to set up master/client type


communications with slaves/servers between intelligent devices.

MODBUS is independent of the physical layer. It can be


implemented on RS232, RS422, or RS485 links as well as on a
wide variety of other media (e.g.: optical fibre, radio, etc.).

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4.2 Modbus serial link and the ISO model


MODBUS on a serial link operating at 1200 to 56 Kbps with a
master/slave access method.

4.3 Modbus Plus and the ISO model


MODBUS PLUS is a bus operating at 1 Mbps based on a token
ring access method which uses the MODBUS message handling
structure.

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4.5 MODBUS Ethernet TCP/IP

MODBUS Ethernet TCP/IP uses TCP/IP and Ethernet 10 Mbps


or 100 Mbps to carry the MODBUS message handling structure.

4.6 RS485 physical layer

Medium: Shielded twisted pair

Topology: Bus type With tap links and line terminators

Maximum distance:1300 m without repeaters

Speed:19,200 bps (56 Kbps on some products)

Max. no. of devices: 32


1 master and 31 slaves

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4.6 RS485 physical layer


Connectors recommended by Schneider

4.6 RS485 physical layer


Example architecture

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4.6 Link layer

Medium access method: Master/slave

Transmission method: Client/server

Max. size of useful data: 120 PLC words

Transmission security: LRC or CRC


Start and stop delimiters
Parity bit
Continuous flow

4.6 Link layer


Modbus ASCII and Modbus RTU

Modbus RTU
There are 2 versions of the MODBUS protocol:
-ASCII mode Each byte in the frame is sent in 2-character ASCII
format.

-RTU mode Each byte in the frame is sent in 2-character 4-bit


hexadecimal format. The main advantage of RTU mode is that it
sends data more quickly. ASCII mode allows the insertion of a
time interval of one second between 2 characters without
generating a transmission error.

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4.6 Link layer


Modbus frame

The structure of a Modbus frame is the same for requests (message


from the master to the slave) and responses (message from the
slave to the master)

4.6 Link layer


Modbus Example

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4.6 Link layer

PDF format version of the MODBUS Protocol

The original was found at:

http://irtfweb.ifa.hawaii.edu/~smokey2/software/about/sixnet/m
odbus/modbus_protocol.pdf

4.7 Application layer

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