06 Tech Modbus En

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Section 6 - Slide 1 / 27P&T - GPS - Training

PhW - 06_TECH_Modbus_en 06/ 2004

Modbus trainingPart 1 : Reference documents - WEB sites

Part 2 : Definitions

Part 3 : Modbus frame description

Part 4 : Security of transmission

Part 5 : Physical layer

Part 6 : Main characteristics resume
http://www.modicon.com/techpubs/toc6.htmlhttp://www.transparentfactory.com/http://www.modbus.org/


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Modbus User guide

Reference : TSX DG MOD *

Detailed description of the Modbus protocol.

X-WAY communication user guideReference : TSX DR NET *

This guide gives common characteristics to X-WAY Schneider networks.

Communication application specific functions PL7 Micro/Junior/Pro

Reference : TLX DS COM PL7 42 *

Volume 1 : Common communication application

Volume 2 : Modbus bus

Part 1 : Reference documents - WEB sites

Schneider documents


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WEB sites

Modbus.org :http://www.modbus.org

Modicon site for Modbus users.

Transparent Factory :

http://www.transparentfactory.comSchneider Electric site for Transparent Factory users.

Modbus Plus :

http://www.modicon.com/techpubs/toc6.html

Modbus Plus Planning and installation.

Part 1 : Reference documents - WEB sites
http://www.modbus.org/http://www.modbus.org/


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Modbus protocol

MODBUS Protocol is a messaging structure created by MODICON

company to connect PLC to programming tools.

It is now widely used to establish master-slave communication between

intelligent devices.

MODBUS is independent of the physical layer.

It can be implemented using RS232, RS422, or RS485 or over a variety

of media (e.g. fiber, radio, cellular, etc...).

Part 2 : Dfinitions


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Modbus Serial line RS485

MODBUS Serial line RS485 is a low cost network using a master/slavemedium access with a transmission speed from 1,200 to 115 Kbits/s.

Application

Presentation

Session

Transport

Network

Link

Physical

7

6

5

4

3

2

1

Master - Slave

Modbus

RS485

Part 2 : Dfinitions


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Modbus TCP/IP

MODBUS TCP/IP uses TCP/IP and Ethernet 10 Mbit/s or 100Mbits/s to carry the MODBUS messaging structure.

Application

Preentation

Session

Transport

Network

Link

Physical

7

6

5

4

3

2

1

CSMA / CD

ETHERNET V2 ou 802.3

Modbus

TCP

IP

Part 2 : Dfinitions


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Modbus Plus

MODBUS PLUS is a higher speed network 1 Mbit/s token passing derivativethat uses the MODBUS messaging structure.

Application

Presentation

Session

Transport

Network

Link

Physical

7

6

5

4

3

2

1

802.4 Token passing

Modbus

RS485

Part 2 : Dfinitions


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Modbus ASCII and Modbus RTU

The MODBUS protocol comes in 2 versions :

ASCII transmission mode

Each eight-bit byte in a message is sent as 2 ASCII characters.

RTU transmission mode

Each eight-bit byte in a message is sent as two four-bit hexadecimal

characters.

The main advantage of the RTU mode is that it achieves higher throughput.

ASCII mode allows time intervals of up to 1 second to occur betweencharacters without causing an error.

Part 2 : Dfinitions


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Modbus frame structure

Address ChecksumDataFunction

The Modbus frame structure is the same for requests (master to slave

messages) and responses (slave to master messages).

Modbus ASCII

Modbus RTU

: CR LF

3A Hex 0D Hex 0A Hex


Address ChecksumDataFunctionsilence silence

Silence >= 3,5 characters


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Address field


Valid slave device addresses are in the range of 0 ... 247 decimal.

The individual slave devices are assigned addresses in the range of 1 ... 247.Value 0 is reserved for broadcast messages (no response).

Request :

A master addresses a slave by placing the slave address in the address field of

the message.

Response :When the slave sends its response, it places its own address in this address field

of the response to let the master know which slave is responding.



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Valid codes are in the range of 1 ... 255 decimal.

Request :

The function code field tells the slave what kind of action to perform.

Response :

For a normal response, the slave simply echoes the original function code.

For an exception response, the slave returns a code that is equivalent to

the original function code with its most significant bit set to a logic 1.

Function field




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Request :

The data field contains additional information which the slave must use to takethe action defined by the function code. This can include items like register

addresses, quantity of items to be handled, etc...

Response :

If no error occurs, the data field contains the data requested.

If an error occurs, the field contains an exception code that the master

application can use to determine the next action to be taken.

Data field




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Modbus RTU uses CRC : Cyclycal Reduncy Check (2 byte)

Modbus ASCII uses LRC : Longitudinal Redundancy Check (1 bytes)

Request :

The checksum is calculated by the master and sends to the slave.

Response :

The checksum is re-calculated by the slave and compared to the value sentby the master.

If a difference is detected, the slave will not construct a response to the master.

Checksum field




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Request :

Frame exemple in RTU mode

Function code = 03 : Read Holding Registers

Slave

AddressCRC16

First word

address

Function

code = 03

Number of

words to read

1 byte 1 byte 2 bytes 2 bytes 2 bytes

Response :

SlaveAddress

CRC16Number ofbytes read

Functioncode = 03

Value of thefirst word


Value of thelast word

2 bytes



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Request :


Function code = 06 : Write Single Register

Slave

AddressCRC16

Word

address

Function

code = 06

Value of

word


Response :


Slave

AddressCRC16

Word

address

Function

code = 06

Value of

word



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Request :


Function code = 16 (dcimal) : Write Multiple Registers

Slave

AddressCRC16

First word

address

Function

code = 16

Number of

words to write


Response :

SlaveAddress

CRC16Functioncode = 16



Value of the

first word

Number of

bytes

1 byte 2 bytes

First wordaddress

Number ofwords to write


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Function code exemples


Code Function

01 (0x01) Read Coils02 (0x02) Read Discrete Inputs

03 (0x03) Read Holding Registers

04 (0x04) Read Input Registers

05 (0x05) Write Single Coil

06 (0x06) Write Single Register15 (0x0F) Write Multiple Coils

16 (0x10) Write Multiple Registers

23 (0x17) Read/Write Multiple Registers

43 (0x2B) Read Device Identification

The complete description of all Modbus request is freely available on the

Modbus.org web site : http://www.modbus.org


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Parity checking

Even or odd can be optionally applied to each character.

Frame checking

LRC or CRC is applied to the entire message.

Continuous stream

The entire message frame must be transmitted as a continuous stream.

If a silent interval (more than 1.5 character times RTU mode or 1 second ASCII mode) occurs

before completion of the frame, the receiving device flushes the incomplete message and

assumes that the next byte will be the address field of a new message.

Error checking methods


P t 4 S it f t i i


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The master is configured by the user to wait for a predetermined timeout

interval before aborting the transaction.

This interval is set to be long enough for any slave to respond normally.

If the slave detects a transmission error, the message will not be acted upon.

The slave will not construct a response to the master.

Thus the timeout will expire and allow the master's program to handle the error.

Error checking methods


P t 5 Ph i l l


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RS485 physical layer


RS485 is the most common physical layer used on Modbus.

The RS485 standard allows variants of different characteristics :

polarisation

line terminator

distribution of a reference potential

number of slaves

length of the bus



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The various versions of the RS485 physical layer


The new Telemecanique devices conform to the Modbus specification

published in 2002 on the Modbus.org web site.

But some Schneider older devices comply with earlier specifications :

Uni-Telway

Jbus



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New Modbus RS485 standard schematic


Master

Slave 1 Slave 2

D1

D0

Common

5 V

650

650

120 1 nF

120 1 nF

Maximum length of bus 1000 m at 19200 bps

Maximum number of stations (without repeater) 32 (31 slaves)

Maximum length of tap links 20 m for one tap link

40 m divided by the number of tap linksBus polarisation 650 at 5V and common for the master

Line terminator 120 - 0,25Wm in series with 1nF 10V

Common polarity Yes (Common) connected to the PG



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Uni-Telway RS485 schematic


Master

Slave 1 Slave 2

D(B)

D(A)0 VL

5 V

4,7 K

120 1 nF

120 1 nF




40 m divided by the number of tap links

Bus polarisation 4,7 K at 5V and common for the master and slaves


Common polarity Yes (0 VL) and high impedance place between 0 VLand the ground in each station

4,7 K

4,7 K4,7 K4,7 K4,7 K5 V5 V



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Jbus RS485 schematic


Master

Slave 1 Slave 2

L-(B/B )

L+(A/A )

5 V

470 470

150 150



Maximum length of tap links 3 m

Bus polarisation 470 at 5V and common for the masterLine terminator 150

Common polarity No



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Mixed RS485 schematic

a t 5 ys ca aye

Master

Slave 1 Slave 2

D1

D0

Common

5 V

Rp

120

1 nF

120 1 nF


Maximum number of stations (without repeater) At most 32 stations (depending on Rp and the number of

4,7 K


40 m divided by the number of tap links

Bus polarisation Rp should be validated by calculating the equivalent polarisation Reaccording to the polarisation of the master and slave stations. Remust be between 162 and 650


Common polarity Yes (Common) connected to the PG

Rp

4,7 K4,7 K

5 V

Suitable polarization Rp

must be calculated

If the master is fitted with a 470 polarisation,it s possible to connect a maximum of

18 slaves with 4,7 K polarisation



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Main characteristics resume

Topology: Bus with line terminations

Maximum distance: With RS485 : 1000 m without repeater

Data rate: From 1,200 to 115 Kbits/s

Max. no. of devices: With RS485 : 32 master included




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Method of accessing the medium: Master slave

Transmission method: Messaging

Max. useful data size: 120 words

Transmission security: LRC or CRCStart and stop delimitersParity bit

Continuous stream

Main characteristics resume

06 Tech Modbus En

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