Section 6 - Slide 1 / 27 P&T - GPS - Training PhW - 06_TECH_Modbus_en 06/ 2004 Modbus training Part 1 :Reference documents - WEB sites Part 2 :Definitions

Section 6 - Slide 1 / 27P&T - GPS - Training PhW - 06_TECH_Modbus_en 06/ 2004

Modbus trainingModbus 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


Modbus User guideReference : 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/ProReference : TLX DS COM PL7 42 *

Volume 1 : Common communication applicationVolume 2 : Modbus bus

Part 1 : Reference documents - WEB sitesPart 1 : Reference documents - WEB sites

Schneider documentsSchneider documents


WEB sitesWEB sites

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

Modicon site for Modbus users.

Transparent Factory :http://www.transparentfactory.com

Schneider 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 sitesPart 1 : Reference documents - WEB sites


Modbus protocolModbus protocol

MODBUS Protocol is a messaging structure created by MODICONcompany 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 : DéfinitionsPart 2 : Définitions


Modbus Serial line RS485Modbus Serial line RS485

MODBUS Serial line RS485 is a low cost network using a master/slave

medium 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 - SlaveMaster - Slave

Modbus

RS485-RS 232RS485-RS 232



Modbus TCP/IPModbus TCP/IP

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

Application

Préeentation

Session

Transport

Network

Link

Physical

7

6

5

4

3

2

1

CSMA / CDCSMA / CDETHERNET V2 ou 802.3ETHERNET V2 ou 802.3

Modbus

TCP

IP



Modbus PlusModbus Plus

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

Application

Presentation

Session

Transport

Network

Link

Physical

7

6

5

4

3

2

1

802.4 Token passing802.4 Token passing

Modbus

RS485RS485



Modbus ASCII and Modbus RTUModbus 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 modeEach eight-bit byte in a message is sent as two four-bit hexadecimalcharacters.

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.



Modbus frame structureModbus 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

Part 3 : Modbus frame descriptionPart 3 : Modbus frame description

Address ChecksumDataFunctionsilence silence

Silence >= 3,5 characters


Address fieldAddress 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 ofthe message.

Response :When the slave sends its response, it places its own address in this address fieldof the response to let the master know which slave is responding.



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 fieldFunction field





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 registeraddresses, 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 masterapplication can use to determine the next action to be taken.

Data fieldData field





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 fieldChecksum field




Request :

Frame exemple in RTU modeFrame exemple in RTU mode

Function code = 03 : Read Holding Registers

SlaveAddress

CRC16First word address

Function code = 03

Number of words to read

1 byte 1 byte 2 bytes 2 bytes 2 bytes

Response :

SlaveAddress

CRC16Number of bytes read

Function code = 03

Value of the first word


Value of the last word

2 bytes



Request :


Function code = 06 : Write Single Register

SlaveAddress

CRC16Word

addressFunction code = 06

Value of word


Response :


SlaveAddress

CRC16Word

addressFunction code = 06

Value of word



Request :


Function code = 16 (décimal) : Write Multiple Registers

SlaveAddress

CRC16First word address

Function code = 16

Number of words to write


Response :

SlaveAddress

CRC16Function code = 16



Value of the first word

Number of bytes

1 byte 2 bytes

First word address

Number of words to write


Function code exemplesFunction code exemplesPart 3 : Modbus frame descriptionPart 3 : Modbus frame description

Code Function

01 (0x01) Read Coils

02 (0x02) Read Discrete Inputs

03 (0x03) Read Holding Registers

04 (0x04) Read Input Registers

05 (0x05) Write Single Coil

06 (0x06) Write Single Register

15 (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


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 methodsError checking methodsError checking methodsError checking methodsPart 4 : Security of transmissionPart 4 : Security of transmission


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 methodsError checking methodsError checking methodsError checking methodsPart 4 : Security of transmissionPart 4 : Security of transmission


RS485 physical layerRS485 physical layer

Part 5 : Physical layerPart 5 : 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


The various versions of the RS485 physical layerThe 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


New Modbus RS485 standard schematicNew Modbus RS485 standard schematic


Master

Slave 1 Slave 2

D1

D0Common

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 links

Bus 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


Uni-Telway RS485 schematicUni-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





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 K

5 V5 V


Jbus RS485 schematicJbus 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 master

Line terminator 150

Common polarity No


Mixed RS485 schematicMixed RS485 schematic


Master

Slave 1 Slave 2

D1

D0Common

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 of4,7 K



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 Rpmust 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


Main characteristics resumeMain 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

Part 6 : Main characteristics resumePart 6 : Main characteristics resume


Method of accessing the medium: Master slave

Transmission method: Messaging

Max. useful data size: 120 words

Transmission security: LRC or CRC Start and stop delimiters

Parity bitContinuous stream

Main characteristics resumeMain characteristics resume

Part 6 : Main characteristics resumePart 6 : Main characteristics resume

Documents

Section 6 - Slide 1 / 27 P&T - GPS - Training PhW - 06_TECH_Modbus_en 06/ 2004 Modbus training Part 1 :Reference documents - WEB sites Part 2 :Definitions