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7/29/2019 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/7/29/2019 06 Tech Modbus En
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Section 6 - Slide 2 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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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Section 6 - Slide 3 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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
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Section 6 - Slide 4 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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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Section 6 - Slide 5 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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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PhW - 06_TECH_Modbus_en 06/ 2004
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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Section 6 - Slide 8 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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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Section 6 - Slide 9 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
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
Part 3 : Modbus frame description
Address ChecksumDataFunctionsilence silence
Silence >= 3,5 characters
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Address field
Address ChecksumDataFunction
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.
Part 3 : Modbus frame description
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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
Address ChecksumDataFunction
Part 3 : Modbus frame description
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Section 6 - Slide 12 / 27P&T - GPS - Training
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Valid codes are in the range of 0 ... 255 decimal.
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
Address ChecksumDataFunction
Part 3 : Modbus frame description
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Valid codes are in the range of 0 ... 255 decimal.
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
Address ChecksumDataFunction
Part 3 : Modbus frame description
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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
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Value of thelast word
2 bytes
Part 3 : Modbus frame description
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Request :
Frame exemple in RTU mode
Function code = 06 : Write Single Register
Slave
AddressCRC16
Word
address
Function
code = 06
Value of
word
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Response :
Part 3 : Modbus frame description
Slave
AddressCRC16
Word
address
Function
code = 06
Value of
word
1 byte 1 byte 2 bytes 2 bytes 2 bytes
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Section 6 - Slide 16 / 27P&T - GPS - Training
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Request :
Frame exemple in RTU mode
Function code = 16 (dcimal) : Write Multiple Registers
Slave
AddressCRC16
First word
address
Function
code = 16
Number of
words to write
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Response :
SlaveAddress
CRC16Functioncode = 16
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Part 3 : Modbus frame description
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
Part 3 : Modbus frame description
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
Part 4 : Security of transmission
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
Part 4 : Security of transmission
P t 5 Ph i l l
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RS485 physical layer
Part 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
Part 5 : Physical layer
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The various versions of the RS485 physical layer
Part 5 : 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
Part 5 : Physical layer
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PhW - 06_TECH_Modbus_en 06/ 2004
New Modbus RS485 standard schematic
Part 5 : Physical layer
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
Part 5 : Physical layer
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Uni-Telway RS485 schematic
Part 5 : Physical layer
Master
Slave 1 Slave 2
D(B)
D(A)0 VL
5 V
4,7 K
120 1 nF
120 1 nF
Maximum length of bus 1000 m at 19200 bps
Maximum number of stations (without repeater) 29 (28 slaves)
Maximum length of tap links 20 m for one tap link
40 m divided by the number of tap links
Bus polarisation 4,7 K at 5V and common for the master and slaves
Line terminator 120 - 0,25Wm in series with 1nF 10V
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
Part 5 : Physical layer
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Section 6 - Slide 24 / 27P&T - GPS - Training
PhW - 06_TECH_Modbus_en 06/ 2004
Jbus RS485 schematic
Part 5 : Physical layer
Master
Slave 1 Slave 2
L-(B/B )
L+(A/A )
5 V
470 470
150 150
Maximum length of bus 1300 m at 19200 bps
Maximum number of stations (without repeater) 32 (31 slaves)
Maximum length of tap links 3 m
Bus polarisation 470 at 5V and common for the masterLine terminator 150
Common polarity No
Part 5 : Physical layer
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Section 6 - Slide 25 / 27P&T - GPS - Training
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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 length of bus 1000 m at 19200 bps
Maximum number of stations (without repeater) At most 32 stations (depending on Rp and the number of
4,7 K
Maximum length of tap links 20 m for one tap link
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
Line terminator 120 - 0,25Wm in series with 1nF 10V
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
Part 6 : Main characteristics resume
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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
Part 6 : Main characteristics resume
Part 6 : Main characteristics resume
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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