P

1
Modbus training
Part 1 Reference documents - WEB sites Part 2
Definitions Part 3 Modbus frame
description Part 4 Security of
transmissionPart 5 Physical layer Part 6
Main characteristics resume
2
Part 1 Reference documents - WEB sites
Schneider documents
Modbus User guide Reference TSX DG MOD
Detailed description of the Modbus
protocol. X-WAY communication user
guide Reference 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
3
Part 1 Reference documents - WEB sites
WEB 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/tec
hpubs/toc6.html Modbus Plus Planning and
installation.
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Part 2 Définitions
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...).
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Part 2 Définitions
Modbus 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.
7 6 5 4 3 2 1
Application
Modbus
Presentation
Session
Transport
Network
Master - Slave
Link
Physical
RS485
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Part 2 Définitions
Modbus TCP/IP
MODBUS TCP/IP uses TCP/IP and Ethernet 10 Mbit/s
or 100 Mbits/s to carry the MODBUS messaging
structure.
7 6 5 4 3 2 1
Application
Modbus
Préeentation
Session
Transport
TCP
Network
IP
Link
CSMA / CD ETHERNET V2 ou 802.3
Physical
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Part 2 Définitions
Modbus Plus
MODBUS PLUS is a higher speed network 1 Mbit/s
token passing derivative that uses the MODBUS
messaging structure.
7 6 5 4 3 2 1
Application
Modbus
Presentation
Session
Transport
Network
802.4 Token passing
Link
Physical
RS485
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Part 2 Définitions
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 between
• characters without causing an error.

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Part 3 Modbus frame description
Modbus frame structure
The Modbus frame structure is the same for
requests (master to slave messages) and
responses (slave to master messages).
Modbus RTU
Silence gt 3,5 characters
Modbus ASCII
Address
Checksum
Data
Function

CR
LF
0D Hex
0A Hex
3A Hex
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Part 3 Modbus frame description
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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Part 3 Modbus frame description
Function field
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.
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Part 3 Modbus frame description
Data field
Valid codes are in the range of 0 ... 255
decimal. Request The data field contains
additional information which the slave must use
to take the 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.
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Part 3 Modbus frame description
Checksum field
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 sent by the master. If a
difference is detected, the slave will not
construct a response to the master.
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Part 3 Modbus frame description
Frame exemple in RTU mode

• Function code 03 Read Holding Registers

Request
1 byte
1 byte
2 bytes
2 bytes
2 bytes
Slave Address
CRC16
First word address
Function code 03
Number of words to read
Response
1 byte
1 byte
2 bytes
2 bytes
2 bytes
2 bytes
Slave Address
CRC16
Number of bytes read
Function code 03
Value of the first word
Value of the last word
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Part 3 Modbus frame description
Frame exemple in RTU mode

• Function code 06 Write Single Register

Request
1 byte
1 byte
2 bytes
2 bytes
2 bytes
Slave Address
CRC16
Word address
Function code 06
Value of word
Response
1 byte
1 byte
2 bytes
2 bytes
2 bytes
Slave Address
CRC16
Word address
Function code 06
Value of word
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Part 3 Modbus frame description
Frame exemple in RTU mode

• Function code 16 (décimal) Write Multiple
  Registers

Request
1 byte
1 byte
2 bytes
2 bytes
2 bytes
1 byte
2 bytes
Slave Address
CRC16
First word address
Function code 16
Number of words to write
Value of the first word
Number of bytes
Response
2 bytes
1 byte
1 byte
2 bytes
2 bytes
CRC16
Number of words to write
Slave Address
Function code 16
First word address
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Part 3 Modbus frame description
Function code exemples
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
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Part 4 Security of transmission
Error checking methods

• 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.

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Part 4 Security of transmission
Error checking methods
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.
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Part 5 Physical layer
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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Part 5 Physical layer
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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Part 5 Physical layer
New Modbus RS485 standard schematic
5 V
Master
650 ?
650 ?
D1
120 ?
120 ?
1 nF
1 nF
D0
Common
Slave 1
Slave 2
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Part 5 Physical layer
Uni-Telway RS485 schematic
5 V
Master
4,7 K?
4,7 K?
D(B)
120 ?
120 ?
1 nF
1 nF
D(A)
0 VL
5 V
5 V
4,7 K?
4,7 K?
4,7 K?
4,7 K?
Slave 1
Slave 2
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Part 5 Physical layer
Jbus RS485 schematic
5 V
Master
470 ?
470 ?
L-(B/B )
150 ?
150 ?
L(A/A )
Slave 1
Slave 2
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Part 5 Physical layer
Mixed RS485 schematic
Suitable polarization Rp must be calculated
5 V
Master
Rp
Rp
D1
120 ?
120 ?
1 nF
1 nF
D0
Common
If the master is fitted with a 470 ?
polarisation, it s possible to connect a
maximum of 18 slaves with 4,7 K? polarisation
5 V
4,7 K?
4,7 K?
Slave 2
Slave 1
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Part 6 Main characteristics resume
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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Part 6 Main characteristics resume
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 bit
• Continuous stream