Data Communication And Networking

Data Communication

and

Networking
Dr. –Ing.
Vo Que Son
Email:


Telecomm. Dept.
Faculty of EEE

DCN-2013
HCMUT

1


Content
 Chapter 3: Data Link Layer Protocols
Flow Control
Error Control
Connection Management
Data link layer Protocols

 Chapter 4: Industrial Networks
MODBUS

Controller Area

Network

Applications

Telecomm. Dept.
Faculty of EEE

DCN-2013
HCMUT

2









MODBUS
MODBUS Protocol is a Serial Communication Protocol created
by MODICON company to connect PLC to programming tools.
It is now widely used to establish master-slave communication
between intelligent devices.
It can be implemented using RS232, RS422, or RS485
variety
(e.g. fiber,
MODBUSofismedia
independent

of the radio,
physicalcellular,
layer. etc...).

Telecomm. Dept.
Faculty of EEE

or over a

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HCMUT

3


MODBUS
 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

Telecomm. Dept.
Faculty of EEE

Applicatio
Application

Modbus

Presentation
Session
Transport
Network

Link

Physical

Maste

Slave
r -

RS485

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HCMUT

4

MODBUS
 MODBUS TCP/IP uses TCP/IP and Ethernet 10 Mbit/s
or 100 Mbits/s to carry the MODBUS messaging
structure.
7

Applicatio
Application

6

Préee
Préeentation

5

Session

4
3

2

Modbus

Transport

TCP

Network

Link

IP

CSMA / CD
ETHERNET V

1

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2
o 802.3

r

Physical

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5


MODBUS
 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

Telecomm. Dept.
Faculty of EEE

Applicatio
Application

Modbus

Presentation
Session
Transport
Network

Link

Physical

802.4 Token passing

RS485

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6


MODBUS ASCII and 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.
Telecomm. Dept.
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DCN-2013
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7


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

Address

Function

Data

Checksum

silence

Silence >= 3,5 characters

Modbus ASCII
:

Address

0x3A
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Function

Data

Checksum

CR

0x0D

LF

0x0A
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Address field
Address

Function

Data

Checksum

 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
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slave is responding.
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9


Function field
Address

Function

Data

Checksum

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
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code with its most significant bit set to a logic 1.
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10


Data field
Address

Function

Data

Checksum

 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 an error occurs, the field
contains an exception code that
If no error occurs, the data field contains the data
the master application can
requested.

use to determine the next

action to be taken.
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11

Data field
Address

Function

Data

Checksum

 Valid codes are in the range of 0 ... 255 decimal.
 Modbus RTU uses CRC :
Cyclic Redundancy 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
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response
to the master.
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12


Frame example in RTU mode
 Function code

= 03 : Read

Holding

2 bytes

2 bytes

Registers

Request :
1 byte

1 byte

Slave

Address

Function code =
03

First word
address

Number of words to
read

2 bytes

CRC16

Response :
1 byte

1 byte

Slave

Address

Function code =
03

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2 bytes

Number of bytes
read

2 bytes

2 bytes

Value of the first

Value of the last

word

word

2 bytes

CRC16

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13


Frame example in RTU mode
 Function code

= 06 : Write

Single

Register

Request
1 byte

1 byte

Slave

Address

Function code =
06

2 bytes

Word

2 bytes

Value of word

address

2 bytes

CRC16

Response:
1 byte

1 byte

Slave

Address

Function code =
06

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2 bytes

Word
address

2 bytes

Value of word

2 bytes

CRC16

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14


Frame example in RTU mode
 Function code = 16 (decimal) :

Write Multiple Registers

Request :
1 byte

1 byte

Slave

Address

Function code =
16

2 bytes

First word
address

2 bytes

Number of words to

1 byte

Number of bytes

write

2 bytes

Value of the first
word

2 bytes

CRC16

Response :
1 byte
Slave

Address

1 byte

Function code =
16

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2 bytes

First word
address

2 bytes

Number of words to

2 bytes

CRC16

write

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15


Function Code Example
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

(0x06)

Write

Single Register Multiple Coils

(0x0F)

Write

Multiple Registers

(0x10)

Write

06
15
16
23
43

(0x17)
(0x2B)

Read/Write Multiple Registers
Read Device Identification

The complete description of all Modbus request is freely
the Modbus.org

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web site

:

available on



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

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

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byte will be the address field of a new message.

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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
program to handle the error.

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allow the master's

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18


RS485 physical layer
 RS485 is the most common physical layer used
Modbus.
 The RS485 standard allows variants

on

of different

characteristics :
polarization
line terminator

distribution of a reference

potential

number of slaves

length of the bus

Telecomm. Dept.
Faculty of EEE

DCN-2013
HCMUT

19


New Modbus RS485 standard

schematic

5V

Master

650 Ω

650 Ω

D1
120 Ω

120 Ω

1 nF

1 nF
D0
Common

Slave 1

Slave 2

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 Ω at 5V and common for the master

Line terminator

120 Ω - 0,25Wm in series with 1nF 10V

Common polarity

Yes (Common) connected to the PG

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MODBUS Protocol
 Master polls slave for
communication by sending

Master

Slave

requests
 Slave replies responses to

Master
the behaviors

 Function code will define

of selected

slave

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21


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
 Method of accessing the
medium: Master slave
 Max. no. of devices: with RS485 : 32 master included
 Transmission method:
Messaging
120 words
 Max. useful data size:
 Transmission security:
 LRC or CRC

Continuous stream
Start and stop delimiters
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Parity
bit

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MODBUS TCP
 The MODBUS continues to survive in automation
world more interested with interconnecting to
Ethernet networks and more specially, IP/Ethernet
 networks
Using MODBUS TCP

and Client/Server

model for

communication

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23

MODBUS TCP - Interconnection

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


MODBUS TCP Operation
A Request is sent by the client to initiate a transaction
An Indication is sent by the server to confirm that the request





was received
A Response is sent by the server to comply with the client
A Confirmation is sent by the client to acknowledge receipt of
request
response

Telecomm. Dept.
Faculty of EEE

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25