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9. MODBUS RTU FUNCTIONS AND RESOURCES
9.1 Introduction
Modbus RTU (Remote Terminal Unit) protocol is a means of communication which allows data exchange between a computer
and programmable logic controllers.
This protocol is based on the exchange of messages between master-slave and client-server devices. Master devices can
receiveinformationfromtheslavesandwriteintheirregisters,whileslavedevicescannotstartanytransferofinformation
until they receive a request from the slave device.
Modbus communication is used in industrial automation systems (IAS) and in the construction of building management systems
(BMS).Modbusprotocoliswidelyutilisedduetothefactitiseasytouse,veryreliableandhasanopensourcecodethatcanbe
used royalty-free on any application or device.
Modbus RTU is the most common application and uses CRC error detection and binary encoding.
Master
ModbusprotocolestablishesaProtocolDataUnit(PDU)independentfromthecommunicationlayerbelowit,introducingsome
additional fields specified on the Application Data Unit (ADU) ("FIG. 25. FRAMING OF A MESSAGE USING MODBUS PROTOCOL"
ON PAGE 30)tospecificbusesandnetworks.
DevicessuchasPLCs(ProgrammableLogicController),HMIs(HumanMachineInterface),controlpanels,drivers,motion
controllers,I/Odevices,etc.canuseModbustobeginaremoteprocedure,andtheprotocolisoftenusedtoconnecta
supervisingcomputerwithaRemoteTerminalUnitinasupervision,controlanddataacquisition (SCADA) system.
ForfurtherinformationrelatingtoModbusprotocol,visittheofficialModbuswebsite:www.modbus.org.
9.2 Modbus message structure
Modbus RTU protocol requires the message to start with a silent time interval of at least 3.5 character times. This feature
isoftenimplementedbyexecutingatimeintervalofmultipleofcharactertimesatthebaudrateusedinthenetwork.The
characters available for each field are in binary form.
A description of the structure of a Modbus RTU message is provided below.
Start
3.5 x character time
Time period in which
Corresponds to the address
data must not be
for the device with which
exchanged over the
the master has established
communicationbus,
dialogue; this is a value
to allow the connected
between 1...247. The
instruments to recognise
address 0 is reserved for
the end of one message
the broadcast message sent
and the start of the next
to all slave devices
9.3 Modbus functions and registers
TheModbusregistersforthedeviceareorganisedaroundthefourtypesofbasicdatareferenceindicatedabove,andthistype
of data is further identified by the first number of the address.
924VHE4.02 - VEH series humidifiers
Request
Response
Fig. 24. Diagram showing message exchange in a Modbus communication
Address
Function code
Fig. 25. Framing of a message using Modbus protocol
Address
8bit
Code for the
function to
execute or
which has been
executed
Modbus RTU
Response
Slave 1
Slave 2
ADU
Data
PDU
Function
Data
8bit
(Nx8bit)
Contains the
data sent by the
master or sent
backbytheslave
as a response to a
question
| 30 |
Read/write
request
Slave "n"
Error check
CRC
16 bit
3.5 x character time
Allows the master
Time period in which data
and the slave to
must not be exchanged
checkwhether
over the communication
any errors are
bus,toallowthe
present during
connected instruments to
communication,
recognise the end of one
andifthereare,to
message and the start of
ignore the message
the next
received
Stop
02/2022
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