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12.1 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
receiveinformationfromslavesandwritetotheirregisters,whileslavedevicescannotinitiateanyinformationtransferuntil
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,veryreliableandhasanopensourcecode
that can be used royalty-free on any application or device.
Modbus RTU is the most common application and uses CRC error detection and binary encoding.
Master
Fig. 73. Diagram showing message exchange in a Modbus communication
ModbusprotocolestablishesaProtocolDataUnit(PDU)independentfromthecommunicationlayerbelowit,introducingsome
additional fields specified on the Application Data Unit (ADU) ("FIG. 74. FRAMING OF A MESSAGE USING MODBUS PROTOCOL"
ON PAGE 92)tospecificbusesandnetworks.
DevicessuchasPLCs(ProgrammableLogicController),HMIs(HumanMachineInterface),controlpanels,drivers,motion
controllers,I/Odevices,etc.canuseModbustobeginaremoteprocedure,andtheprotocolisoftenusedtoconnecta
supervisingcomputerwithaRemoteTerminalUnitinasupervision,controlanddataacquisition (SCADA) system.
ForfurtherinformationrelatingtoModbusprotocol,visittheofficialModbuswebsite:www.modbus.org.
12.2 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
data must not be
address for the device
exchanged over the
with which the master has
communicationbus,
established dialogue; this
to allow the connected
is a value between 1...247.
instruments to
The address 0 is reserved
recognise the end of one
for the broadcast message
message and the start of
sent to all slave devices
the next
12.3 Modbus functions and registers
TheModbusregistersforthedeviceareorganisedaroundthefourtypesofbasicdatareferenceindicatedabove,andthis
type of data is further identified by the first number of the address.
924EHPNE4.01 - EHPN series humidifiers
Request
Response
Slave 1
Address
Function code
Fig. 74. Framing of a message using Modbus protocol
Address
Function
8 bit
Code for the
function to
execute or
which has been
executed
| 92 | Modbus RTU functions and resources |
Modbus RTU
Read/write
Response
Slave 2
ADU
Data
PDU
Data
8 bit
(N x 8 bit)
Contains the
data sent by the
master or sent
backbytheslave
as a response to a
question
request
Slave "n"
Error check
CRC
16 bit
3.5 x character time
Allows the master
Time period in which
and the slave to
data must not be
checkwhether
exchanged over the
any errors are
communicationbus,
present during
to allow the connected
communication,
instruments to recognise
andifthereare,to
the end of one message
ignore the message
and the start of the next
received
Stop
02/2024
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