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13. MODBUS RTU FUNCTIONS AND RESOURCES
13.1 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,veryreliableandhasanopensourcecodethatcanbe
used royalty-free on any application or device.
Modbus RTU is the most common application and uses CRC error detection and binary encoding.
Request
Read/write
request
Modbus RTU
Master
Slave 1 Slave 2 Slave “n”
Response
Response
Fig. 48. Diagram showing message exchange in a Modbus communication
Mo dbusprotocolestablishesaProtocolDataUnit(PDU)independentfromthecommunicationlayerbelowit,introducingsome
additional fields specified on the Application Data Unit (ADU) ( "FIG. 49. FRAMING OF A MESSAGE USING MODBUS PROTOCOL"
ON PAGE 57 )tospecificbusesandnetworks.
DevicessuchasPLCs(ProgrammableLogicController),HMIs(HumanMachineInterface),controlpanels,drivers,motion
controllers,I/Odevices,etc.canuseModbustobeginaremoteprocedure,andtheprotocolisoftenusedtoconnecta
supervisingcomputerwithaRemoteTerminalUnitinasupervision,controlanddataacq
uisition (SCADA) system.
Address Function code Data Error check
ADU
PDU
Fig. 49. Framing of a message using Modbus protocol
ForfurtherinformationrelatingtoModbusprotocol,visittheofficialModbuswebsite: www.modbus.org .
13.2 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 Address Function Data CRC Stop
3.5 x character time 8 bit 8 bit (N x 8 bit) 16 bit 3.5 x character time
Time period in which
data must not be
exchanged over the
communicationbus,
to allow the connected
instruments to recognise
the end of one message
and the start of the next
Corresponds to the address
for the device with which
the master has established
dialogue; this is a value
between 1...247. The
address 0 is reserved for
the broadcast message sent
to all slave devices
Code for the
function to
execute or
which has been
executed
Contains the
data sent by the
master or sent
backbytheslave
as a response to a
question
Allows the master
and the slave to
checkwhether
any errors are
present during
communication,
andifthereare,to
ignore the message
received
Time period in which data
must not be exchanged
over the communication
bus,toallowthe
connected instruments to
recognise the end of one
message and the start of
the next
13.3 MODBUS FUNCTIONS AND REGISTERS
TheModbusregistersforthedeviceareorganisedaroundthefourtypesofbasicdatareferenceindicatedabove,andthistype
of data is further identified by the first number of the address.