Modbus

https://www.halvorsen.blog

Hans-PetterHalvorsen

WhatisModbus?• Modbusisaserialcommunicationsprotocoloriginally

publishedbyModicon (nowSchneiderElectric)in1979forusewithitsprogrammablelogiccontrollers(PLCs).

• Simpleandrobust,ithassincebecomeadefactostandardcommunicationprotocol,anditisnowacommonlyavailablemeansofconnectingindustrialelectronicdevices

• ThedevelopmentandupdateofModbusprotocolshasbeenmanagedbytheModbusOrganizationsinceApril2004,whenSchneiderElectrictransferredrightstothatorganization

• Modbusbecamethefirstwidelyacceptedfieldbusstandard.

https://en.wikipedia.org/wiki/Modbus

Modbus• ThenicethingoftheModbusstandardistheflexibility,butatthesametimetheeasyimplementationofit.

• Notonlyintelligentdeviceslikemicrocontrollers,PLCsetc.areabletocommunicatewithModbus,alsomanyintelligentsensorsareequippedwithaModbusinterfacetosendtheirdatatohostsystems.

• WhileModbuswaspreviouslymainlyusedonwiredserialcommunicationlines,therearealsoextensionstothestandardforwirelesscommunicationsandTCP/IPnetworks.

http://www.lammertbies.nl/comm/info/modbus.html

Modbus• Modbusisa“master-slave”system,wherethe“master”

communicateswithoneormultiple“slaves.”• ThemastertypicallyisaPLC(ProgrammableLogicController),

PC,DCS(DistributedControlSystem)orRTU(RemoteTerminalUnit)

• ModbusRTUslavesareoftenfielddevices,allofwhichconnecttothenetworkinamulti- dropconfiguration

• WhenaMODBUSRTUmasterwantsinformationfromadevice,themastersendsamessagethatcontainsthedevice’saddress,dataitwants,andachecksumforerrordetection.Everyotherdeviceonthenetworkseesthemessage,butonlythedevicethatisaddressedresponds.

UsingModbusforProcessControlandAutomation(PDF):http://www.miinet.com/Portals/0/articles/Using_MODBUS_for_Process_Control_and_Automation.pdf

Modbus

Master

Slave

Slave

Slave

RTUPLC

PC

ProcessMeasurements

ThemastertypicallyisaPLC(ProgrammableLogicController),PCorDCS(DistributedControlSystem)

ModbusRTUslavesareoftenfielddevices,allofwhichconnecttothenetworkinamulti- dropconfiguration

Aremoteterminalunit(RTU)isamicroprocessor-controlledelectronicdevicethatinterfacesobjectsinthephysicalworldtoaDCSorSCADASystem

SCADA DSC

RTU

SensorsandActuators

Master/Slave• Modbusprotocolisdefinedasamaster/slaveprotocol,meaningadeviceoperatingasamasterwillpolloneormoredevicesoperatingasaslave.

• Thismeansaslavedevicecannotvolunteerinformation;itmustwaittobeaskedforit.

• Themasterwillwritedatatoaslavedevice’sregisters,andreaddatafromaslavedevice’sregisters.Aregisteraddressorregisterreferenceisalwaysinthecontextoftheslave’sregisters.

Master/Slave

Master SlaveRequest

Response

TheModbusprotocolfollowsamasterandslavearchitecturewhereamastertransmitsarequesttoaslaveandwaitsfortheresponse.

Master/Slave

MasterSlave

Master DataStoredinRegisters

WriteData

ReadData

Modbusprotocolisdefinedasamaster/slaveprotocol,meaningadeviceoperatingasamasterwillpolloneormoredevicesoperatingasaslave.Thismeansaslavedevicecannotvolunteerinformation;itmustwaittobeaskedforit.Themasterwillwritedatatoaslavedevice’sregisters,andreaddatafromaslavedevice’sregisters.Aregisteraddressorregisterreferenceisalwaysinthecontextoftheslave’sregisters.

Polling/Request

Registers

ModbusRegisterTypes1. Coil(DiscreteOutput)– Coilsare1-bitregisters,usedtocontroldiscreteoutputs,ReadorWrite

2. DiscreteInput– 1-bitregisters

3. InputRegister(ReadOnly)4. HoldingRegister(Read/Write)

16-bit data registers

RegisterAddresses

0x=Coil,AddressRange:00001-099991x=DiscreteInput,AddressRange: 10001-199993x=InputRegister,AddressRange:30001-399994x=HoldingRegister,AddressRange: 40001-49999Whenusingtheextendedreferencing,allnumber referencesmustbeexactlysixdigits.Thisavoidsconfusionbetweencoilsandotherentities.Forexample,toknowthedifferencebetweenholdingregister#40001andcoil#40001,ifcoil#40001isthetarget,itmustappearas#040001.

AccessLevels

• InSCADAsystems,itiscommonforembeddeddevicestohavecertainvaluesdefinedasinputs,suchasgainsorproportionalintegralderivative(PID)settings,whileothervaluesareoutputs,likethecurrenttemperatureorvalveposition.Tomeetthisneed,Modbusdatavaluesaredividedintofourranges

• Inmanycases,sensorsandotherdevicesgeneratedataintypesotherthansimplyBooleansandunsignedintegers.Itiscommonforslavedevicestoconverttheselargerdatatypesintoregisters.Forexample,apressuresensormaysplita32-bitfloatingpointvalueacrosstwo16-bitregisters.

IntroductiontoModbus(NationalInstruments):http://www.ni.com/white-paper/7675/en/

MemoryType DataType MasterAccess SlaveAccess

Coils Bit(Boolean) Read/Write Read/Write

DiscreteInput Bit(Boolean) Read-only Read/Write

InputRegister UnsignedWord Read-only Read/Write

HoldingRegister UnsignedWord Read/Write Read/Write

RegisterReferencing40001:7• Thisisacommonlyusednotationforreferencingindividualbitsinaregister.

• Thisparticularexamplereferencesregister40001(whichisaHoldingRegister),bit7.

• Bitsaregenerallynumberedstartingatbit0,whichistheleastsignificantorrightmostbitinthefieldof16bitsfoundinaModbusregister.

ModbusProtocols• ModbusASCII• ModbusRTU(RemoteTerminalUnit)–ModbusRTUusesRS-485orRS-232

• ModbusTCP/IP–ModbusTCPusesEthernet

ModbusASCIIandModbusRTUaresimpleserialprotocolsthatuseRS-232orRS-485totransmitdatapackets.ModbusTCP/IPfollowstheOSINetworkModelandcanbeusedinanordinaryEthernetnetwork

ModbusCommunication

RS-232 RS-485 TCP/IP

Master Slave Master Slave Master Slave

Slave

Slave

...

128 Master Slave

SlaveMaster

... ...

Multi-drop network

ModbusASCII• ASCIIcharactersforprotocolcommunication

ModbusRTU• Binaryrepresentationofthedataforprotocolcommunication.

• ModbusRTUusesRS-485orRS-232• ModbusRTUrequiresthatyouknowordefinebaudrate,characterformat(8bitsnoparity,etc.),andslaveID(akaslaveaddress,unitnumber,unitID).Amismatchinanyofthesewillresultinnocommunication.

ModbusTCP/IP• ModbusTCP/IPfollowstheOSINetworkModelandcanbeusedinanordinaryEthernetnetwork

• ModbusTCPrequiresthatyouknowordefineIPaddressesonthenetwork

• ModbusTCP/IPusesPort502• InModbusTCP/IPweusethetermServer/ClientinsteadofSlave/Master– Slave->Server– Master->Client

ModbusRTUvs.ModbusTCP/IP

ThemostbasicdifferencebetweenModbusRTUandModbusTCP/IPisthatModbusTCP/IPrunsonanEthernetphysicallayerandModbusRTUisaserialprotocol

ModbusRTUFrameFormat

ModbusRTUFrameFormat

ModbusASCII/RTUMessageProperties

ModbusinLabvIEW

Hans-PetterHalvorsen,M.Sc.

ModbusinLabVIEW3waystouseModbusinLabVIEW:• Useahigh-levelOPCServer• UseModbusI/OServer• UsetheLabVIEWModbusAPI

IntroductiontoModbus(NationalInstruments):http://www.ni.com/white-paper/7675/en/

“LabVIEWReal-TimeModule”or“LabVIEWDSCModule”required

LabVIEWModbusAPI

LabVIEWExample

Master(Client) Slave

(Server)

Master(Client) DataStoredinRegisters

WriteData

ReadData

Polling/Request

Registers

LabVIEWApp#1LabVIEWApp#2

LabVIEWApp#3

InthisExampleweCreate3differentLabVIEWApplications

LabVIEWExample

LabVIEWApp#1(Slave)

LabVIEWApp#2(Master)

LabVIEWApp#3(Master)

Note!Youneedtostart/runtheModbusSlaveAppbeforeyoustarttheModbusMasterApps

WriteDatatoSlave

ReadDatafromSlave

ModbusSlave

ThispartisjustforreadingtheinternalCoilData.Itcanberemoved.ItisjusttoseewhennewdatafromtheMastershasbeenreceived

ModbusMaster(Write)

ModbusMaster(Read)

LabVIEWModbusSimulator

TheLabVIEWModbusSimulatorisintegratedwith“LabVIEWReal-TimeModule”or“LabVIEWDSCModule”

Itcanbeusedfortestpurpose,etc.

TheLabVIEWModbusSimulatorisaModbusSlave(Server)

FindModbusExampleswithNIExampleFinder

LabVIEWModbusSimulatorExample

References• ModbusOrganization:http://www.modbus.org• Modbus(Wikipedia):https://en.wikipedia.org/wiki/Modbus• IntroductiontoModbus(NationalInstruments):

http://www.ni.com/white-paper/7675/en/• ConnectLabVIEWtoAnyPLCWithModbus(NationalInstruments):

http://www.ni.com/tutorial/13911/en/• Modbus101- IntroductiontoModbus:

http://www.csimn.com/CSI_pages/Modbus101.html• ModbusTCP/IP:http://www.rtaautomation.com/technologies/modbus-tcpip/• ModbusRTU:http://www.rtaautomation.com/technologies/modbus-rtu/• UsingModbusforProcessControlandAutomation(PDF):

http://www.miinet.com/Portals/0/articles/Using_MODBUS_for_Process_Control_and_Automation.pdf

Hans-PetterHalvorsen

UniversityofSoutheastNorwaywww.usn.no

E-mail:hans.p.halvorsen@usn.noWeb:https://www.halvorsen.blog