MODBUSisanapplicationlayermessagingprotocol,positionedatlevel7oftheOSImodel,whichprovidesclient/servercommunicationbetweendevicesconnectedondifferenttypesofbusesornetworks.Theindustry’sserialdefactostandardsince1979,MODBUScontinuestoenablemillionsofautomationdevicestocommunicate.Today,supportforthesimpleandelegantstructureofMODBUScontinuestogrow.TheInternetcommunitycanaccessMODBUSatareservedsystemport502ontheTCP/IPstack.MODBUSisarequest/replyprotocolandoffersservicesspecifiedbyfunctioncodes.MODBUSfunctioncodesareelementsofMODBUSrequest/replyPDUs.TheobjectiveofthisdocumentistodescribethefunctioncodesusedwithintheframeworkofMODBUStransactions.