Chapter 9. Modbus/TCP Client140 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QStructure of the Communications RequestThe Communications Request is made up of the following elements:▪ The COMMREQ Function Block (ladder instruction)▪ The COMMREQ Command Block▪ The Channel Command▪ Status Data (COMMREQ Status word, LAN Interface Status and Channel Status bits)▪ The logic program controlling execution of the COMMREQ Function BlockThe figure below illustrates the relationship of these elements:INPUTSANDOUTPUTSFOR COMMREQFUNCTIONCOMMANDBLOCKADDRESSCOMMREQFUNCTION BLOCKDETAILSOF THECHANNELCOMMANDCOMMREQCOMMAND BLOCK COMMREQSTATUS WORDSTATUSCODESCOMMREQSTATUSWORDADDRESSSTATUS BITSLAN INTERFACE STATUSAND CHANNEL STATUSBITSINITIATESCOMMREQFUNCTIONBLOCKCONTROLLOGICLocation in PLC memoryspecified when configuringthe Interface usingConfiguration SoftwareFigure 54: Phases of a COMMREQ ExecutionCOMMREQ Function BlockThe COMMREQ Function Block is the ladder instruction that triggers the execution of the Channel command. Inthe COMMREQ Function Block, you specify the rack and slot location of the Ethernet interface, a task value, andthe address of a location in memory that contains the Command Block. There is also a fault output on theCOMMREQ Function Block that indicates certain programming errors.COMMREQ Command BlockThe COMMREQ Command Block is a structure that contains information about the Channel command to beexecuted. The Command Block consists of two parts:Common Area - includes the address of the COMMREQ Status word (CRS word).Data Block Area - describes the Channel command to be executed.When the COMMREQ function is initiated, the Command Block is transferred to the Ethernet interface for action.Modbus/TCP Channel CommandsThe Channel commands are a set of client commands used to communicate with a server. Up to 3216 channelscan be established. The channel number is specified in the Command Block for the Channel command. Thechannel can be monitored using the Channel Status bits. The 32 Client connections of an Ethernet interface areshared between all Client protocols. For example, if 16 Client connections are used for SRTP Channels, thereare 16 Client connections available for Modbus/TCP Channels. Any given channel can be assigned to only oneprotocol at a time.
