User Guide - Eurotherm Ltda

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nanodac RECORDER/CONTROLLER: USER GUIDE5 MODBUS TCP SLAVE COMMS5.1 INSTALLATIONThe installation of the Modbus link consists of connecting a standard Ethernet cable between the RJ45 connectorat the rear of the unit to a host computer either directly or via a network. A ‘straight-through’ cablecan be used in either case (i.e. a cross-over cable is not required).5.2 INTRODUCTIONMODBUS TCP allows the instrument to act as a 'slave' device to one or more host computers connected viathe RJ45 connector at the rear of the recorder. Each recorder must have a unique Internet Protocol (IP) address,set up as described in Section 4.2.1 (Network.Interface).MODBUS TCP (Transmission Control Protocol) is a variant of the MODBUS family of communications protocolsintended for supervision and control of automated equipment specifically covering the use of MODBUSmessaging in an intranet or internet environment, using TCP/IP protocols. Much of the MODBUS detail inthis manual is derived from the document openmbus.doc, available at http://www.modbus.org/default.htmThe above mentioned document also includes implementation guidelines for users.Note: The Modbus protocol allows a maximum of 255 data bytes to be read from or written to inone transaction. For this reason, the maximum number of standard (16 bit) registers that can beaccessed in one transaction is 255/2 = 127 and the maximum number of IEEE (32-bit) registers is127/2 = 63.5.2.1 Function CodesMODBUS function codes 3, 4, 6, 8 and 16, defined in table 8.2.1a below, are supported and are fully describedin section 5.5, below.Code Modbus definition Description0304Read holding registersRead input registersReads the binary contents if holding registers. In this implementationcodes 3 and 4 are identical in operation.Reads the binary contents if holding registers. In this implementationcodes 3 and 4 are identical in operation.06Preset single registerWrites a single value to a single register.08DiagnosticsPerforms a simple loop back test.16Preset multiple registersWrites values to multiple holding registers.Table 5.2.1a MODBUS Function code definitionDIAGNOSTIC CODESFunction code 08, subfunction 00 (Return query data) echoes the query (Loop back).Page 170HA030554Issue 7 Nov 12
nanodac RECORDER/CONTROLLER: USER GUIDE5.2.1 Function Codes (Cont.)EXCEPTION CODESMODBUS TCP provides reserved codes used for exceptions. These codes provide error information relatingto failed requests. Exceptions are signalled by hex 80 being added to the function code of the request,followed by one of the codes listed in table 8.2.1b, below.CodeDec Hex01 01Modbus definitionIllegal functionDescription (see Modbus specification for full details)An invalid function code was received0202Illegal Data AddressAn invalid data address was received0303Illegal Data ValueAn invalid data value was received0404Slave Device FailureAn unrecoverable error occurred in the instrument0909Illegal Sub FunctionAn invalid sub function was received100AGateway path unavailableGateway misconfigured or overloaded110BGateway target device failedto respondDevice not present on the networkTable 5.2.1b Exception codes5.2.2 Data typesThe following data types are supported:1. 2's complement signed 16-bit analogue values with implied decimal point. The decimal point positionmust be configured in both the recorder and the host computer.2. 16, 32 and 64 bit signed integers.3. 16-bit unsigned integer values.4. 32 bit IEEE Floating point values.5. Strings of limited size, can be transferred across Modbus TCP in Unicode format using a single nonmultiplexedset of consecutive registers.DATA ENCODINGMODBUS uses what is called a 'Big endian' representation for addresses and data items. This means thatwhen a numerical quantity larger than a single byte is transmitted, the most significant byte is sent first. Forexample a 32-bit hex value of 12345678 would be transmitted as 12, followed by 34, followed by 56 andfinally 78.5.2.3 Invalid multiple register writesWhen a recorder receives a multi-register write request, it is possible that one or more requests will be rejected.Under such a circumstance, the recorder accepts all valid write requests and ignores any invalidwrites. No error response is produced.5.2.4 Master communications timeoutWhilst the instrument is archiving, it is possible that communications responses slow sufficiently to causecommunications timouts. The Modbus master device should be configured with a timout value largeenough to ensure against nuisance timeouts during archiving.HA030554Issue 7 Nov 12Page 171
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