Modbus Protocol for ION 7500, ION 7600 and ION 8000 series

More documents

Recommendations

Info

Description of the Modbus packet structure Modbus Register MapDescription of the Modbus packet structureEvery Modbus packet consists of four fields:Slave Address FieldFunction FieldData FieldError Check Field (Checksum)Slave Address FieldThe slave address field of a Modbus packet is one byte in length and uniquelyidentifies the slave device involved in the transaction. Valid addresses rangebetween 1 and 247. A slave device performs the command specified in the packetwhen it receives a request packet with the slave address field matching its ownaddress. A response packet generated by the slave has the same value in the slaveaddress field.Function FieldThe function field of a Modbus request packet is one byte in length and tells theaddressed slave which function to perform. Similarly, the function field of aresponse packet tells the master what function the addressed slave has justperformed. “Table 2: Modbus Functions Supported by the Meters” on page 8 liststhe Modbus functions supported by the meter.Data FieldThe data field of a Modbus request is of variable length, and depends upon thefunction. This field contains information required by the slave device to performthe command specified in a request packet or data being passed back by the slavedevice in a response packet.Data in this field is contained in 16-bit or 32-bit registers. Registers are transmittedin the order of high-order byte first, low-order byte second. This ordering of bytesis called “Big Endian” format (see example below).Example (Big Endian):A 16-bit register contains the value 12AB Hex. This register is transmitted:High order byte = 12 HexLow order byte = AB HexThis register is transmitted in the order 12 AB.Error Check Field (Checksum)The checksum field lets the receiving device determine if a packet is corruptedwith transmission errors. In Modbus RTU mode, a 16-bit Cyclic RedundancyCheck (CRC-16) is used.Page 6
Modbus Register MapException ResponsesThe sending device calculates a 16-bit value, based on every byte in the packet,using the CRC-16 algorithm. The calculated value is inserted in the error checkfield.The receiving device performs the calculation, without the error check field, on theentire packet it receives. The resulting value is compared to the error check field.Transmission errors occur when the calculated checksum is not equal to thechecksum stored in the incoming packet. The receiving device ignores a badpacket.The CRC-16 algorithm is detailed in “Appendix A: CRC-16 Calculation” onpage 24.Exception ResponsesCode Name MeaningIf a Modbus master device sends an invalid command to a meter or attempts toread an invalid holding register, an exception response is generated. Theexception response follows the standard packet format. The high order bit of thefunction code in an exception response is set to 1.The data field of an exception response contains the exception error code. The tablebelow describes the exception codes supported by the meter and the possiblecauses.Table 1: Exception Codes supported by the meter01 Illegal Function02 Illegal AddressAn Invalid command is contained in the function field of the request packet. The meter only supportsModbus functions 3 and 16.The address referenced in the data field is an invalid address for the specified function. This could alsoindicate that the registers requested are not within the valid register range of the meter.03 Illegal Value The value referenced in the data field is not allowed for the referenced register on the meter.Broadcast PacketsThe ION Modbus protocol supports broadcast request packets. The purpose of abroadcast request packet is to allow all Slave devices to receive the same commandfrom the Master station.A broadcast request packet is the same as a normal request packet, except the slaveaddress field is set to zero (0). All Modbus slave devices receive and execute abroadcast request command, but no device will respond. The Preset MultipleRegisters command is the only command supporting broadcast packets.Page 7
Page 1 and 2: 7500, 7600 and8000 Series MeterAdva
Page 3 and 4: Modbus Register Map7500, 7600 and 8
Page 5: Modbus Register MapIntroductionIntr
Page 9 and 10: Modbus Register MapFunction 16: Pre
Page 11 and 12: Modbus Register MapMeter Modbus Reg
Page 13 and 14: Modbus Register MapModbus Slave Mod
Page 15 and 16: Modbus Register MapION External Con
Page 17 and 18: Modbus Register MapModbus Configura
Page 19 and 20: Modbus Register MapModbus Register
Page 25 and 26: Modbus Register MapPseudocode For C
Page 27 and 28: Modbus Register MapAppendix B: Modb
Page 47 and 48: Modbus Register MapAppendix C: Data
Page 49 and 50: Modbus Register MapAppendix C: Data
Page 51: Modbus Register MapAppendix D: Modb

Modbus Protocol for ION 7500, ION 7600 and ION 8000 series

Create successful ePaper yourself

Delete template?

Save as template?