GE PACSystems RXi User Manual

Chapter 8. Gigabit Ethernet (GbE) Interface Overview and OperationGFK-2816F August 2015 169Read Data from a Modbus TCP Device (3003)The Read Data from a Modbus TCP Device COMMREQ requests a data transfer from aModbus TCP device to the controller. The Read Data COMMREQ must reference an activeModbus TCP channel previously established with the Open Modbus TCP Client ConnectionCOMMREQ.Registers, Coils or Exception Status data may be read from the remote Modbus TCP device.The Modbus Function Code specifies the data type. Valid Function Codes for the Read DataCOMMREQ are presented in the following table.FunctionCodeDescription Modbus ServerMemory RegionAccessedData UnitSizeMaximum DataUnits1 Read Coils Internal Bits orPhysical coilsBit 20002 Read Input Discretes Physical DiscreteInputsBit 20003 Read MultipleRegistersInternal Registers orPhysical OutputRegistersRegister(16-bit Word)1254 Read Input Registers Physical InputRegistersRegister(16-bit Word)1257 Read ExceptionStatusServer ExceptionMemoryByte Not Applicable24 Read FIFO Queue Internal Registers orPhysical OutputRegistersRegister(16-bit Word)32The table above describes the general Modbus server memory areas. The actual memoryaccessed is dependent on how the server maps the Modbus memory regions to the server’slocal memory.An Address and Length specify the location of the data in the remote device and the numberof data units to transfer. The Length is the number of Registers or Coils to transfer. ModbusFunction Code 7, Read Exception Status does not require the address as the remote deviceretrieves the exception status from an internal location.When transferring data between server bit or coil memory to RXi Controller bit memory, onlythe number of bits specified is transferred. For example, if the COMMREQ requests to readnine coils from the Remote Device and requests to put the data at %M00001 in the LocalController (using a bit type memory type), %M00001 through %M00009 will be updated withthe data from the Remote Device and %M00010 through %M00016 will be unaffected.However, if server bit or coil memory is transferred to the controller byte or word memory, thefollowing rules apply:1. Transferring discrete data from the Remote Device to Local Controller Word (16-bit)memory: If the number of requested coils is not a multiple of 16, the data is padded with0s to a 16-bit boundary. For example if the COMMREQ requests reading 17 coils fromthe Remote Device and requests to place this data at %R00010, %R00010 (all 16 bits)and bit 0 of %R00011 will be updated with values from the Remote Device and bits 1through 15 of %R00011 will be set to 0.2. Transferring discrete data from the Remote Device to Local Controller byte memory(using byte type memory type): If the number of requested coils is not on an 8-bitboundary, the data is padded with 0s to an 8-bit boundary. For example if theCOMMREQ requests 9 coils from the Remote Device and requests to place this data at%M00001, %M00001 through %M00009 will be updated with values from the RemoteDevice and %M00010 through %M00016 will be set to 0.