Chapter 9. Modbus/TCP ClientGFK-2224Q January 2017 155Command 3004, Example 1 – Set Single RegisterWrite one register from %AI10 to register address 200 in the remote Modbus/TCP server. Return the COMMREQStatus word to %R10. Use channel 6, a channel previously opened with the Open Modbus/TCP ClientConnection COMMREQ.Dec (Hex)Word 1 00008 (0008) Length of Channel command Data BlockWord 2 00000 (0000) Always 0 (no-wait mode request)Word 3 00008 (0008) Memory type of CRS word (%R)Word 413 00009 (0009) CRS word address minus 1 (%R10)Word 5 00000 (0000) ReservedWord 6 00000 (0000) ReservedWord 7 03004 (0BBC) Write to a Modbus/TCP DeviceWord 8 00006 (0006) Channel number (6)Word 9 00006 (0006) Modbus Function Code – Write Single RegisterWord 10 00010 (000A) Local PLC Memory TypeWord 11 00010 (000A) Local PLC Starting AddressWord 12 00200 (00C8) Address in the Remote DeviceWord 13 00001 (0001) Number of Registers in the Remote DeviceWord 14 00001 (0001) Unit Identifier(Word 7) Channel Command Number: Word 7 identifies the COMMREQ as a Write Data to remoteModbus/TCP device.(Word 8) Channel Number: Word 8 identifies the channel number previously allocated for communication withthe remote Modbus/TCP server.(Word 9) Modbus Function Code: Word 9 specifies Function Code 6, Write Single Register.(Word 10) Local PLC Memory Type: Words 10–11 specify the location in the local PLC from where the Ethernetinterface will get the data to be written to the remote PLC. Valid values for Word 10 are listed on page 150.(Word 11) Local PLC Starting Address: Word 11 determines the starting address in the local PLC from whichthe data is to be written. The value entered is the offset (1-based) from the beginning of PLC memory for thememory type and mode specified in Word 10. This offset will be either in bits, bytes, or words depending on themode specified. Valid ranges of values depend on the PLC’s memory ranges.(Word 12) Remote Device Address: specifies the destination register in the remote device.(Word 13) Number Registers in Remote Device: Word 13 specifies the quantity of registers to write to theremote device. For Function Code 6, Write Single Register this must be set to 1.(Word 14) Unit Identifier: This field is typically used by Ethernet to Serial bridges to specify the address of aModbus Slave on a multi-drop link. The Modbus/TCP Unit Identifier is a special control code used in aModbus/TCP message block.
