Chapter 9. Modbus/TCP Client
GFK-2224Q January 2017 157
Command 3004, Example 3 – Set Multiple Registers
Write the four registers from Discrete Input Memory (%I40 to) address 200 in the remote Modbus/TCP server.
Return the COMMREQ Status word to %R10. Use channel 6, a channel previously opened with the Open
Modbus/TCP Client Connection COMMREQ.
Length of Channel command Data Block
Always 0 (no-wait mode request)
Memory type of CRS word (%R)
CRS word address minus 1 (%R10)
Write to a Modbus/TCP Device
Modbus Function Code – Write Multiple Registers
Address in the Remote Device
Number of Registers in the Remote Device
(Word 7) Channel Command Number: Word 7 identifies the COMMREQ as a Write Data to Modbus/TCP device.
(Word 8) Channel Number: Word 8 identifies the channel number previously allocated for communication with
the remote Modbus/TCP server.
(Word 9) Modbus Function Code: Word 9 specifies Modbus Function Code 16, Write Multiple Registers
(Word 10) Local PLC Memory Type: Words 10–11 specify the location in the local PLC where the Ethernet
interface will get the data to be written to the remote PLC. Values for Word 10 are listed on page 150. The
value 16 specifies Discrete Input Memory %I (byte mode).
(Word 11) Local PLC Starting Address: Word 11 determines the starting address in the local PLC from which
the data is to be written. The value entered is the offset (1-based) from the beginning of PLC memory for the
memory type and mode specified in Word 10. This offset will be either in bits, bytes, or words depending on the
mode specified. Valid ranges of values depend on the PLC’s memory ranges.
(Word 12) Remote Device Address: Word 12 specifies the destination register in the remote Modbus/TCP
device.
(Word 13) Number Registers in Remote Device: Words 13 specifies the quantity of registers to write to the
remote device.
(Word 14) Unit Identifier: This field is typically used by Ethernet to Serial bridges to specify the address of a
Modbus Slave on a multi-drop link. The Modbus/TCP Unit Identifier is a special control code used in a
Modbus/TCP message block.
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