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GFK-1541B Chapter 4 Programming Modbus/TCP Channel Commands 4-17
4
Command 3004, Example 1 – Set Single Register
Write one register from %AI10 to address 40200 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.
Dec (Hex)
Word 1
00008 (0008) Length of Channel command Data Block
Word 2 00000 (0000) Always 0 (no-wait mode request)
Word 3 00008 (0008) Memory type of CRS word (%R)
Word 4 00009 (0009) CRS word address minus 1 (%R10)
*
Word 5 00000 (0000) Reserved
Word 6 00000 (0000) Reserved
Word 7 03004 (0BBC) Write to a Modbus/TCP Device
Word 8 00006 (0006) Channel number (6)
Word 9 00006 (0006) Modbus Function Code – Preset Single Register
Word 10 00010 (000A) Local PLC Memory Type
Word 11 00010 (000A) Local PLC Starting Address
Word 12 00200 (00C8) Address in the Remote Device
Word 13 00001 (0001) Number of Registers in the Remote Device
Word 14 00001 (0001) Unit Identifier
* Word 4 (CRS word address) is the only zero-based address in the Command Block. Only this
value requires subtracting 1 from the intended address.
(Word 7) Channel Command Number:
Word 7 identifies the COMMREQ as a Write Data to
remote 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 6, Preset Single
Register.
(Word 10) Local PLC Memory Type:
Words 10–11 specify the location in the local PLC from
where the Ethernet Interface will get the data to be written to the remote PLC. Valid values for
Word 10 are listed on page 4-14.
(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
device.
(Word 13) Number Registers in Remote Device:
Word 13 specifies the quantity of registers to
write to the remote device. For Modbus Function Code 6, Preset Single Register this must be set
to 1.
(Word 14) Unit Identifier:
Default is 1. This field is typically used by Ethernet to Serial bridges to
specify the address of a Modbus Slave on a multidrop link. The Modbus/TCP Unit Identifier is a
special control code used in a Modbus/TCP message block. This value is 1 for most Modbus/TCP
messages.