Modbus TCP Explicit Message Example: Write Registers
(Original Document)
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Overview
The following example shows you how to use the SEND_REQ function block to send a generic explicit message that will write two (2) registers to a remote device at IP address 192.168.1.6, beginning at register address 10 in the remote device.
Implementing the SEND_REQ Function Block
To implement the SEND_REQ function block, you need to create and assign variables for the following blocks:
Configuring the Address Variable
The Address variable identifies the explicit message originating device—-in this example, the TSX ETC 101 communication module—and the target device. Note that the Address variable does not include the Xway address elements {Network.Station} because we are not bridging through another PLC station. Use the ADDR function to convert the following character string to an address:
ADDR(‘0.3.0’), where:
Configuring the ReqCode Variable
The ReqCode variable identifies the function type for the SEND_REQ function block—in this case, a generic Modbus request:
Variable
Description
Value (hex)
ReqCode
Code identifies a generic Modbus request
16#0050
Configuring the ManagParam Variable
The ManagParam[3] setting defines the length of the DataToSend variable for the SEND_REQ function block. In this example, 14 bytes of data will be sent:
Variable
Description
Value (hex)
ManagParam[3]
Length of the DataToSend variable
16#000E
Configuring the DataToSend Variable
The DataToSend variable identifies the generic Modbus request:
Variable
Description
Value (hex)
DataToSend[0]
First two octets of the target device IP address (192.168.1.6):
  • High byte = byte 4: 16#C0 (192 decimal))
  • Low byte = byte 3: 16#A8 (168 decimal)
16#C0A8
DataToSend[1]
Last two octets of the target device IP address (192.168.1.6):
  • High byte = byte 2: 16#01 (1 decimal)
  • Low byte = byte 1: 16#06 (6 decimal)
16#0106
DataToSend[2]
  • High byte = Modbus function code: 16#10 (16 decimal)
  • Low byte = the most significant byte (MSB) of the starting write address: 16#00 (0 decimal)
16#1000
DataToSend[3]
  • High byte = the least significant byte (LSB) of the starting write address: 16#0A (10 decimal)
  • Low byte = Quantity of registers to write (MSB): 16#00 (0 decimal)
16#0A00
DataToSend[4]
  • High byte =Quantity of registers to write (LSB): 16#02 (2 decimal)
  • Low byte =Byte count: 16#04 (4 decimal)
16#0204
DataToSend[5]
  • High byte =Register value to write (MSB): 16#01 (1 decimal)
  • Low byte =Register value to write (LSB): 16#02 (2 decimal)
16#0102
DataToSend[6]
  • High byte =Register value to write (MSB): 16#03 (3 decimal)
  • Low byte =Register value to write (LSB): 16#04 (4 decimal)
16#0304