Sending Explicit Messages to Modbus TCP Devices
Original instructions
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Overview
Use the Modbus Explicit Message window to send an explicit message from Control Expert to a Modbus TCP module or device on the network.
You can use explicit messaging to perform many different services. Not all Modbus TCP devices support all services.
NOTE: Before you can perform explicit messaging, connect the DTM for the upstream communication module to the module itself. To do this, select the module node in the DTM Browser, then select Edit → Connect.
The Modbus TCP Explicit Message window, below, presents an example of both the configuration of a Modbus TCP explicit message, and the response. In this example, the explicit message is used to read 2 registers in the remote STB NIP 2212 network interface module, starting at offset 5391.
Sending Explicit Messages
To send an explicit message to a target Modbus TCP device:
Step
Action
1
In the DTM Browser, select the communication module that is upstream of the target device.
2
Click the right mouse button, and in the pop-up menu select Device menu → Modbus Explicit Message.
Result: The Modbus Explicit Message window opens.
3
Configure the explicit message using the following fields:
IP Address
The IP address of the target device, used to identify the target of the explicit message. In this example: 192.168.1.7.
Start Address
A component of the addressing path. In this example 5391.
Quantity
A component of the addressing path. In this example 2.
Read Device Id Code
(read-only) The service the explicit message is intended to perform. In this example Basic Device Identity. Not used in this example.
Object Id
(read-only) Specify the object the explicit message is intended to access. In this example 0. Not used in this example.
Refer to your Modbus TCP device user manual for Start Address, Quantity, Read Device Id Code, and Object Id values.
Unit Id
The number of the device, or module, that is the target of the connection. A value of:
  • 255 (the default) used to access the Ethernet communication module itself
  • 0...254 identifies the device number of the target device, behind a Modbus TCP to Modbus gateway
Number
(read-only) The integer associated with the service to be performed by the explicit message. An integer from 0...255.
Name
Select the service the explicit message is intended to perform. In this example ReadHoldingRegisters
Repeat 500ms
Select this check box to re-send the explicit message every 500 ms. Leave this check box de-selected.
4
After your explicit message is configured, click Send to Device.
The Response area displays any data sent to the configuration tool by the target device in hexadecimal format.
The Status area displays messages indicating whether or not the explicit message has succeeded.
5
Click Close to close the window.