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Configuring a Modbus Unit Identifier
The Modbus Unit Identifier (Unit ID) is equivalent to the Modbus Slave Address used by Modbus RTU. The
primary purpose of the Unit ID is to provide a means to route requests to a Modbus slave device attached to a
Modbus Ethernet to Serial gateway. If you are communicating via a gateway, you will need to create and
configure one Modbus Unit ID object for every slave you will access.
If you are communicating with a Modbus TCP/IP device that is not a gateway, you will need to create and
configure one Unit ID object. Some of these devices require a specific Unit ID in order to communicate with
the device. Other devices may ignore the Unit ID value completely, in which case you can use the default value
of 1. Before designating a Unit ID, you should consult the user documentation for the Modbus device you are
using.
To create an Open Modbus Unit ID object, right click on the Modbus Connection object and select the “Add
Unit ID” option. The Protocol Selection panel should appear as shown below.
Clicking on the Unit ID object will allow you to enter the applicable parameters in the configuration panel.
Description: You can enter up to 40 characters
of descriptive text. This description will appear
beside the Unit ID object in the Protocol
Selection panel.
Unit ID: The primary use of the Unit ID is to
address Modbus slave devices that are
connected using a Modbus Ethernet to Serial
Gateway. If you are using a gateway, enter the
Modbus address of the target slave device. If
you are communicating with a Modbus TCP/IP
device that requires a Unit ID, enter the appropriate Unit ID value. Otherwise, use the default value of 1.
CTI 2500P-ECC1 User Manual V1.6
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