Unique Modbus Addressing does not apply to PSTN sets.
Use the Unique Modbus Addressing check box on the Modbus tab to specify whether the controllers associated with a set need to have a unique Modbus Address.
This check box can only be cleared if the channel associated with a set uses TCP Protocol Type (see Specify the Required Modbus Protocol and Packet Size).
The Modbus TCP standard specifies that a unique Modbus Address is not required for a device, as the TCP/IP address can be used for identification.
Select the check box if Unique Modbus Addressing is to be maintained for Modbus TCP controllers associated with the set. For example, if a Modbus TCP to Modbus RTU converter is being used.
Clear the check box if Unique Modbus Addressing is not required for these devices.