Issue
Confirming Modbus address and Unit ID

Product Line
ION

Environment
Modbus

Cause
Determining Modbus specifications

Resolution
The Modbus address is dependent on how the meter and communication system is configured:
1. If using the serial Modbus protocol, the Modbus unit ID is the unit ID of the meter as configured on the relevant serial communication port.

2. Using Modbus TCP, the inclusion of the  Modbus Client Options module changes how the meter responds to modbus requests by changing the Unit ID limitation.

The default setting for the "Modbus Gate Connection" is Gateway Disabled, which disables the gateway functionality and allows the meter to respond to any Unit ID for Modbus TCP (port 502) or Modbus RTU (port 7701) requests.

Note that the ION8650 meter does not have this gateway functionality and thus responds to modbus requests on any Unit ID.
This is different from the GATEWAY ENABLED setting, which sets the meter to respond only to Unit ID 255 and enables the gateway functionality even if no downstream devices are setup.

3. For ION7650/7550 meters with firmware preceding V350 and  legacy ION meters (ION7700, ION8600, ION8500), the meter default server ID is 100. Since all Modbus TCP devices have unique IP addresses it allows the default to be used for all units.

NOTE:
The above statements are excerpted directly from the "Modbus Gateway Connection" subsection of the "Modbus Client Options Module" section of the ION Reference document.