Each "logical" Unit ID configured in the ECOM100 has
1. IP Address
2. TCP Port number (502 is Modbus/TCP)
3. (RTU) ID
This 3rd Modbus/TCP parm is normally 255 or 1 for Modbus/TCP devices, but for any Modbus/TCP->RTU Gateway, you would set it to the RTU Unit ID off the gateway with THAT IP address. Obviously, this is how you would use the ECOM100 with our latest MB-GATEWAY product.
As an aside, for the 275e MX Controller, the IP Address, TCP Port Number, and (RTU) ID are just parameters in the MRX/MWX instructions. The 275 CPU is still in development, and is part of the MX Controller and DirectSOFT/MX products that are currently in Beta release.