The T3-nano will be a great candidate for your project. It can read the modbus registers over RS485 and report them back to your application over Ethernet using Bacnet protocol, it is depicted at Tab8 in the diagram below. You will need a small program running on the T3-nano to read the modbus parameters and save them as VARS.
10 VAR1 = 2.7.MB_REG100
Where 2 is the ‘Panel ID’ you assign to the Nano and 7 is the subnet modbus ID you assign to your device. 100 is the register you want to read from the modbus device.
You would then read the bacnet object from the T3-nano by referring to XXXXAV1, where XXXX is the bacnet instance of the nano.
You can read more about network programming here:
https://forums.temcocontrols.com/t/network-programming-overview/316
