Iono Uno speaks Modbus too!
We have developed a Modbus RTU slave sketch for Iono Uno. Although Iono Uno doesn’t have a RS485 port, you can still use Modbus RTU via its TTL serial port or a USB cable. Using the serial port, and a few extra components, you can easily connect multiple Iono Uno modules to the master using a single TTL serial line.

You can find the sketch in the examples of the iono libraries.

Of course, as usual, you can always modify it to add your custom logic!