32U4 and Arduino modbus libraries

Hi There,
I have been using Atmega 328P's and a number of Modbus RTU libraries successfully during a number of years. I mostly use RS485 with the MAXX485 chip in half-duplex mode.

So I decided to check out the Pololu A-star 32U4 with integrated USB. I hooked up everything and half duplex comms simply don't work. I tried all the digital pins as transmit pin... nothing. Modbus over USBserial works like a charm, but as I use industrial HMI's, RS485 has preference.

Why would the Pololu A-star 32U4 not work? I can't see any reason as the library is written using any digital output as transmit pin, Rx and Tx. Very strange. Any help would be much appreciated.

Cheers, Jan

It looks like different controller chips, possibly different rigging like timers.... That's the unit the compiler makes the code for, not the complete board.