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.