Thank you very much for all suggestions.

Craig wrote: I've usually seen RS485 used for rather simple protocols that send packets of relatively small amounts of data (less than 256 bytes) and then wait for some kind of ACK

My packet size is 64 bytes maximum, and the slave returns the complete packet as an acknowledge BUT before the slave can return any data the communication direction has to be reversed by an RTS or DTR signal depending on the interface used. I don't know how to check if the last byte was sent.