The AN-B-001 Booting from Serial Interfaces explains the UART Boot protocol.
A) Are there any timing for answering on STX (or a time window within the answer has to be done) ?
115.200kbps / 57.600kbps
Using smart snipped on UART 1 (Standard PC UART Port without USB) does not work on a very fast PC at 115.200 kbps.
It works fine with 57.600kbps
V24 Level Converter (Maxim) seems to be OK (Signals verified by Scope)