whileLoop: Thank you for your help. I believe there is really a little problem with FTDI code. I played a little bit, and was able to do application with UART communication, but communication only worked if I connected Tx and Rx directly on a board. If I connected external device to it, it did not work anymore. Actually it worked for one short period, but then it stopped. I have to investigate a little bit more to find a problem.
By the way, did you found a way how to change strings inside FT311 device? There is a FTDI software, but no information where to connect, and which cable to use. I'm using FT311 Dev board from FTDI, which by default uses its own strings, but I would like to change them to do some more further tests.