Donatas wrote,
"in the description it is writen, that it works with X-ON ONLY ... I could not find any information about such form of flow control. Everyone is speaking about normal x-on x-off."

Patrick replied,
"I guess that the description means that the device uses only X-on/X-off flow control and is unable to use hardware (rts/cts) flow control.".

No - that was Donatas' whole point: he knows what XON/XOFF is (or has found suitabel references) his qestion was what this "X-ON ONLY"

Unfortunately, I can't think how XON only flow control might work; the whole point of flow control is that you need to be able to turn the flow both on and off!