I am sorry, I still don't get what you want to say...
>> But if you in hurry it makes no sense e.g. to call ReadFile with prioritiy 251 or only decrease Priority256 below 103. In this case I would call ReadFile and following actions with priority 104 (most important read data from HW (103) than process data (104)).
<< I don't understand this sentence(s). Both reading and writing on UART is done in a single thread, first we read data from UART, if it is what we expect, then we write some data, and then continue loop with reading. So, if we don't set Priority256 registry value, then the DispatchThread has priority 103. If we set priority of our read/write application thread to 104, then our application could be delayed (by whatever interrupt) to drive RTS on time after serial driver has finished with sending data. Right?
>> Assume it is another reason. Because here we have a handshake and our partner will nothing send as long as RTS is not clear.
Do you have this problem only while connect or also while normal modem operation?
<< Ok, thanks. I am still investigating this issue. It happens that during normal modem operation, dial up connection gets broken, but our software does not get notified on this (software is based on .NET compact framework).