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You need to operate in a handshake mode. These may be classified as hardware (RTS/CTS), or software (DC codes, XonXoff, Xmodem). Also you need FIFOs disabled at your computer.
You may not want to use a USB to RS232 serial adapeter even if it appears to work for normal transfers. Many do not even work for normal transfers.
See my web site www.beta-a2.com and you will find a phone number at the bottom of almost any page.