Check using MSD.EXE (supplied with MS DOS V5 or later that you do in fact
have 16550A UARTs installed.
If this OK check whether you have enabled hardware flow control and not
software (XON/XOFF) flow control.   If you are using XON/XOFF switch to
CTS/RTS.   
If this does not help post some more info re your settings etc. 
Russell

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