Can't read from COM-Port

I have to read timecritical data from com1 and depending on the receiced
data send an answer within 3ms. So I decided to build an own wdm driver
(generic function driver). I have created my own driver’s class. The
ressources needed (IRQ and portrange) are described using the inf’s
LogConfig statement. After installing my driver it seems everything to be
OK: the hardware manager shows my driver in an extra section and the
needed ressources are OK. But it seems the driver has NO access to the
com-port. each port I read gets FF. What is wrong? Do I need to handle
some PnP IRP’s? Even when I use the windbg’s command “ib 3fa” I get FF.