WDMWIZ based driver can't read from COM ports

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’ve used Walter Oney’s WDMWIZ to create a
skelleton for a new wdm function driver. I have created my own driver’s
class in order to get the interrupt handled in my driver (therefore I
don’t use serial.sys). The ressources needed (IRQ and portrange) are
described using the inf’s LogConfig statement. I build the driver using
DDK2.6’s build command (XP checked). After installing my driver (on XP) 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 might be
wrong? Do I need to handle some more PnP IRP’s? I’ve tried to explicit
power on the UART using “SendDeviceSetPower(pdx, PowerDeviceD0, TRUE);”. I
also took a look in the DDK’s serial sample. I can’t find where to power
up the UART in the sample.
It’s the first time for me to write a driver for XP, so I think I make
some basically mistake… Do I have to take the same class-ID as serial?