I found out.
You don't need to set any encoding to the port. Just define the data as i.e.
01 ReadSerialNo type Byte[] value x"04FF11EA". *> Read Transponder No.
and send it:
invoke serialPort1::Write(ReadSerialNo, 0, 4)
of course 4 is BytesToWrite.
I was confused by intellisense which explains the "Write" function with "Text as String": Should be corrected.
Thanks anyway
Rainer