
the interface to port A of the DUART is realisied in
basic/io_duart.h. There are two iobuf structures, rdioin and rdioout.
rdioout can be written with the function rdioput(char), rdioin can be 
read by using cahr rdioget(). The isr writing to rdioin can be enabeled by
calling rdio_enable_rx() and disabeled by calling rdio_disable_rx().

NOTE: These functions do not set the output ports on the DUART to
enable/disable the receiver on the RadioMetrix module. These have to be
handeled specially, since enabeling both rx and tx enables the test mode
of the module, feeding back the output to the input. There are inline
functions in rdio.h which control the output bits and allow to read the input
bit connected to the CD output.

NOTE2: See NOTE. Using the test mode of the module might be a nice way to 
detect its presence.

