I have a PIC that is producing a serial data stream that I want to
superimpose on a constant DC source such that a 0 bit is 12V and a 1 bit is
17V.  The DC source is provided by a LM338 which is a three pin adjustable
voltage regulator.  I want to connect an output pin from the PIC to the
reference pin on the LM338 in some way, but I have no idea what would be the
correct way to do this.

A related question:

How would you extract the logic level voltage from this DC source just
described so that another PIC could read the data stream?  A zener diode maybe?

I am trying to provide power to a motor and serial commands to a PIC using
the same two
wires.  The PIC would sit between the motor and the power and issue the
appropriate PWM signals to the motor as directed by the data stream.

Does anyone have any ideas please?

Thanks - DRC