The Protocol
Nothing special. Microcontroller requests a new point, computer sends a new point. If there are no new points, computer sends an end transmission code.
The microcontroller also buffers a set of points. Currently, it is set to store 256 points before requesting more. Pretty arbitrary. So the program flow is:
while stop flag not set
If buffer is empty
reload buffer
if received end transmission
set stop flag
draw all points in buffer
One More Thing
The Z height needs to be adjustable. Get a couple buttons to adjust the drawing height, so the pen touches, not misses or jams into, the paper.
With that done, its picture time!