Connect 7220C to modem port. No null modem cable needed.

Use hp-plot.sh to convert PostScript to HPGL using GhostScript and
hpgl.ps

** WARNING: coordinates may come out negative. If they do add a
   translate PS command after the comments in the PS file.

Use cat-serial to send HPGL output to plotter. It will automatically
buffer.

If output is too faint slow the pen down by adding a VS command to the
HPGL just after the IN; command. VS1 worked well last time with how
dried the pens were.