Ken,
The program to convert ppm files to HP RTL for use on a HP 650c plotter can be
gotten from anonymous ftp on:
speclab.cr.usgs.gov - 136.177.80.5 in the directory: /pub/p3
This directory contains the source code and a README file. The source code is
compressed in a file called part01.Z
The program was originally meant for an HP 300XL PaintJet, but has been
modified for the HP 650c. Be sure to use the option -DJET to select the
650c. The alternate version of the option -d does not work.
Good luck,
Ron Wahl, USGS