This program is part of Netpbm.
pnmtorastreads a PPM image as input and produces a Sun rasterfile as output.
Color values in Sun rasterfiles are eight bits wide, so pnmtorast will automatically scale colors to have a maxval of 255. An extra pamdepth step is not necessary.
The -standard option forces the result to be in RT_STANDARD form; the -rle option, RT_BYTE_ENCODED, which is smaller but, well, less standard. The default is -rle.
All options can be abbreviated to their shortest unique prefix.