This area is not entered into lightly. Look for a commercial or free
package that supports your device.  Failing that, try to acquire
(legally!) the source code for another device and modify it.  The AT&T
documentation "Troff User's Manual" has a Reference section that
completely documents the ditroff language.  SunOS and Psroff 3.0
documentation contain descriptions of the CAT troff language.  Psroff
3.0 is designed for drop-in drivers, and may be the easiest to extend.
Coupled with hp2pbm, psroff3.0 can probably drive just about any kind
of printer that supports raster graphics with relatively little trouble
(see Epson 24 pin driver for guidance).

If you can get a PostScript driver and are running on current SunOS,
you could consider developing a NeWSprint driver for your printer.
NeWSprint is Sun's NeWS (PostScript language interpreter extended for
driving displays; not the same thing as "Display PostScript"(tm).  This
may be easier than developing a troff driver, since all NeWSprint
really needs is the ability to send a bitmap to a printer. Source and
developer's documentation are shipped standard with NeWSprint.

Thack is ideal for quick hacks with CAT troff for printers that have
dynamically scalable fonts (aside from the fact that thack has no
facilities for building width tables).