During their weekly Take Apart Tuesday, the fine folks at the Crash Space hackerspace in LA were planning to convert an old printer into a pen plotter, by disabling the original electronics and hooking directly to the stepper motors that controlled the print head. Unfortunately, things became more difficult when the discovered that the printer actually used a DC motor and optical encoder instead of stepper motors. Fear not, they eventually figured out how to control it, but not before smoking a potentiometer, and causing some other mischief. Their project write-up has an interesting discussion about why the DC motor might have been chosen over a stepper, and the issues involved in driving them directly.
Reverse engineering old printers