Looks like printer was actually modified to achieve this. Now it's much less interesting... An Atmega8 and an FPGA are connected to various parts of the origin

Looks like printer was actually modified to achieve this. Now it's much less interesting... An Atmega8 and an FPGA are connected to various parts of the original printer main board. The Atmega handles the incoming MIDI messages, communicates with the FPGA and drives the stepper motors for the print head and paper feed. The FPGA is configured to generate lots of pulse-width modulation signals with independent frequency and duty cycle to drive the individual printer pins.