MICR Printer

A document printer comprising:

• a print wheel rotatable about an axis and having a plurality of raised character disposed at predetermined angular positions about the periphery of said wheel;
• electrically actuable print hammer disposed radially of said print wheel and operable between retracted and print wheel impacting positions;
• document transport means for supporting said document and for selectively positioning said document between said print wheel and said print hammer;
• print wheel stepper motor for rotating said print wheel in incremental angular steps about said axis and having means for positioning a selected one of said raised characters in registrations with said print hammer;
• driver means for controlling said print wheel stepper motor including means for determining the number of angular steps required to position said selected one of said characters in registration with said print hammer;
• said drive means further including means for generating a plurality of current pulses for driving said print wheel stepper motor, the period between said pulses being selectively variable in accordance with said number of said incremental angular steps needed to rotate said print wheel to the next character to be printed;
• interrupt controller providinga high priority timer A interrupt signal and a low priority timer B interrupt signal;
• means for performing a timer A interrupt routine in response to said timer A interrupt signal, said timer A interrupt routine being responsive to said number of angular steps requried and determining a table of ordered pairs of values, one value of each pair representing an interrupt rate value and the other value representing a number of interrupt cycles;
• said timer A interrupt routine having an odd cycle for selectively altering the rate of said timer A interrupt signal in accordance with said interrupt rate value;
• said timer A interrupt routine having an even cycle for selectively causing said driver means to activate said stepper motor;
• means for performing a timer B interrupt routine in responsse to said timer B interrupt signal, said timer B interrupt routine determining if said document is positioned and providing an indication thereof to said timer A interrupt routine;
• means responsive to said timer A interrupt routine and said timer B interrupt routine for selectively operating said hammer.