Bidirectional serial printer

Claim 1

A serial printer comprising: a paper feeding mechanism for feeding paper in a paper feeding direction; a print head having a plurality of dot printing elements arranged in the paper feeding direction; a head moving mechanism for moving said print head in a direction substantially perpendicular to the paper feeding direction to scan the paper; a raster buffer for storing bit map pattern data for a scanning area of said print head; and a control section for causing said print head to print a dot image corresponding to the bit map pattern data stored in said raster buffer while moving said print head in a printing direction and reversing the printing direction after printing the dot image; wherein said control section includes text processing means for detecting that printing data externally supplied is constructed in a text form containing character codes, determining a coordinate position for each character code, selecting each group of character codes whose printing area is set within the scanning area of said print head, producing bit map pattern data in said raster buffer by converting the character codes of a selected group of character codes into a dot data form, the produced bit map pattern data representing a whole image of the characters of the selected group of character codes, driving said dot printing elements according to the bit map pattern data while moving said print head, and feeding the paper to set the printing area to the scanning area of said print head, wherein said control section further includes: upper and lower work buffers each for storing lines of dot data the number of which is equal to half the number of dot printing elements; and graphic processing means for detecting that the printing data externally supplied is constructed in a graphic form containing lines of dot data, determining a coordinate position for each dot data line, selecting each group of sequential dot data lines the number of which is equal to half the number of dot printing elements of said print head, storing the sequential dot data lines of a selected group in said lower work buffer, producing bit map pattern data in said raster buffer by selectively masking, with a mask pattern, a combination of dot data items retrieved from said upper and lower work buffers, driving said dot printing elements according to the bit map pattern data while moving said print head, feeding the paper by a distance equal to half the printing height of said print head determined by the number of dot printing elements and the pitch thereof, saving the lines of dot data stored in said lower work buffer into said upper work buffer, and reversing the mask pattern.