Abstract: A dot printing matrix head having printing elements on multiple rows and columns is moved so the columns of the head are translated relative to a medium on which characters are printed. The elements have a predetermined arrangement so that: (1) in each column and row there is at least one element, and (2) there are no elements in adjoining row positions whereby there is no interference between the elements in response to simultaneous activation of any pair of the elements. The elements in different columns of the matrix are activated at differing times in response to signals from at least one shift register bank, a data bit generator and a shift column control circuit as the different matrix columns become aligned with the same column of dots to be printed.

Abstract: The disclosure describes apparatus for checking the operation of control circuits of the type used to actuate the hammers of a printer. A first gating matrix is addressed by a selector so that the control circuits are operated in groups during a plurality of sub-cycles of operation. Detectors which generate an error signal in response to the erroneous operation of each control circuit are connected to the gates of a second gating matrix arranged in the same manner as the first gating matrix. The selector also addresses the second gating matrix so that the detectors which generate error signals can be identified in an error register connected to the second gating matrix.