Line printers operate by the random selection and firing of individual print hammers in a row of hammers which effect a sharp impact between a print medium and the type characters on a continuously moving type carrier. The type carrier may be a revolving, flexible type belt, band, chain or rotating drum. Various numbers of print hammers can be turned on each print cycle. This is dependent on the data to be recorded in a given line. The rate at which hammers are turned on depends on the distribution of characters on the type carrier in the print line. Controlling the maximum number of hammers which can be turned on at any time in the course of the print line cycle is a problem with this type of printer. The problem arises from the fact that hammers when fired remain turned on for an interval of several print scans or cycles of a print line cycle during which random numbers of other print hammers are being fired and turning off. Hammer number limit control schemes which depend on detecting and counting actual hammer firing pulses versus hammer reset pulses or pulses of a pulse rate generator are not satisfactory especially in very high speed printers and particularly where the controls utilize delay or other circuitry as a means for compensating for flight time variations of the hammers. Limit controls which count hammer drive pulses and clock or pulse rate generation for limit detection and control require circuit redesign where printers are to be operated at different speeds. Likewise, where print hammers are delayed different amounts before firing difficulty can be experienced in the circuitry for detecting when the hammers are turned off and conventional counter means are not effective to detect simultaneous or near simultaneous driving pulses and reset pulses.
USPTO Backgrounds
USPTO Backgrounds