History Control by Driver IC

The KCE and KHT printhead series have a history control feature built into their driver ICs. This section describes the additional control signals present on these printheads. Please first see Controlling the Thermal Printhead to understand the relationship of the \STROBE, BEO and the data bit in the latch register.

History control is a set of techniques for reducing the pulse width when it is known that a heater element has retained heat from firing on the previous line. If history control is not used, the printer must wait long enough after a line has printed to allow the heaters which fired to cool down to the temperature of the heaters which did not fire. Otherwise, heaters which fire frequently grow hotter over the length of the page, resulting in poor print quality from larger or darker printed dots and possible shorter lifetime due to operating at excessive temperatures. History control uses different pulse widths on a dot-by-dot basis, depending on the heat assumed to be present from having fired on previous print lines.

The conventional \STROBE signal fires all selected heaters for the same amount of time. The printer controller can implement history control by, during a single printed line, loading the printhead several times with different data and briefly firing the heaters, resulting in a total pulse width that varies by dot. The burden is on the controller to compute which heaters are to receive which supplemental pulses. History control as implemented by Kyocera's driver ICs has five \CONT signals, which the printer controller holds low for progressively shorter amounts of time. The driver IC chooses which \CONT pulse width to use for each dot based on that dot's immediate history and the immediate history of the adjacent dots.

Each heater that is going to fire can be unambiguously assigned to one of five categories as shown in the diagram. The lowermost squares are black, indicating that these heaters are selected by their data bits to fire on this print line. The two squares above them show what had happened on the previous lines. The arrow shows the direction of the printhead relative to the paper. The heater in category "cont.1" is relatively cold because it did not fire in the preceding two lines. The heater in "cont.5" is relatively hot. To print these two dots with the same optical density, the controller must hold \CONT1 low for a longer time than \CONT5.

The sketch above is meant to show that when a heater qualifies for "cont.3," it cannot simultaneously be in "cont.2." Kyocera chose this algorithm after experience with other algorithms in prior generations of history control driver ICs. Note that it takes some time for heat to diffuse from a heater that fired to an adjacent heater that did not fire. At higher print speeds it is more important to consider the contribution from adjacent dots on the prior line than on the present line. As shown in the diagram below adjacency on the present line is no longer considered.

The driver IC will switch a heater on as long as the appropriate \CONT is low, \STROBE is low, BEO is high, and the data bit in the latch register is high. Data is loaded and latched in the same way as for other printheads. The \CONT.1 - \CONT.5 signals determine the 5 different pulse widths. \STROBE is still provided as a convenience.

The \LATCH pulse separates print lines for purposes of history control. The printhead has no internal clock or reset. When the printer pauses or advances the form, the controller should load a line of low data bits and pulse \LATCH twice to force the printhead to assume that all heaters are cold.