JPH01241463A - Thermal head drive circuit - Google Patents

Abstract

PURPOSE:To enable the optimum control of a heat pulse width by providing an inverted circulation channel for a shift register holding a heat printing information. CONSTITUTION:Printing informations (black and white) are read out serially from a memory 1 by addresses from an address generating circuit 2 and transferred to a shift register 6 through a switching unit 4. A parallel output of the shift register 6 is passed through a latch circuit 7 and a heat pulse is impressed on a heat resistance element 9 from a drive circuit 8. The heat resistance element 9 is disposed in a column. A thermal head is moved in the direction perpendicular to the column and image printing is conducted by a dot matrix. A serial output of the shift register 6 is passed through an inverter 5 and a (b)-side contact of the switching unit 4 and fed back to the serial input of the shift register 6, so as to control a heat pulse width. When a preceding printing dot is white, the heat pulse width of the following dot is widened by a certain period, while the heat pulse width of the following dot is narrowed when the preceding printing dot is black, and thereby the nonuniformity in a printing density is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a thermal head drive circuit for thermal printing, and is particularly suitable for application to image printing.

[Summary of the invention]

A serial circulation path including an inverter is installed in the shift register that outputs heated printing information in parallel, and immediately before the contents of the shift register are updated, inverted information of the retained contents is output for a certain period of time. This thermal head drive circuit widens the heating pulse width of a dot by the above-mentioned fixed period, and narrows the heating pulse width of a subsequent dot when the previous printed dot is black, thereby reducing unevenness in print density.

[Conventional technology]

In a thermal printing device that obtains a printed image by heating thermal paper or a thermal transfer ink ribbon based on printing information using a heating dot, uneven print density tends to occur due to the thermal inertia of the head. In other words, the temperature of the head tends to rise insufficiently at the beginning of writing, and if the heating power is increased to compensate for this, the temperature will rise too much when printing continuous black dots, and the temperature will rise too much in the next non-printed area. Pulling (unnecessary color development) occurs.

Japanese Patent Publication No. 57-11784 discloses that a wide heating pulse is applied to the head and throat when printing at the beginning of continuous printing dots and when printing at non-continuous independent printing dots. It has been disclosed that a narrow warming pulse is applied for do-no-do printing.

[Problem to be solved by the invention]

The thermal head heating circuit disclosed in the above-mentioned publication requires a determination circuit for determining whether printed dots are continuous, independent, or the beginning of a continuous dot, and the circuit configuration is complicated.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal head drive circuit that can perform optimal heating control with an hour wheel circuit configuration.

[Means to solve the problem]

The thermal head drive circuit of the present invention includes a shift register 6 that holds thermal printing information and is capable of serial shifting, and an inverter 5 that inverts the serial output of this shift register and feeds it back to its serial input.

Shift control and inversion of the shift register is performed so that print heating pulses are output in parallel to the thermal head by updating the contents of the shift register at regular intervals, and inversion information of the held contents is output in parallel during a predetermined period immediately before the update. It is characterized by performing circulation control.

[Effect]

When the information held in the shift register 6 is "white", "black" information for a predetermined period is added before the next heating pulse.
The heating power is increased for independent points of black dots or leading dots of continuous black dots. Furthermore, when the information held in the shift register 6 is "black", the width of the next addition and Q pulses is narrow, so for a series of black dots, the minimum heating power required to maintain the temperature required for printing is required. is given. Therefore, there is no lack of color development in the leading black dots or tailing of white parts due to overheating when black dots are continuous, so that a uniform printed image without density unevenness can be obtained.

〔Example〕

FIG. 1 shows a thermal head drive circuit according to an embodiment of the present invention.

Image information to be printed is stored in the buffer memory l, and one column 6 is stored according to the address from the address generation circuit 2.
Four pieces of print information (black and white) are serially read out from the memory 1 and transferred via the switch 4 to the shift register 6 for serial-to-parallel conversion. The 64 parallel outputs of the shift register 6 are applied to the drive circuit 8 via the latch circuit 7, and heating pulses corresponding to print information are applied from the drive circuit 8 to the 64 heating resistance elements 9 in parallel. The heating resistance elements 9 constituting the thermal head are arranged in columns, and by moving the thermal head in a direction perpendicular to the columns,
Image printing is performed using a dot matrix. Note that the timing signal generation circuit 3 generates various clocks, latch pulses, strobe pulses, etc.

The serial output of the shift register 6 is configured to be fed back to the serial input of the shift register 6 via the inverter 5, through the b-side contact of the switch 4. This configuration controls the heating pulse width.

FIG. 2 shows the operation timing of the drive circuit shown in FIG. 1, and FIG. 3 shows the state of controlling the heat generated by the head.

A strobe pulse S1 in FIG. 2A indicates a basic printing cycle for one column, and is supplied from the timing signal generation circuit 3 to the drive circuit 8. The switch control signal S4 in FIG.
When "b" (high level), the switch 4 is connected to the a contact, and when "b" (high level), the switch 4 is connected to the b contact.

Therefore, in each interval "a", print information is transferred from the buffer memory 1 through the switch 4 to the shift register 6, and its parallel output is applied to the heating resistor element 9 via the latch circuit 7 and the drive circuit 8.

Every time printing of one column in the "a" period is completed, the switch 4 is switched to the b contact side in the "b" period, and 64 shift clocks S5 (D in FIG. 2) are applied to the shift register 6. serial shift is performed. The serial output passes through the inverter 5 and the switch 4 and is fed back to the shift register 6 as a serial input, so that the shift register 6 receives an inverted signal of the print information in the "a" section.

When one cycle of the shift register 6 is completed, the first launch pulse S2 is applied to the latch circuit 7 at the leading edge of the strobe pulse S1.
The parallel outputs of the shift register 6 are taken into the launch circuit 7. The output of this launch circuit 7 is
Since ΔT is held for a short time until the launch pulse S3 is applied, the black dots in the previous row of printing will be white (
(no heating), white dots are black (heated).

On the other hand, at the leading edge of the "a" section, the changeover switch 4 switches to the contact a side, and then 64 shift clocks S6 are applied to the shift register 6, and the print information for the next column is transferred from the memory 1. be taken in. Furthermore, immediately after that (after ΔT), the second
A latch pulse S3 is applied to the latch circuit 7, and new print information is transferred from the latch circuit 7 to the drive circuit 8 in parallel.

As shown in FIG. 2 E-H, when the output of the shift register 6 (data DI, D2, etc.) is 0'' during printing of the next column, as a result of the inversion cycle of the shift register 6, the printing section of the next column is In the section of ΔT at the beginning, the latch circuit 7
A pulse of "1" shown by P in FIG. 2F and H is always added to the output of the circuit.

In other words, the front row has white dots (no heating) and the next row has black dots (
heating), the heating pulse is widened by ΔT from the basic heating pulse width T, as also shown in FIG. 3A. Therefore, for an independent black dot or a leading dot of continuous black dots, the heating power is increased as shown in FIG. 3B, and the heat generation temperature is rapidly raised to the color development level L (or transferable level) or higher.

On the other hand, the output of the shift register 6 is “1” when printing the column consisting of
At this time, as a result of the inversion circulation of the shift register 6, the output of the latch circuit 7 becomes "0" for a period of .DELTA.T at the beginning of the next printing period, as shown by Q in FIG. 2F and H.

That is, if the previous row is a black dot and the next row is a black dot, the heating pulse is narrowed to the basic width T as shown in FIG. 3A,
Heating power sufficient to maintain the temperature above the color development level L is provided.

Note that when white dots are continuous as shown in FIGS. 2F and 3) and FIG. 3A, R
“1” with a width of ΔT as shown in FIG. 1 appears at the output of the launch circuit 7. However, this pulse R with a width of ΔT is
As shown in FIG. 3, since heating power sufficient to reach the coloring level L is not applied to the resistance element 9, the white dots are not colored. Rather, by applying the pulse R, preheating is performed, which improves the heating start-up of the next black dot, which helps eliminate printing unevenness.

Figure 3C is a comparative example in which the heating pulse width is not corrected; in this case, the heat generation temperature gradually increases when black dots are continuous, and a tail trail occurs until the head temperature drops sufficiently at the next white dot. .

Note that in the above-described embodiment, the additional period ΔT is adjusted depending on the printing speed, but the ratio to the basic heating pulse width T may be constant.

〔Effect of the invention〕

As described above, the present invention makes it possible to optimally control the heating pulse width by providing a reversing circulation path in the shift register that holds thermal printing information. Therefore, there is no need for a conventional determination circuit that performs pulse width control by determining whether printed dots are continuous, independent, or the beginning of continuous dots, and the circuit configuration becomes extremely simple.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a thermal head drive circuit showing one embodiment of the present invention, FIG. 2 is a drive timing diagram, and FIG. 3 is a waveform diagram showing heat generation control. In addition, in the symbols used in the drawings, 1---------1・-Buffer memory 2-・-
−−−−−−−−−−−−− Address generation circuit 3−
−−−−−−−−・−・−・−Timing signal generation circuit 4−−・−−−−−−−−−−−・Switcher 5・
−−−−−−−−−−−1・−・−−Inverter 6・
・−−−−〜・−−１１１１・・・−Shift register 7−
・−・・=・−・−・・Latch circuit 8−・−・・−・・
−−−Drive circuit 9・・−−−−−−−−−1−
--Heating resistance element.

Claims (1)

[Scope of Claims] A shift register capable of holding thermal printing information and serially shifting; and an inverter that inverts the serial output of this shift register and feeds it back to its serial input; Shift control and reversal circulation control of the shift register are performed so that printing heating pulses are output in parallel to the thermal head by updating the contents at each time, and inversion information of the retained contents is output in parallel during a predetermined period immediately before the update. Features a thermal head drive circuit.