JP2820020B2 - Thermal head drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printer and, more particularly, to a thermal head driving circuit for driving a line thermal head by dividing it into two blocks.

[0002]

2. Description of the Related Art Conventionally, a thermal head drive circuit requires a large-capacity power supply to simultaneously drive all the heating elements of a line thermal head for one line. (For example, JP-A-63-76668).

In order to further increase the overall printing speed, a plurality of blocks are driven in a time-division manner by switching a plurality of strobe signals, and apparently a plurality of blocks are controlled so as to be driven simultaneously. For example, JP-A-2-81649.

In the above-described thermal head drive circuit, for example, a basic strobe signal is simply input to a switching circuit, and a strobe A signal and a strobe B signal that are alternately enabled for a heating element divided into two blocks. Output to drive the thermal head,
If the time during which the basic strobe signal is enabled is divisible by doubling the switching switching time, the time during which the strobe A signal and the strobe B signal are enabled becomes equal as shown in FIG. The current values flowing through the heating elements of the blocks are equal, and no shading appears in the printing result.

Further, in order to improve the printing result, the time during which the basic strobe signal is enabled may be shortened as the head temperature increases. This is because if the head temperature rises due to heat storage,
This is to avoid a phenomenon in which the print result becomes darker when a current is applied to the heating element with the same strobe width.

When this control is performed, the time during which the basic strobe signal is enabled may not be divisible by twice the switching switching time as shown in FIG. In this case, the enabled time differs between the strobe A signal and the strobe B signal. In other words, the printing result of the block with the longer enable time becomes darker than that of the block with the shorter enable time.

[0007]

In the above-described conventional thermal head drive circuit, when the thermal head is driven in a time-division manner, the strobe A can be reduced by shortening the time-division cycle.
The difference between the enable time of the signal and the enable time of the strobe B signal is also small. However, if the time division cycle is too short, the temperature of the heating element does not rise to the expected temperature due to the heat capacity of the head.
There has been a problem that the strobe time must be extended, resulting in a decrease in printing speed.

When the time-sharing cycle is lengthened, depending on the relationship between the enable time of the basic strobe signal and the time-sharing cycle, the enable time of one strobe signal may be longer than that of the other strobe signal. There is a problem that shading appears in the printing result.

SUMMARY OF THE INVENTION Therefore, the present invention solves such a problem and provides a switching control circuit for setting the time until the switching circuit starts operating, and enables the strobe signal for both switching. It is an object of the present invention to provide a thermal head drive circuit that makes the same.

[0010]

In order to achieve the above object, a thermal head drive circuit according to the present invention is a thermal head drive circuit for driving a line thermal head by dividing the line thermal head into two blocks. An output means for outputting a basic strobe signal for determining a time during which a current flows in the body; and a basic strobe signal being input, and the heating elements of the two blocks are alternately turned on while the basic strobe signal is enabled. The switching circuit includes a switching circuit that alternately generates two strobe signals to be driven, and a switching control circuit that sets a time from when the basic strobe signal is enabled until when the switching circuit is operated.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a thermal head drive circuit showing one embodiment of the present invention, and FIGS. 2 to 4 are waveform diagrams of strobe signals.

As shown in FIG. 1, the present invention provides a thermistor 2 for detecting the temperature of a thermal head 8 and an A / D converter 3 for A / D converting a detection value obtained by the thermistor 2.
A ROM 4 storing an enable time of a basic strobe signal for the next printing corresponding to the A / D-converted A / D-converted value; In response, the enable time of the basic strobe signal is read from the table in the ROM 4.
PU1, a basic strobe signal generating circuit 5 for outputting a basic strobe signal in which the basic strobe signal is enabled for a time during which a current flows through the heating element 20 of the thermal head 8 selected by the CPU 1, and a basic strobe signal While the basic strobe signal is enabled, the switching circuit 6 alternately generates the strobe A signal and the strobe B signal so that the enable state appears alternately, and operates the switching circuit 6 from the time when the basic strobe signal is enabled. A switching control circuit 7 for setting a time until the data is input, a data A terminal 15 to which serial data is input from outside, and a data B
A terminal 16, a shift register 9 for inputting the serial data and converting the data into parallel, and a latch A for latching parallel data of the shift register 9;
To obtain the logical product of the output of the latch A unit 10 and one of the two outputs (strobe A) of the switching circuit 6, and the output of the latch B unit 11. AND gate 12 for obtaining the logical product of the output of the switching circuit 6 and the other output (strobe B)
And a CLK signal terminal 13 to which a CLK signal used for serial-parallel conversion of the data input to the shift register 9 is supplied. LAT signal terminal 14 for supplying the LAT signal used when
And a transistor 17 for controlling ON / OFF of a current flowing through the heating element 20.

The data A terminal 15 and the data B terminal 16
And the CLK signal terminal 13 are connected to the input of the shift register 9, and the data A and B are serial-parallel converted by the shift register 9 in synchronization with the CLK signal from the CLK signal terminal 13.

A parallel data output terminal of the shift register 9 is connected to input terminals of a latch A unit 10 and a latch B unit 11. An LAT signal for latching the parallel data output of the shift register 9 is input to the latch A unit 10 and the latch B unit 11.

The outputs of the latch A section 10 and the latch B section 11 are data for determining whether or not to drive each heating element, and the output terminal thereof is connected to one input terminal of the AND gate 12. The strobe A signal output terminal 18 of the switching circuit 6 is connected to the other input terminal of the AND gate 12 to which the output terminal of the latch A unit 10 is connected. Similarly, the strobe B signal output terminal 19 of the switching circuit 6 is connected to the other input terminal of the AND gate 12 to which the output terminal of the latch B unit 11 is connected.

Further, an output terminal of the AND gate 12 is connected to a base terminal of a transistor 17 for controlling ON / OFF of a current flowing through the heating element 20.

The basic strobe signal, which is output from the output terminal 21 of the basic strobe signal generation circuit 5 and determines the time during which current flows through the heating element 20, is supplied to the switching circuit 6
Is input to the switching control circuit 7.

While the basic strobe signal output from the basic strobe signal generation circuit 5 is enabled, the switching circuit 6 alternately outputs the strobe A signal and the strobe B signal output from the output terminals 18 and 19, respectively. Enable. Further, the internal switching switching time of the two strobe signals is normally a fixed value, and in this embodiment, the switching switching time is set in the switching circuit 6 by the CPU 1 in advance.

The switching control circuit 7 supplies a trigger signal to the switching circuit 6 to activate the switching circuit 6 after a switching operation start time.

Assuming that the switching operation start time is X, the time during which the basic strobe signal is enabled (hereinafter referred to as the basic strobe enable time) is Y, and the internal switching switching time is Z, the following equation is obtained. More required.

X = {Y− (Z × 2) × int [Y / (Z × 2)]} / 2 (1) where int is a function for extracting an integer part of division in [].

The operation of the present invention in the above configuration will be described with reference to FIGS.

First, when the basic strobe signal supplied from the basic strobe signal generation circuit 5 is enabled, the switching control circuit 7 starts operating, and at the same time, the switching circuit 6 enables the strobe A signal.

When the switching operation start time set by the CPU 1 elapses, the switching control circuit 7 outputs a trigger signal to the switching circuit 6. The switching circuit 6 starts the internal switching operation from the time when the trigger signal is received.

The data A and data B fetched into the shift register 9 by the CLK signal from the CLK terminal 13 are converted into parallel data,
The signal is latched by the latch A unit 10 and the latch B unit 11 by the AT signal. Thereafter, when the strobe A signal is enabled, a current flows through the heating element 20 in which the AND output between the output of the latch A unit 10 and the strobe A signal becomes “H”, and printing is performed. Next, when the strobe A signal is disabled and the strobe B signal is enabled, a current flows through the heating element 20 in which the AND output between the output of the latch B unit 11 and the strobe B signal becomes “H”. Printing is performed.

At this time, the strobe A signal and the strobe B
The signal timing will be described with reference to FIG.

The basic strobe enable time Y set by the basic strobe signal generation circuit 5 is equal to the internal switching switching time Z set by the switching circuit 6 by two.
If the value is divisible by the multiplied value, the switching operation start time X set in the switching control circuit 7 is 0.
μs, the switching control circuit 7 immediately outputs a trigger signal from the trigger signal output terminal 34 to the switching circuit 6 when the basic strobe signal is enabled, and the switching circuit 6 starts operating. Therefore, the strobe A signal and the strobe B signal are enabled for the same time as shown in FIG.

Here, when the printer prints a predetermined line, the temperature of the thermal head 8 is detected by the thermistor 2, the detected value is A / D-converted by the A / D converter 3, and the converted value is transmitted to the CPU 1 by the CPU 1. Read.

The CPU 1 refers to the table in the ROM 4 based on the converted value, determines the optimum enable time Y of the basic strobe signal for driving the current thermal head 8, and sets it in the basic strobe generating circuit 5. I do.

When such control is performed, the basic strobe enable time Y becomes equal to the switching switching time Z.
May not be divisible by a value twice as large. FIG. 3 shows an example.

In FIG. 3, the basic strobe enable time Y is not divisible by a value obtained by doubling the internal switching switching time Z, but the switching operation start time X is calculated and set in the switching control circuit 7, so that the strobe A The enable time of the signal and the strobe B signal can be made equal as shown in FIG.

For example, assuming that the switching switching time Z set to the switching circuit 6 as a fixed value is 25 μs and the basic strobe enable time Y of the basic strobe signal selected by the CPU 1 is 75 μs, the switching operation start time X is ( 1) From equation 1
2.5 μs. At this time, the enable time of the strobe A signal and the enable time of the strobe B signal are both equal to 37.5 μs.

The operation at this time will be described in more detail with reference to FIG.

First, when the basic strobe signal supplied from the basic strobe signal generation circuit 5 is enabled, the switching control circuit 7 starts operating.

Here, when the basic strobe signal is enabled, the switching circuit 6 enables one strobe signal. In the case of the present embodiment, the strobe A signal is enabled.

The switching control circuit 7 has a CPU 1
When the calculation result of the expression (1) set by the above, that is, the switching operation start time X elapses, a trigger signal is supplied to the switching circuit 6. The switching circuit 6 that has received the trigger signal starts the internal switching operation at this time, that is, 12.5 μs after the basic strobe signal is enabled, and the enabled strobe A signal is output at the timing of the trigger signal. Disabled, and the strobe B signal is enabled at the same time.

Thereafter, the strobe A signal and the strobe B signal are alternately enabled every 25 μs, which is the set internal switching switching time.

Then, the basic strobe signal is disabled, that is, as soon as 75 μs elapses after the basic strobe signal is enabled, both the strobe A signal and the strobe B signal are disabled. As a result, the enable times of the strobe A signal and the strobe B signal are both 37.5 μs, which are completely equal.

By appropriately setting the switching operation start time X in this manner, the enable times of the strobe A signal and the strobe B signal can be equalized,
Good printing results can be obtained.

[0040]

As described above, according to the present invention, by providing the switching control circuit for setting the time until the operation of the switching circuit is started, the enable time of both switching strobe signals is always equalized. Therefore, high-quality printing can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a thermal head drive circuit showing one embodiment of the present invention.

FIG. 2 is a waveform diagram of a strobe signal.

FIG. 3 is a waveform chart when enable times of two strobe signals are different.

FIG. 4 is a waveform diagram of a strobe signal output from a switching circuit used in an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 CPU 2 Thermistor 3 A / D converter 4 ROM 5 Basic strobe signal generation circuit 6 Switching circuit 7 Switching control circuit 8 Thermal head 9 Shift register 10 Latch A section 11 Latch B section 12 AND gate 17 Transistor 20 Heating element

Claims (4)

(57) [Claims] 1. A thermal head drive circuit for driving a line thermal head by dividing the line thermal head into two blocks, comprising: an output unit for outputting a basic strobe signal for determining a time during which a current flows through a heating element of the line thermal head. A switching circuit for inputting the basic strobe signal and alternately generating two strobe signals for alternately driving the heating elements of the two blocks while the basic strobe signal is enabled; And a switching control circuit for setting a switching operation start time from the time when the switching circuit becomes to the time when the switching circuit is operated. 2. The output means comprises: a thermistor for detecting a temperature of the thermal head; a control means for determining an optimum basic strobe time for printing in accordance with a detection value obtained by the thermistor; 2. A thermal head drive circuit according to claim 1, further comprising a basic strobe generating circuit for outputting a basic strobe signal for a time determined by the means. 3. The switching control circuit sets an optimum basic strobe time for printing according to a detection value obtained by a thermistor for detecting a temperature of the thermal head from a time when a basic strobe signal is enabled. 2. The thermal head drive according to claim 1, wherein a trigger signal for starting the switching operation of the switching circuit is output to the switching circuit after a lapse of the switching operation start time set by the determining means. circuit. 4. A switching operation start time set in the switching control circuit is X, a time when a basic strobe signal provided from the basic strobe signal generation circuit is enabled is Y, and the switching circuit start time is X. X = 内部 Y− (Z × 2) × int [Y /
2. The thermal head drive circuit according to claim 1, wherein the value is obtained from (Z × 2)]｝ / 2.