JPH0321463A - Thermal head drive system - Google Patents

Abstract

PURPOSE:To supply a constant energy to heating elements to prevent the occurrence of unevenness in density and to obtain a power source of longer life by a method wherein resistors are connected to the heating elements in parallel so that an apparent resistant value viewed from the power source can be made constant corresponding to the number of the heating elements to be electrically conducted. CONSTITUTION:Printing data is transmitted to a buffer memory 10. The data for one line is once stored and read out. Then, the data is transmitted to a thermal head 11 and successively stored in a shift register. At the completion of the data transmission for one line, a counting means 12 is reset, and repeats an action of counting bits having a logical value '0' in next data for one line. Based on the number of bits having a logical value '0' counted by the counting means 12, an impedance selection means 13 selects an impedance to be connected to a power source VD in parallel with a thermal head 11. On a strobe signal, the impedance selection means 13 opens a gate to pass a current to the selected impedance. In this manner, an electric current of an invariantly constant value can be supplied from the power source VD when data for one line is printed.

Description

[Detailed Description of the Invention] [Summary] Regarding a thermal head drive method that prevents the occurrence of print density unevenness when printing using a line thermal head, the present invention aims to prevent the occurrence of density unevenness and extend the life of the power supply. The purpose is to arrange N heating elements in a line, and generate heat by supplying current from a power supply to the heating element that prints dots corresponding to data among the N heating elements based on a printing signal. Accordingly, in an apparatus that performs printing using a thermal head that simultaneously prints a plurality of dots on paper, there is provided a counting means for counting the number of dots to be printed simultaneously among data to be printed by the thermal head, and the counting means. Corresponding to the number counted,
An impedance selection means for selecting an impedance connected in parallel with the thermal head is provided to the power source, and when printing a single or plural dots, current is supplied to the power source to the N heating elements. The plan is to send out the same current as in the case.

[Industrial application field]

The present invention relates to a thermal transfer printing device using a thermal head, and particularly when printing using a line thermal head,
The present invention relates to a thermal head drive method that prevents uneven printing density.

Because line thermal heads are quiet when printing, they are used in thermal transfer printing devices such as facsimile machines and line printers used in offices.

In this line thermal head, hundreds to thousands of heating elements are lined up, and by controlling the energization of each heating element, each heating element generates heat, melting the ink on the ribbon and printing dots on the paper. By transferring or heating the thermal paper to form dots, one line of dots is simultaneously formed on the paper, and by continuously feeding the paper, characters, figures, etc. can be printed on the paper at high speed. is being considered for printing.

By the way, line thermal heads have a large number of heating elements,
To print one line of data at a time, the current capacity of the power supply that supplies the heating element becomes large, so it is divided into multiple blocks and the current is supplied at different times for each block. If the number of heating elements driven by each block differs, the current value supplied to each block will vary, resulting in fluctuations in the power supply voltage and a difference in the energy supplied to the heating elements, resulting in a concentration difference between blocks. There will be a difference in the printing density, and uneven printing density will become noticeable.

This printing density unevenness is unsightly and must be prevented.

[Conventional technology]

FIG. 3 is a block diagram illustrating an example of the prior art.

Data to be printed in one block constituting the line thermal head enters the shift register 9 from the terminal DATA, and is sequentially shifted and assigned to the block by the clock that enters the shift register 9 from the terminal CLK.
Data for printing lines is stored.

Each bit of data stored in the shift register 9 is latched by a latch circuit 8 corresponding to a plurality of heating elements R, to R7, respectively, by a latch signal input from a terminal LACTH.

The bit latched by the latch circuit 8 is sent to a plurality of AND circuits 1 to 7, respectively, and the logic “1#” input from the terminal STB.
AND circuits 1 to 7 while the strobe signal continues.
In the logic "1" bit, a manually generated AND circuit sends out a logic "1", which is sent to each of the corresponding transistors among the plurality of transistors Q1 to Q7, and turns on the corresponding transistor. do.

Therefore, a current flows from the voltage -aV to the heating element whose corresponding transistor is turned on among the plurality of heating elements R1 to R, and the heating element through which the current flows prints dots on the paper.

[Problem to be solved by the invention]

Conventionally, multiple heating elements R were incorporated into one block,
-During RnO, current flows to the heating element corresponding to the logical "1" bit in the data, so current is supplied to one heating element, and current is supplied to N heating elements. Although there may be cases, there is a large difference in the current value supplied by the power supply between this one heating element and N heating elements.

That is, N heating elements R1 to R. When supplying current from a power source to t. ”-r., then r = 1 / (1/r+ +1/rt+1/r3+-
---+l/rn). For example, if we consider a case where the resistance value of one heating element is IKΩ and the total number of heating elements is 11)00, when electricity is applied to one heating element and when electricity is applied to 11) 00 heating elements, The change in total resistance varies from 1Ω to 1KΩ. Therefore, the power supply voltage is 11)
V, the current will vary from IOA to 0.01A.

For this reason, there is a large difference in voltage drop due to the internal resistance of the power supply and the resistance of wiring, etc.
1) When printing 00 dots, a large difference occurs in the energy supplied to each heating element.

Therefore, when printing characters, etc., differences in print density are often relatively unnoticeable, but some blocks print all dots, and other blocks print half of all dots. In such a case, there is a problem that a difference occurs in printing density between blocks, and density unevenness becomes noticeable and unsightly, and the power supplied by the power source fluctuates rapidly, shortening the life of the NB.

In view of these problems, the present invention connects a resistor in parallel with the heating elements so that the apparent resistance value seen from the power source becomes constant according to the number of heating elements that are energized. Ensure that the voltage applied to the energized heating element does not fluctuate,
The purpose of this is to supply a constant amount of energy to the heating element to prevent uneven concentration, and to maintain a constant current value sent out by the power source to extend the life of the power source.

[Means to solve the problem]

FIG. 1 is a block diagram illustrating the principle of the present invention.

The print data is sent to the puffer memory 11) from a head control circuit controlled by a processor (not shown),
Data for one line is stored once and then outputted one after another. The signals are then sent to the thermal head 11 and sequentially stored in the shift register as explained in FIG.

The counting means 12 is activated when the transfer of data for one line is started by a start signal from the head control circuit,
For example, count the logical 0''' bits in the print data. Then, when data transfer for one line is completed,
A reset signal sent from the head control circuit resets the logic “O” in the next line of data.
Repeat the operation of counting the bits of . The impedance selection means 13 selects the power source ■ according to the number of logic "0" bits counted by the counting means 12.

On the other hand, an impedance connected in parallel with the thermal head 11 is selected. That is, N of the thermal head 11
Assuming that a current is supplied to M heating elements among the heating elements, an impedance through which a current flows corresponding to the current flowing through (N-M) heating elements is selected.

Then, in response to the strobe signal described in FIG. 3, the impedance selection means 13 opens the gate that allows current to flow through the selected impedance, so the power source 1 is turned on while the dots are being printed. A current flows through the thermal head l1 and the impedance selection means 13, but this current value is the same as the current value flowing through the N heating elements.

[Function] With the above-described structure, the impedance selection stage 13 selects a portion where the current flowing through the thermal head 11 is smaller than the current flowing through the N heating elements based on the result counted by the counting means 12. In order to compensate when opening the power supply V. When printing one line of data from
It is possible to always send out a constant value of current. Therefore, the energy supplied to each heating element of the thermal head 11 becomes constant, preventing the occurrence of density unevenness, and since the current value sent out by the power supply is constant, it is possible to extend the life of the power supply. .

〔Example〕

FIG. 2 is a block diagram of a circuit showing one embodiment of the present invention.

The same reference numerals as in FIG. 3 indicate the same functions. In this embodiment, one line of data will be explained as 16 bits. The print data is stored in the buffer memory 11 as explained in FIG.
), one line of data is once stored in the register 9, and then read out, sent to the shift register 9, sequentially shifted by a clock, and stored in the shift register 9.

At this time, the hexadecimal counter l5 is activated by the start signal.
Count the logical "0" bits in the print data.

Then, when the counted number of logical “0” bits is 1 bit, the counter 15 turns on ■ and turns the logical “1” into A.
The signal is sent to the ND circuit 16. Therefore, when the strobe signal is input from the terminal STB, the AND circuit 16 has a logic "1".
” to the transistor 20, the transistor 2
0 is turned on, and current flows from the power supply Vl through the resistor 24.

At this time, as explained in FIG. 3, the data stored in the shift register 9 is latched by the latch circuit 8, and the strobe signal causes a current to be applied to the heating element of the thermal head corresponding to the logical "L" bit. flows.

Here, assuming that the average resistance value of one heating element is R, the resistance value of the resistor 24 is R. The fact that the number of logical "0" bits counted by the counter 15 is 1 bit means that 15 bits are printed, so current flows through 15 heating elements, and the resistance 24 are connected in parallel to the heating elements, 16 heating elements are connected to the power supply VD
It will be the same as if it were connected to. Therefore, the apparent resistance value from the power source (2) is R/16, which is the same resistance value as when supplying current to 16 heating elements.

When the number of logical “0” bits counted by the counter 15 is 2 bits, turn on ■ and output the logical “1” to the AND circuit 1.
Send to 7. Therefore, when the AND circuit 17 receives the strobe signal from the terminal STB, it sends a logic "1" to the transistor 21, so the transistor 21 is turned on and current flows from the power supply VD through the resistor 25.

The resistance value of this resistor 25 is R/2. That is, since the logic "O" counted by the counter 15 is 2 bits, 13
This shows that bits are being printed, and at this time, current flows through 13 heating elements. Therefore, when the resistance values of two heating elements are connected in parallel, 16 heating elements become the power source ■. It will be the same as if it were connected to. Therefore, by connecting the resistor 25 in parallel to the heating element, the power supply ■. The apparent resistance value is R/16, which is the same resistance value as when current is supplied to 16 heating elements.

When the number of logic "O" bits counted by the counter 15 is 3 bits, the logic "1" is sent to the AND circuits 16 and 17 by turning on ■ and ■. Therefore, AND circuit l6 and 1
When a strobe signal is input from the terminal STB, the transistors 20 and 21 are turned on, and the transistor 20 is connected from the power supply VD through the resistor 24 to the resistor 25. Current flows through the transistors 2l through the respective transistors 2l.

Since the logic "O" counted by the counter l5 is 3 bits, this indicates that 12 bits are printed, and at this time, current flows through 12 heating elements. Therefore, if the resistance values of three heating elements are connected in parallel, it will be the same as if 16 heating elements were connected to the power supply VD. Therefore, resistors 24 and 2
5 are connected in parallel to the heating elements, the apparent resistance value from the power source VD becomes R/16, which is the same resistance value as when supplying current to 16 heating elements.

When the number of bits of the logic "θ' counted by the counter l5 is 4 bits, the AND circuit 1 turns on ■ and outputs the logic "1".
Send to 8. Therefore, when the AND circuit 18 receives a strobe signal from the terminal STB, it sends a logic "1" to the transistor 22, so the transistor 22 is turned on and current flows from the power supply vI through the resistor 26.

The resistance value of this resistor 26 is R/4. That is, since the logic "On" counted by the counter 15 is 4 bits, 12
This shows that bits are being printed, and at this time, current flows through 12 heating elements. Therefore, if the resistance values of four heating elements are connected in parallel, it will be the same as if 16 heating elements were connected to the power supply VD. Therefore, by connecting the resistor 26 in parallel to the heating element, the power supply ■. The apparent resistance value is R/16, which is the same resistance value as when supplying current to 16 heating elements.

When the number of logic "0" bits counted by the counter 15 is 5 bits, the logic "1" is sent to the AND circuits 16 and 18 by turning on the circuits 1 and 2. Therefore, AND circuit 16 and 1
When the strobe signal is input from the terminal STB, the transistor 8 sends a logic "1" to the transistors 20 and 22, so the transistors 2o and 22 are turned on, and the power supply is turned on. Current flows from the resistor 24 to the transistor 20 and the resistor 26 to the transistor 22, respectively.

Since the logical "0" counted by the counter 15 is 5 bits, this indicates that 11 bits are printed, and at this time, current flows through 11 heating elements. Therefore, if the resistance values of 5 heating elements are connected in parallel, 16 heating elements will be
It is the same as if it were connected to 0. Therefore, resistors 24 and 26
are connected in parallel to the heating elements, so that the apparent resistance value from the power source VD becomes R/16, which is the same resistance value as when supplying current to 16 heating elements.

When the number of logic "0" bits counted by the counter 15 is 6 bits, 2 and 3 are turned on and a logic 1 is sent to the AND circuits 17 and 18. Therefore, AND circuit 17 and 1
When the strobe signal is input from the terminal STB, the transistors 21 and 22 are turned on, and the resistor 8 is connected to the transistor 21 through the resistor 25 from the power supply ■. A current flows through each transistor 22 via 26.

Since the logic "0" counted by the counter 15 is 6 bits, this indicates that 11) bits are printed, and at this time, current flows through 11) heating elements. Therefore, the heating element 6
When the resistance values of 16 heating elements are connected in parallel, it becomes the same as if 16 heating elements were connected to the power supply v8. Therefore, by connecting resistors 25 and 26 in parallel to the heating element, the apparent resistance value from the electric current Bvo becomes R/16, which is the same resistance value as when supplying current to 16 heating elements. Become.

When the number of logical “0” bits counted by the counter 15 is 7 pits, turn on ■, ■, and ■, and set logical “1” to AN.
It is sent to D circuits 16, 17 and l8.

Therefore, when the strobe signal is input from the terminal STB, the AND circuits 16, 17, and 18 send out a logic "1" to the transistors 20, 21, and 22.
0, 21, and 22 are turned on, and the resistor 24 is turned on from the power supply V.
Current flows through the transistor 20 through the resistor 25, the transistor 21 through the resistor 26, and the transistor 22 through the resistor 26, respectively.

Since the logic "0" counted by the counter 15 is 7 bits, this indicates that 9 bits are printed, and at this time, current flows through 9 heating elements. Therefore, when the resistance values of seven heating elements are connected in parallel, it becomes the same as if 16 heating elements were connected to the power supply va.

Therefore, by connecting the resistors 24, 25, and 26 in parallel to the heating element, the apparent resistance value from the power source VD is R/1.
6, which is the same resistance value as when supplying current to 16 heating elements. When the number of logic "O" bits counted by the counter 15 is 8 bits, turn on ■ and apply logic "l" to the AND circuit 1.
Send on 9th. Therefore, when the AND circuit 19 receives a strobe signal from the terminal STB, it sends a logic "1" to the transistor 23, so the transistor 23 is turned on and current flows from the power supply V through the resistor 27.

The resistance value of this resistor 27 is R/8. That is, since the logic "O" counted by the counter 15 is 8 bits, it indicates that 8 bits are printed, and at this time, the heating element is 8 bits.
Current flows through each individual. Therefore, if the resistance values of 8 heating elements are connected in parallel, 16 heating elements become the power source ■. It will be the same as if it were connected to.

Therefore, by connecting the resistor 26 in parallel to the heating element,
Power supply V. The apparent resistance value from is R/16,
The resistance value is the same as when current is supplied to 16 heating elements.

Counter l5 counts the number of logical “O” bits by 9 bits, turns on ■ and ■, and increases the number of logical “0” bits to 11.
) When counting bits, turn on ■ and ■ and set logic “O”.
When counting the number of bits of 11 bits, turn on ■ and ■, and when counting the number of bits of logic "0" by 12 bits, turn on ■ and ■, and set the number of bits of logic "0" to 13 bits. When counting bits, turn on ■, ■, and ■. When counting the number of logical "O" bits, turn on ■, ■, and ■, and count the number of logical "0" bits by 15 bits. When this happens, turn on ■, ■, ■, and ■. Therefore, in any of the above cases, the power supply ■. The apparent resistance value is R/16, which is the same resistance value as when current is supplied to 16 heating elements, but the reason is the same as described above, and detailed explanation will be omitted.

When the number of logical "O" bits is counted as 16 bits, (1) to (2) remain off.

When the counter 15 receives a reset signal, it resets the counting result, and when the next start signal is received, it counts the number of logic "0" bits in the print data stored in the buffer memory 11).

〔Effect of the invention〕

As explained above, in the present invention, when current is applied from the power supply to each heating element of the thermal head for printing, the load resistance value is kept constant, so there is no voltage fluctuation and the energy supplied to the heating elements is constant. , it is possible to prevent the occurrence of density unevenness. Furthermore, since the current value sent out by the power source is constant, the life of the power source can be extended.

[Brief explanation of the drawing]

FIG. 1 is a block diagram illustrating the principle of the present invention, FIG. 2 is a block diagram of a circuit illustrating an embodiment of the present invention, and FIG. 3 is a block diagram illustrating an example of the prior art. In the figure, l~7, 16~19 are AND circuits, 8 is a latch circuit, 9 is a shift register, 11) is a buffer memory, l1 is a thermal head, 12 is a counting means, l3 is an impedance selection means, 15 is a counter, 20-23. Q, ~Q, are transistors 24 to 27
is a resistor, and R and ~R7 are heating elements. of

Claims (1)

[Scope of Claims] N heating elements are arranged in a line, and a current is supplied from a power source to the heating element that prints dots corresponding to data among the N heating elements to generate heat based on a printing signal. This makes it possible to print multiple dots on paper simultaneously using a thermal head (1
1), a counting means (12) for counting the number of dots to be simultaneously printed in the data to be printed by the thermal head (11);
and impedance selection means (1) for selecting an impedance connected to the power source in parallel with the thermal head (11) in accordance with the number counted by the counting means (12).
3), and when printing a single dot or a plurality of dots, the power source is configured to send out the same current as when supplying current to N heating elements. .