JPS58162360A - Circuit for preventing burning of thermal head - Google Patents

Abstract

PURPOSE:To ensure the protection even at the time of CPU failure, by providing a time constant determining circuit, which varies the width of an output control pulse that is inversely proportional to a temperature, in a one shot multivibrator, which controls the driving time for the thermal head. CONSTITUTION:The circuit 70 for preventing the burning of the thermal head comprises an OR circuit 71, a differentiation circuit 72 which detects the rise up of the output, the one shot multivibrator circuit 74 which is started by the differentiated pulse, and the time constant determining circuit 76 which determines the width of the output pulse 75. The OR circuit 71 divides and receives an output 51 of a decoder 50, which specifies a group number to a group driver 40 based on the group address from the CPU30. The time constant varies according to a thermistor 15 in a head 10. Since the control pulse 75 is supplied to an enable terminal of the decoder 50, the actual driving time is adjusted by the output 75 of a burning preventing circuit 70 having a printing density function. In this hardware constitution, even though the faults such as uncontrolled program running occurs, the prevention of the burning is ensured.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a burnout prevention circuit for a thermal head having a print density adjustment function.

Background of the Technology In a thermal recording device that prints on thermal recording paper by selectively driving a large number of heating elements arranged in a thermal head, the temperature of the heating element, the driving time (current application time), and the ambient temperature affect the print density. do. The printing density increases as the driving time of the heating element increases and the ambient temperature rises, and vice versa, so the printing density can be appropriately adjusted by detecting the temperature and adjusting the driving time. In addition, the heating element of the head burns out due to excessive temperature rise due to prolonged energization, so it is necessary to avoid this type of accident as much as possible.

Prior Art and Problems FIG. 1 is a schematic diagram of a conventional thermal recording apparatus, mainly showing the head driving portion. The thermal head 10 is the second
As shown in the figure, a large number (mXn pieces) of heating resistors 11 are connected to one
Arrange them in columns, divide them into n groups, and select them.

The m x n heating resistors 11 print one small line of one line of characters.

In other words, if one character is represented by 5 x 7 dots and there are 20 characters in one line, one line is printed 7 times + 3 times (space between lines), and the number of dots per printing is 5 x 20 ( 1
1 large, and excluding character spaces etc.) is this (7
19.5X20 plus 2X20 for the inter-character space is equal to nxm (this was called one subline above),
Further, m is, for example, (5+2') x 2 (2 is the space between characters) for two characters (this is called one dot line), and n is 10.20.32, etc. m heating elements 11 in the same group can be driven at the same time, but heating elements in different groups are driven sequentially in l groups;
Since the printing time for a dot line is several milliseconds, printing one small line takes 10.20 times that time. 12 is an OR circuit (diode array) corresponding to each group, each of which is connected to one of the group selection terminals 13 of n(i); 14 is an OR circuit (diode array) for selecting a heating element in the group;
These heating element selection terminals are selected according to print data. The selection driver 20 in FIG. 1 selects this heating element selection terminal 14 in accordance with print data given from a CPU (central processing unit) 30. At this time, the group driver 40 selects which group should be energized. For example, one of the group selection terminals 13 and 2--n corresponding to the designated group is grounded. The decoder 50 decodes the group address from the CPU 30 and instructs the group driver 40 of the group number to be selected. With the above-described configuration, for example, the leftmost heating element 11 in FIG. 2 is driven when terminals 14 and 1 of terminals 13 are selected, and the second heating element from the left is driven when terminals 14 and 2 of terminals 13 are selected. The following description applies to this.

The CPU 30 sets the drive time of one dot line of the head 10 to, for example, 2 ms. This value was considered necessary to obtain proper print density at standard temperature. However, as described above, if the ambient temperature, particularly the temperature of the head 10, increases, the print density at 21 will become darker, and if the temperature decreases, the print density will become lighter. Therefore, conventionally, a temperature detection element such as a thermistor is provided in the head 10 to detect the head temperature, and this is fed back to the CPU 30. The pulse@adjustment circuit 60 converts temperature information (analog or log amount) from the temperature detection element into a pulse width and relays the pulse width to the CPU 30. CPU30
detects temperature changes from the length of the pulse width and
Controls the driving time of O.

However, in this system, if a problem such as a program runaway occurs in the CPU 30, the drive time for the head 10 is not controlled (this results in the inconvenience that the heating element II may be burned out in the worst case).

OBJECTS OF THE INVENTION The present invention attempts to control print density and prevent burnout of the head by using hardware rather than software processing, thereby reliably protecting the head even in the event of a CPU failure.

Structure of the Invention The present invention provides a thermal head in which a large number of heating elements are arranged in a plurality of groups, a group driver that selects a group of the heads, and a thermal head that drives the heating elements in the selected group according to print data. burnout of the thermal head of a thermal recording device equipped with a select driver to select a group to select, a decoder to instruct the group driver which group to select, and a central processing unit to provide the group address to the decoder and the print data to the select driver 20. When the prevention circuit includes a one-shot multi-circuit that controls the input time to the group driver by an output control pulse width, and an output control pulse width of the one-shot multi-circuit that is inversely proportional to the temperature of the thermal head. The present invention is characterized by comprising a constant determining circuit and a differentiating circuit that activates the one-shot multi-circuit when the decoder generates a new output. This will be explained in detail.

Embodiment of the Invention FIG. 3 is a schematic block diagram showing an embodiment of the invention. In the figure, 70 is a thermal head burnout prevention circuit independent of the CPU 30, and has a printing density adjustment function.

In this example, the circuit 60 in FIG. 1 is replaced with a circuit 70, and the other configurations are the same. The circuit 70 receives temperature information from the head 10 and the output 51 of the decoder 50, and controls the input time width of the group driver 40. driver 40
The input time width can be controlled by controlling the operable period of the decoder 50 or by controlling the opening and closing of a gate provided at the input stage of the driver 40. FIGS. 4 and 7 are specific examples of the former, and FIGS. 5 and 6 are specific examples of the latter.

The circuit 70 in FIG. 4 includes an OR circuit (diode array) 71 that branches the output 51 of the decoder 50 and takes it in, a differentiation circuit 72 that detects a change in the output (rise), and a one-shot activated by the differentiation pulse 73. It consists of a multi-circuit 74 and a CR time constant determining circuit 76 that determines the width of its output pulse 75. The time constant determined by circuit 76 is head 1
It changes depending on the resistance value of the thermistor 15 installed on the 0 side. The reason why the one-shot multi-circuit 74 is started at the time when the decoder output 51 changes is to synchronize the generation timing of the head driving time control pulse 75 with the group selection by the CPU 30. The control pulse 75 is generated every time one of the decoder outputs 51 (parallel as many as the number of groups) rises, that is, every time a group of heads 10 is selected, but the pulse width is determined by the temperature of the belly button 10 being high. The narrower the In this example, since the control pulse 75 is supplied to the enable terminal of the decoder 50, CP tJ
Even if the head 10 is always driven under the same conditions, the actual driving time is adjusted by the output cuff 5 of the circuit 70 (so that the print density is constant). Particularly effective is that the head burnout prevention circuit 70 is hardware unrelated to the CPU, so it is not affected by program runaway that may occur in the CPU 30, and it reliably eliminates the decoder output after a predetermined period of time after a group is selected. This reduces the complexity of the solution 10 in that if the CPU becomes inoperable and loses output to the decoder 50, the decoder remains in an inoperable state.
burnout is reliably prevented.

In the example shown in FIG. 5, an AND circuit (or circuit if negative logic) 77 is inserted between the decoder 5o and the group driver 40, and the opening and closing of this circuit is controlled by a control pulse 75. In this case, decoder 5o is always active, but is limited by pulse width circuit 70 to group driver 4o. The other configurations are the same as in FIG. 4. In the examples shown in FIGS. 6 and 7, the decoder output 51 is divided into an even group 51a and an odd group 51b. That is, when sequentially selecting heating element groups in the head 10, if the decoder output for the second group rises immediately after the decoder output for the first group falls (second and third, third and fourth - -------Similarly for each group), if all decoder outputs 51 are input to the same OR circuit 71 as shown in FIGS. 4 and 5, the rising edge of the decoder output will be the same as the previous falling edge. This makes it difficult to detect. Therefore, in FIGS. 6 and 7, the decoder output 51 of the even group
An OR circuit 71a for a and an OR circuit 71b for odd-numbered groups of decoder outputs 51b are provided separately, and the output columns of each OR circuit are spaced apart by one group. In this way, the rising edge could be detected reliably.

The other movements 1zo are the same as in Figures 4 and 5. FIG. 8 shows operating waveforms, with solid lines corresponding to FIG. 7 and broken lines corresponding to FIG. 6. If the decoder outputs 51a and 51b are treated together, the operating waveforms will also be as shown in FIGS. 4 and 5. Note that the differentiating circuit 72, one-shot multi-circuit 74, and CR time constant circuit 76 can be configured either linearly or digitally.

Effects of the Invention As described above, according to the present invention, the head drive time can be temperature-compensated without going through the CPU, so the print density can be kept constant, and even if the CPU malfunctions due to program runaway, etc. When the drive time at that point has elapsed, the head drive current is forcibly cut off, thereby preventing a head burnout accident.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of main parts of a conventional thermal recording device, Fig. 2 is an explanatory diagram of a thermal head, Fig. 3 is a schematic block diagram showing an embodiment of the present invention, and Figs. 4 to 7 are A block diagram showing a specific example of the drive time control and protection circuit, and FIG. 8 is an operation waveform diagram thereof. In the figure, lO is the thermal head, ]l is the heating element, 20 is the select driver, 30 is the CPU, 40 is the group driver, 50 is the decoder, 70 is the thermal head burnout prevention circuit, 72 is the differential circuit, and 74 is the one-shot multi circuit,
76 time constant determining circuit. Applicant Fujitsu Ltd. Representative Patent Attorney Minoru Aoyagi 1 H-J Figure 8

Claims (1)

[Claims] A number head in which a large number of heating elements are arranged in a plurality of groups, a group driver that selects a group of the heads, a select driver that drives the heating elements in the selected group according to print data, and a decoder that instructs a group driver to select a group; and a decoder that instructs a group address to a group driver;
A thermal head burnout prevention circuit for a thermal recording device comprising: a central processing unit that supplies the print data to the thermal head; a time constant determining circuit that changes the output control pulse width of the one-shot multi-circuit in inverse proportion to the temperature of the decoder, and a differentiating circuit that activates the one-shot multi-circuit when the decoder generates a new output. characterized by becoming,
Thermal head burnout prevention circuit with print density adjustment function.