Recording head driving device

Abstract: Disclosed herein is a recording head driving device for a thermal printer comprising heating elements (7) energizable over a time interval, for driving a number of recording heads such as thermal heads (6, 7) or the like for printing characters on recording media. A control signal for controlling the time required to energize each of the recording heads (6, 7) is input to recording head driving gate circuits (5) from NAND gates (5) in accordance with the present and/or past recorded information output from a latch circuit (8). A plurality of collating circuits (10) is provided each of which allows each of the latch circuits (8) to retain or hold recorded information on at least past three lines as the previous recorded information and feeds back the past latch outputs (Q6, Q7, Q8) of the latch circuit (8) to any one (Q3) of the outputs of the latch circuit (8).

Description: The present invention relates to a driving device which serves as a printing unit for printing characters on a recording medium, and more specifically to a recording head driving device for driving a thermal head used for printing of a facsimile system, a printer, etc. The European Patent Application EP-A 0 304 916 discloses a control circuit for a printing head in which history data are stored in a shift register over a plurality of cycles. A logic circuit is provided for performing logic operations making use of the history data for the present and previous lines. The current supplied in a given printing cycle for a printing dot is determined by previous energization of that dot as well as adjacent dots on previous cycles. FIG. 1 is a circuit diagram showing a conventional one-dot type thermal head driving circuit which has been illustrated in a catalogue (as entitled "Thermal Head, H-C9683-E" described in P25 and issued on Feb., 1991) produced by Mitsubishi Electric Corp. Thermal heads are arranged in such a manner that the thermal head driving circuit is provided with a predetermined number of dots. In the same drawing, reference numeral 1 indicates a shift register for shifting input data on the present line in accordance with a clock. The shift register 1 has steps corresponding to the number of dots relative to the thermal heads. Designated at numeral 21 is a latch circuit for taking in data which appears at a tap of the shift register 1 so as to retain or hold it therein. Reference numeral 31 indicates a gate signal generating unit for generating three gate signals GA, GB, GC. Designated at numerals 4a, 4b are reverse logical product (hereafter called "NAND") gates serving as gate circuits supplied with latch outputs Q2, Q3 from the latch circuit 21 and gate signals GB, GC from the gate signal generating unit 31. Reference numeral 51 indicates a logical product (hereafter called "AND") gate serving as a gate circuit supplied with the outputs of the NAND gates 4a, 4b, the Q1 output of the latch circuit 21 and the gate signal GA of the gate signal generating unit 31 so as to output a pulse signal indicative of a conductible or energizable state thereof. Designated at numeral 6 is a Darlington transistor serving as a drive circuit for driving or energizing a thermal or heating resistor 7 of a thermal head in response to the pulse signal output from the AND gate 51. The operation of the thermal head driving circuit will now be described below. FIG. 2 is a timing chart for describing the relationship in time among respective signals. The shift register 1 first takes in data shown in FIG. 2(B) as an image signal in response to a clock signal shown in FIG. 2(A) and shifts it to a desired location. The latch circuit 21 successively takes in data from the tap of the shift register 1 corresponding to a dot thereof in response to a latch signal shown in FIG. 2(C). At this time, the latch circuit 21 brings data from the shift register 1 in response to the input latch signal and shifts the latched data one stage. As a result, data on the previous line relative to the given dot appears at the Q2 terminal of the latch circuit 21, whereas data on the line prior to the previous line relative to the given dot appears at the Q3 terminal. On the other hand, the gate signal generating unit 31 generates the gate signals GA, GB, GC represented in the form of given patterns as illustrated in FIGS. 2(D), 2(E) and 2(F). The pulse signal to be sent to the heating resistor 7 is determined by the gate signals GA, GB, GC, the outputs Q1, Q2, Q3 of the latch circuit 21, the NAND gates 4a, 4b and the AND gate 51. The Darlington transistor 6 drives the heating resistor 7 in response to the signal delivered from the AND gate 51 so as to cause the heating resistor 7 to generate heat in proportion to the amount of current which flows into the heating resistor 7 driven by the Darlington transistor 6, thereby subjecting a thermal recording paper located on the heating resistor 7 to colour development. A description will now be made of history control of the amount of current supplied to the heating resistor 7. When the time required for the Darlington transistor 6 to cause the heating resistor 7 to conduct current, i.e., energize the heating resistor 7 as shown in FIG. 3(A) is 1 ms, the temperature of the heating resistor 7 reaches 300°C. When the energization of the heating resistor 7 is repeated in a period corresponding to 2 ms as shown in FIG. 3(B), the heating resistor 7 increases up to a temperature of 500°C. Thus, even if the same amount of current is provided, the temperature of the heating resistor 7 at the time of completion of the energization is also high when the temperature of the heating resistor 7 at the start of the energization is high. That is, a color-developed density becomes high upon energization of the heating resistor 7 in a quick repeating cycle unless the energy supplied to the heating resistor 7 is controlled. It is therefore necessary to control the amount of energy depending on the temperature of the heating resistor at the start of its energization. More specifically, the control for the energization of the heating resistor is performed based on a decision made as to whether or not desired data has been recorded at the line prior to the previous line. This history control is carried out in the following manner. It is necessary to recognize the degree of an increase in temperature with respect to patterns (recorded conditions of dots at the present line, the previous line and the line prior to the previous line) in order to determine in what manner the energy should be supplied to a dot at the present line judging from the recorded conditions of the dots at the previous line and the line prior to the previous line, i.e., the energization with respect to its dot should be done. FIG. 4 is a simplified graph showing the result of simulated increases in temperature with respect to the respective patterns upon non-performance of the history control. In the same drawing, "H" represents that the recording (energization) of dots has been made, whereas "L" represents that the recording (energization) of the dots has not been done. For example, FIG. 4(B) shows that the recording of the dot has been made at the line prior to the previous line and the recording of the dot has not been made at the previous line. In addition, values (each of which represents the degree of increase in temperature, but is now called a point number) obtained by normalizing respective temperatures at the time that the energization has been completed at the present line are shown in FIG. 4. It is understood that the history control should be done in such a manner as to provide large energy when the point number is low such as "1.0" [see FIG. 4(A)]. Also, a small amount of energy should be provided when the point number is as high such as "3.0" as is shown in FIG. 4(D). FIG. 5 is a view showing the relationship between the point numbers shown in FIG. 4 and the data Q1, Q2, Q3 latched in the latch circuit 21. As has already been described above, the latch data Q1, Q2, Q3 represent criteria as to whether or not the dots are recorded at the line prior to the previous line, the previous line and the present line. Now, the number of levels is defined depending on the number of "H". The more the number of "H" occurs in a pattern, the more the number of levels becomes high. The most suitable energized states corresponding to four kinds of patterns shown in FIG. 5 are represented by FIGS. 2(G) to 2(J). In order to establish a suitable current corresponding to the point numbers, the gate signal generating unit 31 generates the gate signals GA, GB and GC shown in FIGS. 2(D), 2(E) and 2(F). As a result, the outputs of the AND gate 51 corresponding to the output patterns of the latch circuit 21 are represented by FIGS. 2(G) to 2(J), and hence the amount of current associated with the point numbers is set. That is, the pattern (L, L, H) representative of the low point number is controlled in such a manner that the amount of current increases. The patterns indicative of the large point numbers are controlled such that the amount of current is reduced. Incidentally, the pulse widths of the gate signals GB, GC are identical to each other. In the case of two patterns in the same level, i.e., in the level 2, the energizing time for one of the two patterns and that for the other are identical in total to each other. Incidentally, techniques related to the conventional thermal head driving circuit have been disclosed as references in Japanese Patent Application Laid-Open Nos. 63-203346, 64-32973 and 64-67365, for example. The conventional thermal head driving circuit has been constructed as described above. It is therefore necessary to increase the number of the outputs of the latch circuit 21 when the history control is strictly performed. Thus, the number of patterns to be controlled increases, thereby causing a difficulty in suitably controlling the patterns. Further, when the respective heating resistors provided adjacent to one another are independently controlled, no attention has been paid to the influence of storage of heat generated between the adjacent respective heating resistors. Accordingly, the control of heat history cannot be performed with high accuracy. An object of the present invention is to provide a recording head driving device capable of suitably realizing history control with less gate signals, even if the number of patterns to be controlled increases as a result of an increase in the number of outputs of a latch circuit. According to the present invention, a driving device for a thermal printer is provided as defined in claim 1. A number of collating circuits allow the latch circuit to retain the recorded information on at least three previous lines; and logically combine the past latch outputs of the latch circuit with any one other of the outputs of the latch circuit. It is possible to control the amount of heat generated by each heating resistor which has referred to the past recorded information. It is thus possible to realize a recording head driving device capable of suitably carrying out history control with a smaller number of gate signals. The above and other objects and advantages of the present invention will become apparent from the following description of embodiments, taken in conjunction with the accompanying drawings. FIG. 1 is a block diagram showing a conventional thermal head driving circuit; FIG. 2 is a timing chart for describing the operation of the thermal head driving circuit shown in FIG. 1; FIG. 3 is a graph for describing the relationship between a pulse applied to each of the thermal resistors employed in the conventional thermal head driving device and the temperature of the thermal resistor; FIG. 4 is a simplified view for describing increases in temperature relative to four kinds of latch patterns output from the latch circuits in the conventional thermal head driving device; FIG. 5 is a view for describing the relationship between latch data representative of the four kinds of latch patterns output from the latch circuits in the conventional thermal head driving device and point numbers relative to increases in temperatures; FIG. 6 is a circuit diagram showing a recording head driving device according to an embodiment of the present invention; FIG. 7 is a view for describing the relationship between latch data indicative of five kinds of patterns output from latch circuits and points numbers relative to increases in temperatures; and FIG. 8 is a view showing one example of a bar-code pattern for describing the operation of the recording head driving device shown in FIG. 6. Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. FIG. 6 is a block diagram showing a thermal head driving circuit according to an embodiment of the present invention. FIG. 6 includes a shift register 1, NAND gates 4 serving as a gate circuit, an AND gate 5 serving as a gate circuit, a darlington transistor 6 serving as a drive circuit and a thermal or heating resistor 7. These components are identical or similar to those shown in FIG. 1 to which the same reference numerals have been applied, and their detailed description will therefore be omitted. Designated at numeral 8 is a latch circuit different from that designated at numeral 21 in FIG. 1, in that recorded information on the present line and record information on the past 7 lines are retained therein. Reference numeral 9 indicates a gate signal generating unit different from that designated at numeral 31 in FIG. 1 in that gate signals GD and GE are generated in addition to the gate signals GA through GC. Designated at numeral 10 is a collating circuit for supplying the past latch outputs Q6, Q7 and Q8 of the latch circuit 8 to the Q3 output terminal of the latch circuit 8. Designated at numeral 11 in the collating circuit 10 is an AND gate supplied with the latch outputs from Q6 to Q8. Reference numeral 12 indicates an OR gate which performs a logical sum (hereinafter called "OR") operation on the output of the AND gate 11 and the Q3 latch output. The operation of the present embodiment will now be described below. The latch circuit 8 takes in data indicative of recorded information from the shift register 1 in response to a latch signal in a manner similar to the conventional latch circuit 21. In this case, the latch circuit 8 is an eight-stage configuration. Therefore, the recorded information held one line before the present line appears at the Q2 terminal, the recorded information held two lines before appears at the Q3 terminal, the recorded information held three lines before appears at the Q4 terminal, etc. When the patterns to be controlled are of the four kinds as in the conventional example (see FIG. 5), they are controlled by the three kinds of gate signals GA through GC as illustrated in FIGS. 2(D) through 2(J). That is, when a pattern is represented by (H, L, H) as shown in FIG. 2(H), it is controlled by the gate signals GA, GB. When, on the other hand, a pattern is represented by (L, H, H), it is controlled by the gate signals GA, GB, GC. By so doing, the energization of each line can be easily carried out. However, when the control for energizing the present line with respect to the corresponding dot is performed in consideration of control information on the past four lines, the patterns to be controlled increase up to 16 kinds as illustrated in FIG. 7. When the patterns as the objects to be controlled are classified into sixteen kinds, the control for energization of each line can be simply carried out so long as the five kinds of gate signals GA to GE are present. However, when many kinds of patterns are further used and have to be controlled, the number of output signal lines of the gate signal generating unit 9 increases, with the result that a suitable control method cannot be provided in practice. Thus, in the present embodiment, only the past recorded information on specific patterns, which is associated with the latch outputs subsequent to the latch output Q6, is fed back to the corresponding Q3 output terminal to thereby perform the current control flow. When a bar-code pattern is used for example, it comprises five thick bars and two thin bars and is regular. Thus, the latch outputs Q1 to Q5 are identical in the recorded information to one another, whereas the latch outputs Q6 to Q8 have completely different information from one another. Accordingly, when the control for energizing each heating resistor is performed only by the latch outputs Q1 to Q5 in this case, the current for generating the same amount of heat is supplied to the heating resistor 7. Thus, in the present embodiment, the latch outputs Q6 to Q8 are supplied to the collating circuit 10. That is, the latch outputs Q6 to Q8 are collectively input to the AND gate 11, which in turn ANDs the inputs. The result of ANDing is input to the OR gate 12 and fed back to the latch output Q3. When all the latch outputs Q6, Q7, Q8 are represented as black (H) as shown in FIG. 8(A) by way of example in this condition, the output of the NAND gate 4 supplied with a signal from the OR gate 12 is turned off at all times during a period in which the gate signal GD is being output to the NAND gate 4 (i.e., it is "H" in level). By so doing, the time interval i.e. the period required to supply the energy corresponding to the amount of the generated heat can be reduced when the long and black "H" has been printed in the past and the stored amount of the generated heat has increased. When, on the other hand, at least one of the latch outputs Q6, Q7, Q8 is represented as white ("L"), the output of the AND gate 11 in the collating circuit 10 is rendered low in level, and the OR gate 12 passes through the latch output Q3 as is. Accordingly, the control for the energization of each heating resistor 7 is carried out in accordance with the patterns of the latch outputs Q1 to Q5. It should thus be apparent that the pattern shown in FIG. 8(A) provided with the continuous black bars in the past makes an increases in the storage of the generated heat as compared with the pattern shown in FIG. 8(B) provided with the continuous white bars in the past. However, the present embodiment can cope with this without increasing the number of the signals output from the gate signal generating unit 9 even in that case. The above embodiment is directed to a case in which the latch outputs Q6 to Q8 are collectively input to the AND gate 11 of the collating circuit 10 where they are collated with the latch output Q3, followed by the control for the energization of the heating resistor. However, the latch circuit 8 may be of a seven-stage configuration, and the latch outputs Q5 through Q7 from the latch circuit 8 may be collectively input to the AND gate 11 of the collating circuit 10. It is also unnecessary to regard the number of the latch outputs, input to the AND gate 11 as three inputs. Further, the output of the OR gate 12 of the collating circuit 10 may also be fed back to specific latch outputs as Q terminals more than or equal to 1 as an alternative to the latch output Q3. The above-described embodiment describes a case in which the collating circuit 10 comprises the AND gate 11 and the OR gate 12. However, the collating circuit 10 may be comprised of other logic circuits. In this case, the same advantageous effect can be obtained. It is therefore possible to suitably control the history without increasing the number of gate signals generated by a gate signal generating unit even if the number of patterns to be controlled increases.

Claims: A driving device for a thermal printer comprising heating elements (7) arranged in a line, the heating elements (7) being energizable over a time interval to produce printed dots, said device comprising: a number of latch circuits (8), each latch circuit associated with one heating element (7) and adapted to retain print information for the present line and previous lines, the information indicative of whether a dot is to be printed or not by its respective heating element (7); a gate signal generating unit (9) for generating gate signals (GA - GE) used in determining drive signals for energizing the heating elements (7); a number of gate circuits (4, 5) connected to receive said print information as latch outputs (Q1 - Q8) from said latch circuits (8) and to receive said gate signals (GA - GE) from said gate signal generating unit (9), the gate circuits (4, 5) adapted to produce drive signals defining the energizing time interval of the heating elements (7); a number of drive circuits (6) for driving the heating elements (7) in response to the drive signals from said gate circuits (4, 5); characterized in that a collating circuit (10) is provided for each heating element (7), the collating circuit (10) connected to receive latch outputs (Q6, Q7, Q8) representing print information on at least three previous lines from said latch circuit (8) and to logically combine the latch outputs (Q6, Q7, Q8) with any one other latch output (Q1 - Q4) of said latch circuit (8). The device according to Claim 1, wherein the gate circuits (4, 5) comprise a plurality of NAND gates (4) connected to the gate signal generating unit (9) and the latch circuit (8) and an AND gate (5) is connected to output said drive signals to the respective drive circuits (6). The device according to Claim 1 or 2, wherein the collating circuit (10) comprises an AND gate (11) connected to receive said at least three latch outputs (Q6, Q7, Q8) of the latch circuit (8) and an OR gate (12) connected to the output of the AND gate (11) and to said any other latch output (Q3) of the latch circuit (8). The device according to Claim 3, wherein the OR gate (12) is connected to receive one or more other outputs (Q3) of the latch circuit (8).

Inventors: ENDO TAKAFUMI, ENDO, TAKAFUMI, Endo, Takafumi, Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: B41J2
IPC codes: B41J2