Thermal recording device
BACKGROUND OF THE INVENTION 
     This invention relates to a thermal recording device. 
     Hitherto, in thermal recording devices for performing recording by the use of heating elements arranged so as to cover entirely the width of a recording paper, a so-called density gradation process is generally employed to perform multi-grade recording in which the density of each dot itself is varied. 
     According to the foregoing process, each picture element is recorded by the use of one dot, so that although the number of heating elements per unit length can be set small, a variation of print density in an intermediate density range is large and hence the number of grades cannot be made larger than 16. 
     Further, the heating elements are divided into a plurality of blocks and driven in a mode of time-sharing. Since the recording of one line is commenced after the print data of the respective picture elements are always written in a shift register through its least significant bit position, the data must be transferred through non-recording blocks up to a block which has to effect recording. Consequently, the data transfer takes much time and a recording speed is decreased. 
     The objects of the present invention are to enhance a data transfer speed and to realize a variety of gradation. 
     SUMMARY OF THE INVENTION 
     The present invention resides in a thermal recording device of the type wherein each picture element is configured by dots arranged in N rows and M columns (N, M: integer not smaller than 2) and a variety of gradation is realized by means of a dot pattern and the density of each dot itself, which is characterized in that heating elements are divided into a plurality of blocks and data are directly distributed to the respective blocks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram showing an embodiment of the configuration of picture elements according to the present invention, 
     FIGS. 2A, 2B and 2C are an explanatory diagram showing an embodiment of dot patterns for determining a variety of gradation, 
     FIG. 3 is a block diagram showing an embodiment of a circuit configuration, 
     FIGS. 4A, 4B and 4C are a flowchart explanatory of the operation, and 
     FIG. 5 is a time chart. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment illustrated, 2560 heating elements are arranged in a row along a line to form a thermal line-printing head so as to cover entirely the width of a recording paper, these heating elements are divided into eight thermal head blocks, each consisting of 320 heating elements, and the heating elements of each block are driven in a of time-sharing made to mark one dot line or one row of dots composed of 2560 dots on the recording paper during one operation of the thermal line-printing head. 
     FIG. 1 shows four dot lines obtained by line-sequentially and successively driving the thermal-printing head four times in the direction transverse to the row of dots D 1  through D 640  designate picture elements or image segments in the form of dot matrixes, each picture element is formed of sixteen dots d, . . . , d arranged in four rows and four columns, and the gradation of each picture element can be varied stepwise within the range of 64 grades, as shown in FIG. 2A, by means of its dot pattern (the number of marked dots included in one picture element) and the density of marked dots (four grades: Density 1 through Density 4). Specifically, a variety of the gradation is achieved by forming the respective dots in the order shown in FIG. 2B and by assigning to each dot one of four-graded densities ranging from Density 1 to Density 4. 
     A practical recording or printing process will now be described. In FIG. 3, 1 designates a RAM for storing the image gradation information of picture elements to be recorded, 2 designates a ROM which stores gradation data relating to the dot patterns of the picture elements and the densities of the dots effective to determine the picture element gradation, and 3 designates a latch circuit for properly reading out the gradation data from the ROM 2 on the basis of data bed from a CPU 4. 5 designates a parallel-serial converter circuit for converting the parallel data read out from the ROM 2 into a control signal of a bit-serial form. 6 designates a data separator for directly distributing data fed from the parallel-serial converter circuit 5 to eight thermal head blocks 1B through 8B Each thermal head block is composed of a shift register SR of 320 bits, a latch circuit L of 320 bits, gate circuits G, . . . , G, and 320 heating elements t 1  through t 320 . 7 designates a thermistor provided in the vicinity of the heating elements for controlling a time duration of energization depending upon a temperature. 8 designates an I/O port, and 9 designates a ROM storing a table of temperature-dependent energization duration. 10 designates a clock pulse generator, and 11 designates a counter for specifying an address of the RAM 1. 
     The operation of the foregoing structure will now be described with reference to FIGS. 4A, 4B, 4C and 5. At first, under the control of the CPU 4, the image gradation information of a first picture element line composed of four successive dot lines and containing 640 picture elements is fetched into the RAM 1. Then, the total number L of picture element lines to be recorded is set and the number l=4 of dot lines of one picture element line is also set. Subsequently, by means of pulses SB 1  shown in FIG. 5, the first thermal head block 1B is selected. Namely, the gate circuits G, . . . , G of the thermal head block 1B shown in FIG. 3 are opened. In addition, by means of the data separator 6 shown in FIG. 3, the block 1B is selected to receive from the parallel-serial converter circuit 5. 
     Then, by means of the output of the counter 11, the gradation information of the first picture element is read out from the RAM 1, and gradation data representative of a dot pattern stored in the ROM 2 is specified or addressed according to the image gradation information. 1st row dot line, of the thus specified dot pattern which is composed of a 4×4 matrix is selected by means of address inputs A8 and A9 fed from the latch circuit 3, and by means of address inputs A6 and A7, the repetition number of operation to be effected in the thus selected first row dot line is specified. That is, the first row dot line is selected repeatedly four times by the address inputs A6 and A7, so that the first row dot line is subjected to an energization duration of four grades and hence each dot of the first row dot line has a four-graded density. 
     By means of the outputs of the RAM 1 and the latch circuit 3, the first dot data (four-bit) of the first dot line is read out from the ROM 2, then, the second dot data of the first dot line is read out, and subsequently and successively, the third dot data through 320th dot data of the first dot line are read out. The thus read-out dot data are converted by the converter circuit 5 into a serial form, and are written in the shift register SR of the thermal head block 1B via the data separator 6. This operation corresponds to Data Transfer of 1st time (a) shown in the flowchart of FIG. 4A. Upon completion of this transfer, the data are latched in the latch circuit L of the first head block B1 by means of pulse P 1  shown in FIG. 5, and the 1st time energization of the heating elements t 1  -t 320  is selectively carried out according to the data latched in the latch circuit L to mark dots having the Density 1. During this energization, the data of 2nd time of the first dot line are read out from the ROM 2 and written in the shift register SR of the first head block B1 (FIG. 4A, b) in a similar manner to Data Transfer of 1st time. 
     As the energization duration of Density 1 based on the data of 1st time terminates, the data of 2nd time are latched in the latch circuit L by means of pulse P 2  shown in FIG. 5. At this moment, marking of the dots of Density 1 in the first dot line terminates, and hereafter, only the dots of Density 2 and higher densities are successively marked on the recording paper. During the 2nd time energization, the data of 3rd time are transferred to the shift register SR (FIG. 4B, c). As the energization of Density 2 terminates during the assigned duration, the data of 3rd time are latched in the latch circuit L by means of pulse P 3  shown in FIG. 5. At this moment, marking of the dots of Density 2 in the first dot line terminates, and subsequently, the dots of Density 3 are marked on the recording paper. 
     During the foregoing energization, the data of 4th time are transferred to the shift register SR (FIG. 4B, d). As the energization duration of Density 3 terminates, the data of 4th time are latched in the latch circuit L by means of pulse P 4  shown in FIG. 5. 
     At this point, the judgment (FIG. 4B, e) is performed as to if all of the eight head blocks have been processed to complete the marking of the four-graded first dot line, and since the answer is NO at this moment, by means of the data separator 6, the second head block B2 to which data are transferred is selected (FIG. 4B, f). Thus, the data of 1st time of the first dot line assigned to the second head block B2 are transferred to a register within the second thermal head block 2B (FIG. 4B, g). Then, after the energization of 4th time of the first head block B1 terminates, the gate circuits within the second thermal head block 2B are opened by means of the pulses SB 2  shown in FIG. 5, and the block to be energized is switched to the second block. 
     Hereafter, in a similar manner to the operation of the first head block B1, the recording of one part of the first dot line assigned to the second head block B2 is performed. 
     In this way, the first block through the eighth block are operated successively to complete the recording of the first dot line, and thereafter, the second dot line is specified (FIG. 4C, h). The second dot line is then recorded similarly to the recording of the first dot line, and subsequently, the third and fourth dot lines are recorded, whereby recording of the first picture element line is completed. 
     As the foregoing recording operation terminates, the number of dot lines is again set to 4 (FIG. 4C, i), and the second picture element line is selected (FIG. 4C, j) and recorded similarly to the recording of the first picture element line. Then, as recording proceeds up to the specified number L of picture element lines, all of recording terminates. 
     In the meantime, since the thermistor 7 is always monitoring the temperature of the thermal heads, an optimum pulse duration is selected from the table of energization duration stored in the ROM 9 on the basis of the monitored temperature information, and the recording operation is performed with the optimum energization duration according to this temperature. 
     Although, in the foregoing embodiment, each picture element is formed of the dots arranged in four rows and four columns, it is to be understood that a dot pattern generally consists of N rows and M columns (N, M: integer not smaller than 2). 
     Further, the number of density grades of the dot should not be limited to that mentioned hereinabove. 
     In the type of recording device wherein each picture element or image segment is configured by a matrix consisting of a dot plurality of dots and a variety of the image segment gradation is given by means of a dot pattern and the density of each dot, the present invention has improved the device by dividing the row of heating elements into a plurality of blocks and directly gradation data to the memory circuits provided in the respective blocks, whereby a variety of the gradation is realized, the information transfer speed is enhanced, and the record of high resolution can be achieved at a high speed.