Double Speed Dot Matrix Printhead

The present invention relates to an apparatus and method for printing a dot matrix of characters on a record member and, more particularly, to an apparatus and method for substantially increasing the speed of such printing without reduction in the quality of the printed characters.

In a printer for printing a dot matrix on a record member, print or output ends of wires are held in a printhead in a fixed array. The printhead is fixed to a carriage which typically moves within a limited range along a track to successive printing stations. At each printing station, a predetermined number of wires are actuated to strike the record member through an inking ribbon to form a portion of a dot matrix of a character on the member. To actuate the print wires, electrical signals are generated to energize a determined number of electromagnets which control hammers or "clappers" which propel the wires to move them towards the record member. Both the wires and their electromagnets are embodied in the printhead, known as a matrix printhead, which typically has a single column of, for example, seven or nine wires, facing the record member.

It is, of course, desirable to design a wire printing system which can form characters as quickly as possible. In the present state of the art, the frequency of impact of a wire on a record member is about 1,000 impacts per second, which produces about 200 characters per second. This operating speed is limited by the minimum interval allowable between successive impacts of each wire, with such minimum interval being limited for a number of reasons. First, a sufficiently efficient electromagnetic material is not presently available to move the clapper fast enough to propel its associated wire to cause an impact rate significantly higher than about 1,000 cycles per second. Second, even if such material were presently available, the dynamics of the wire and clapper would impose upon the design of the printhead severe constraints such as allowable settling time, material strengths and wear characteristics of the wire and clapper. Alternately, to generate a strong magnetic field without an efficient magnetic material would impose severe restrictions on the design of the drive circuit for the clapper, power consumption, reliability and the economics of the printing system.

One system for increasing the rate of character formation includes operating the printhead wires at the present limit of about 1,000 impacts per second, but which provides two separate printheads, each printing half a line, thereby doubling the rate of character formation. In such a system, the two printheads, each having a single column of wires and movable together, are operated simultaneously so that while one printhead is printing the first half of a line, the other printhead is printing the second half of the line. This two printhead system has a number of disadvantages as will now be described.

In many instances, a dot matrix printer may not be printing a full line on a record member such as a sheet of paper. In this case, in the two printhead system the left printhead, for example, will be performing more work than the right printhead, the percentage of more work depending on the percentage of a full line being printed. If only half a line is to be printed, the right printhead will move along the record member, but will not perform any printing. Consequently, it is not unlikely that the left printhead will wear out much faster than the right printhead, thereby requiring significant time and cost in providing a new printhead and readjusting the machine for this new head. Also the use of two separate printheads requires additional manufacturing cost for hardware and additional physical space on the dot matrix printer, thereby adding additional design constraints. Perhaps even more importantly, the throughput or character formation of this machine when printing, for example, half a line, is still only that of the single head machine described above.

Furthermore, with a two printhead printer, line buffering is necessary in order to print a full line. That is, data incoming to the dot matrix printer and representing a full line of characters must be stored in two separate half-line buffers before the printer can be activated to print such a line. This is because the data for the first part of the left half of the line must be sent to the left printhead when data for the first part of the right half of the line is sent to the right printhead since the two printheads must be activated simultaneously to print both halves of the line simultaneously. Also, the dot matrix printer having the two separate printheads is relatively more difficult to control when it is responding to character at a time key-board data entry. To key in a whole line of information, the left printhead moves right to record the left half of the line, then the two printheads are returned to their original left position, printhead selection is then switched to the right printhead, and then keyed in data will be printed on the right half of the line with the right printhead.

In addition, in a dot matrix printer, the gap between the ends of the printhead wires and the record member must be accurately adjusted to achieve suitable print density. That is, this gap must be adjusted so that the wires will strike the record member through the inking ribbon with appropriate force to place a pronounced dark dot on the member. When two separate printheads are used, their gaps should be precisely the same; otherwise, for example, the left printhead might print characters which are darker than those printed by the right printhead, resulting in an overall appearance which is not pleasing. Since these gaps may be, for example, as small as 0.014 inches with tolerances of only .+-. 0.001 inches, it is oftentimes difficult to match accurately the gap distances of both printheads.

Yet another disadvantage of the dot matrix printer having two printheads is that the width of the record member on which it can print is limited if advantage is to be taken of the increased rate of character formation which such printer can provide. If a sufficiently wide paper is not used, then, for example, only the left printhead will be useful while the right printhead will not be energized. Thus, again the rate of character formation of the two printhead machine will be reduced, for example, to that of a single printhead machine described above.

Furthermore, as indicated above, the two separate printheads must be operated simultaneously to print respectively one half of a line. This means that the wires in both printheads will be energized simultaneously, thereby increasing the peak power requirements of the machine.

It is an object of the present invention to provide a novel apparatus and method for printing characters on a record member.

It is another object of the present invention to provide an apparatus and method which overcome the above-mentioned disadvantages of a two printhead dot matrix printer.

It is yet another object of the present invention to provide a novel dot matrix printer and method of operating the same.

A still further object of the present invention is to increase significantly the operating speed of a dot matrix printer without reducing the quality of the printed matter produced by relatively low speed printers.

These and other objects of the invention are obtained with a single printhead having a predetermined number of print wires arranged in two spaced columns and energizers for the wires. Each column has, for example, nine wires, and the wires in a column are moved to impact only alternate dot columns on the record member. The wires in one column are energized to print only odd-numbered dot columns on the record member, while the wires in the other column are energized to print only even-numbered dot columns on the recording member. While the minimum interval between impacts of wires in a given printhead column is the same as in the present state of the art, for example 1,000/second, the two columns of wires reduces by one half the minimum required interval for printing two adjacent dot columns on the record member, thereby doubling the rate of character formation.

In one embodiment the columns of wires are spaced apart a distance equal to an even number multiple of the space between two adjacent dot columns on the record member, center-to-center. In this embodiment, the wires of the respective columns are energized alternately to print, respectively, odd and even dot columns on the record member. In an alternative embodiment, the two multi-wire columns are spaced apart a distance equal to an odd number multiple of the space between two adjacent dot columns, center-to-center. In this latter embodiment, the wires of the respective columns are energized simultaneously to print, respectively, odd and even dot columns on the record member.

FIG. 1 is a perspective view of the single printhead of the present invention.

FIG. 2 is a cross-sectional view taken through lines 2--2 of FIG. 1.

FIG. 3 is a bottom plan view of an armature retainer taken along lines 3--3 of FIG. 2 with one armature mounted therein.

FIG. 4 is an end elevation showing print wires and taken along lines 4--4 of FIG. 1.

FIG. 5 illustrates dot columns on a record member.

FIG. 6 is a schematic diagram of the electrical circuitry for energizing print wires in the printhead of FIG. 1.

FIG. 7 is an alternative embodiment of dot columns on a record member.

With reference to FIGS. 1-3, there is shown a single printhead 10 comprising an odd side assembly 12 and an even side assembly 14, together with a common stylus guide assembly 16 for guiding a plurality of impact wires or styli 18 along predetermined paths. While FIGS. 2 and 3 include sections of assembly 12, it is to be noted that assembly 14 is the same as section 12 and, therefore, need not be separately shown in section.

Each assembly 12, 14 includes a support plate 20 having 9 electromagnets 22 supported on the support plate 20. Each electromagnet includes an inner pole piece 24 upstanding from the surface of the support plate 20 and a coil 26 disposed about the inner pole piece 22. Each coil 26 is electrically connected to a driver circuit (see FIG. 6) which selectively applies a predetermined current flow through the coil. Each electromagnet 22 also comprises an outer pole piece 28 upstanding from the top surface of the support plate 20 adjacent the associated coil 26.

Each assembly 12, 14 also comprises nine armatures or clappers 30 respectively associated with the nine electromagnets 22. Each clapper 30 forms with its associated electromagnet 22 an electromagnetic actuator for converting electrical energy into mechanical energy to move an associated one of the print wires 18. Each armature 30 has an inner end 32 and an outer end 34 extending outwardly from the outer pole piece 28 by a predetermined distance.

An armature retainer 35 for retaining each of the nine armatures 30 includes a relatively rigid disk 36 having a central opening 38 for receiving a screw 40 which is screwed into a cylindrical post 42 of the guide assembly 16 to connect the retainer 35 to the assembly 16. The disk 36 also includes a peripheral portion 44 having, as shown in FIG. 3, depending posts 46 for receiving a pair of notches 48, 50 of each clapper 30 for engagement by two adjacent posts 46, thereby restraining radial movement of the clapper 30 relative to the disk 36.

The retainer 34 also comprises a shock-absorbing member, such as an O-ring 52, together with a relatively resilient biasing member, such as a rubber O-ring 54, mounted to the peripheral portion 44 of the disk 36 between two adjacent circumferential walls 56, 58. The posts 46 are dependent from the wall 58.

As shown in FIG. 2, the cross-sectional diameter of the O-ring 54 is such as to compress normally when the retainer 35 is mounted to the guide assembly 16 with the electromagnet 22 de-energized. The diameter of the O-ring 54 and those of the walls 56, 58 are predetermined relative to the location of the clappers 30 and outer pole pieces 28. The axis 60 of the O-ring 54 is preferably slightly offset outwardly of the pivot line of each clapper 30, this pivot line being at the outermost edge 62 of the associated outer pole piece 28.

The retainer 35 has the primary function of retaining the clappers 30 engaged with their associated outer pole pieces 28. Additionally, and preferably, the retainer 35 also functions to apply a moment of force to each clapper 30 tending to cause the inner end 32 to rotate about the associated outer pole piece 28 to hold normally this inner edge in engagement with the O-ring 52.

As shown in FIG. 2, the disk 36 also includes a pair of walls 64, 66 depending from a central portion of the disk 36 and mounting the O-ring 52 therebetween. The wall 64 has nine spaced grooves 65 formed therein for accommodating respectively the clappers 30 at locations adjacent the inner ends 32. The grooves 65, in cooperation with the posts 46 and notches 48, 50 in each clapper suitably restrain any movement of the clappers in a plane perpendicular to the longitudinal axis 68 of the head 10.

Each assembly 12, 14 has its respective support plate 20 resting on a base plate 70 and is connected thereto by a screw 72 extending through the base plate 70 and support plate 20. As shown in FIG. 2, each wire 18 extends through the guide assembly 16 including plate 70 and into the assembly 12 or 14 through plate 20. A cap 74 is mounted on upper end 18a of the wire 18. A suitable compression spring 76 is coupled between the cap 74 and an upper surface 78 of a hollow extension 79 of plate 70 to force normally the cap 74 into engagement with the lower surface of the clapper 30 adjacent its inner end 32.

Each wire 18 follows a generally curvilinear path as it is guided through the guide assembly 16. The caps 74 of each assembly 12, 14 are arranged in a horseshoe type array, whereas the lower ends 18b of the wires 18, as shown in FIG. 4, are arranged in two vertical columns in a substantially linear array. This is accomplished by providing a plurality of guide members 80, 82 and 84 having holes for each respective wire 18. These hole patterns in the guide members progressively constrict the horseshoe type array down to the linear array shown in FIG. 4. The guide member 84 preferably includes a conventional ruby bearing plate of the variety commonly employed in matrix printheads of this type.

In the operation of this printhead, each wire 18 can be propelled to impact a record member (not shown) through an inking ribbon (not shown) adjacent the lower end 18b of each wire to form one dot of the desired dot matrix. To propel each wire, an associated electromagnet 22 is energized by applying current through the coil 26. This produces a magnetic flux path through the electromagnet in a well-known manner causing the armature or clapper 30 to be attracted to the inner pole piece 24. When this occurs, the inner end 32 of the armature pushes the cap 74, and thus the wire 18, downwardly, as viewed in FIG. 2, causing the wire to be propelled through the guide assembly 16 until printing end 18b impacts the record member via the ribbon. During the latter portion of this action, the armature 30 bottoms on the inner pole piece 24 and the print wire 18 goes into free flight. During the time the wire is in free flight, the electromagnet 22 will be de-energized, so that no magnetic force then exists to hold armature 30 to the inner pole piece 24.

The rebound force on the wire 18, after it strikes the record member, causes the return flight of such wire. When the wire 18 returns, the cap 74 impacts the clapper 30 and both return together to their initial position. The shock absorbing characteristics of the O-rings 52,54 contribute to the desired damping of the clapper 30.

FIG. 4 discloses the alignment of the ends 18b of the print wires 18 to print a dot matrix on the record member. These ends are aligned in two parallel columns, A and B, respectively. Column A comprises nine print wires 18 which are coupled to the assembly 12 and responsive to respective electromagnets 22 in that assembly, while column B comprises nine wires 18 which are coupled to the assembly 14 and responsive to respective electromagnets 22 in the latter assembly. The center-to-center spacing d between the two columns A, B of print wires is predetermined and equal to one of two distances in accordance with the two embodiments of the present invention, as will be more fully described.

FIG. 5 illustrates the relationship of the various print positions on a record member for printing, for example, an upper case letter D or M by the printhead of the present invention. As one example, the printhead of the present invention may be used to print characters which are 10 pitch, i.e., there are 10 characters, such as the letter D, per 1 inch. Furthermore, as one example, such printhead is used to print a 9 by 5 dot matrix in which there are 9 print wires per dot column and five dot spaces or six dot columns per character. This is shown in FIG. 5 in which there are numbered six dot columns C.sub.1 -C.sub.6 comprising five dot spaces between the columns and nine dots D.sub.1 -D.sub.9 per column. For upper case letters, only dots D.sub.1 -D.sub.7 are used whereas for lower case letters having descenders, such as g and j, the last two dots D.sub.8 -D.sub.9 are used to form the characters.

As may be appreciated, a dot column may be defined as a series of dots aligned so that a straight line extending through the centerpoints of the dots is at a fixed angle to the direction of movement of the printhead.

In the specific example shown in FIG. 5, a straight line extending through a dot column is at an angle of 90.degree. in relation to the direction of movement of the printhead. This will be provided when the wire columns A and B are at 90.degree. in relation to printhead movement, as shown in FIG. 4. These wire columns A and B could be placed at any suitable angle to produce a corresponding dot column at such angle; obviously, though, for example, an angle of 0.degree. would not be suitable.

With reference to FIG. 4, again, in either of the two embodiments the wires in column A are used to print only the odd dot columns C.sub.1, C.sub.3, C.sub.5 while the wires in column B are used to print only the even dot columns C.sub.2, C.sub.4, C.sub.6. Thus, the printhead has to move a distance equal to two dot spaces, e.g., the distance between three dot columns C.sub.1 -C.sub.3 or C.sub.2 -C.sub.4, center-to-center before print wires in column A or B may have to be energized again to print dots on a dot column. This means that the printhead of the present invention can move twice as fast as when wires in a printhead having only a single column have to be activated at adjacent dot columns C.sub.1 -C.sub.6 to print the desired character.

In one embodiment of the present invention, the spacing d between columns A and B of print wires is equal to an even number multiple of the space between two adjacent dot columns, center-to-center. For example, if this even number multiple is 2, the distance d is equal to the distance between three dot columns, center-to-center, such as columns C.sub.1 -C.sub.3. Thus, this spacing d is equal to the distance t between columns C.sub.1 and C.sub.3, as shown in FIG. 5. In this first embodiment, and with reference to FIG. 5, the wires in columns A and B are energized alternately to print the dot matrix. As the printhead moves across the dot column C.sub.1, wires in column A will be selectively energized to print a predetermined number of dots for the letter D since this is an odd-numbered column. When the printhead next moves column A over dot column C.sub.2, none of the wires in columns A and B will be energized since column A is across an even-numbered dot column and column B has not yet crossed any dot column (assuming C.sub.1 is the first dot column of a line; while column B is not "energized" in this condition because no dots are to be printed, it may be considered "energized" so that the above stated rule of alternately energizing the columns A and B is satisfied; the reason for this will become more apparent when the energizing circuit of FIG. 6 is discussed.). In the next instance when column A moves across dot column C.sub.3, selected wires in column A will be energized to continue printing the letter D since column A will be across an odd-numbered column; however, the wires in column B, which is now across odd-numbered dot column C.sub.1, will not be energized. In the very next instance, column A will be located across even-numbered dot column C.sub.4 and, therefore, the wires in this column will not be energized, but column B will now be across even-numbered column C.sub.2 and therefore the wires in this column B will be selectively energized to print the required dots for this dot column. This process continues until the matrix for the letter D and similarly letter M is completed. Thus, in this first embodiment, wires in columns A and B are energized alternately since both columns are simultaneously either across an odd- or even-numbered dot column and column A is used only for the odd-numbered dot columns while column B is used only for the even-numbered dot columns.

In a second embodiment, the distance d between the columns A and B of print wires is equal to an odd number multiple of the space between two adjacent dot columns, center-to-center. For example, if this odd number multiple is 1, the distance d is equal to the distance t' between two dot columns, center-to-center, as shown in FIG. 5, i.e., one dot column space. In this second embodiment, while the wires in column A are used to print only odd-numbered dot columns and the wires in column B are used to print only even-numbered dot columns, as in the first embodiment, the wires in the two dot columns are selectively energized simultaneously to print the character. For example, to print the letter D, when column A is across dot column C.sub.1, its wires will be selectively energized to print the dots in this column (assuming C.sub.1 is the first column in the line, then column B wires would not be "energized" because no dots are required; however, the column B wires may be considered "energized" so that the above stated rule of simultaneous energization is satisfied; the reason for this also will become more apparent when FIG. 6 is discussed). In the next instant, column A is across dot column C.sub.2 and column B is across dot column C.sub.1, at which time none of the wires in either column will be energized since column A is across an even-numbered dot column and column B is across an odd-numbered dot column. In the next instant, column A will be across dot column C.sub.3 and column B will be across dot column C.sub.2, in which event wires in both columns A and B will be selectively energized simultaneously to print the dots in these two columns. Thus, it can be appreciated that as the columns A and B cross the dot columns C.sub.1 -C.sub.6 to print letters D or M, the wires in columns A and B will either not be energized or selectively energized simultaneously. It will, therefore, also be appreciated that in the two embodiments described, wires in a given column are moved the distance t before such wires may have to be again energized.

FIG. 6 illustrates, schematically, a diagram of a circuit for energizing the wires in columns A and B in accordance with the two embodiments described above. A position transducer 88 generates a signal each time a column A or column B of print wires traverses a dot column on a record member. Transducer 88 includes a light source 90 and a phototransistor assembly 92 which are attached to a movable carriage (not shown) supporting the printhead, and a scale 94, fixed to a frame (not shown) which interrupts the light path from the source to the phototransistor. The scale 94 is so constructed that each time the carriage crosses a dot column, the light path from source 90 to the transistor 94 is interrupted. Such on and off interruptions of the light path will, when suitably amplified through an amplifier 96, produce a train of square pulses having one cycle per dot column, as shown at the output of amplifier 96.

A microprocessor 98 or other similar logic unit is connected to the output of amplifier 96. Microprocessor 98 stores data representing the dot pattern of the characters desired to be printed, and feeds data for the odd-numbered dot columns to a buffer 100 and the data for the even-numbered dot columns to a buffer 102. The microprocessor 98 also outputs an odd column fire select pulse on line 104 and an even column fire select pulse on line 106. The microprocessor 98 is programmable to output the fire select pulses on lines 104,106 alternately for purposes of the first embodiment described above, or simultaneously for purposes of the second embodiment described above. A more detailed description of the structure and operation of the microprocessor 98 will be given below.

Each buffer 100,102 has nine outputs identified in FIG. 6 as pin 1 - pin 9 corresponding to each of the print wires 18 in a respective column A or column B on the printhead. The nine outputs pin 1 - pin 9 for odd column buffer 100 are connected as one input, respectively, to nine NAND gates 108, only two of which are specifically shown. Each of the nine outputs pin 1 - pin 9 of the even column buffer 102 is connected as one input to nine NAND gates 110, only two of which are specifically shown. The NAND gates 108 receive as a second input the odd column firing select pulse on line 104 from microprocessor 98, while the NAND gates 110 have a second input receiving the even column firing select pulses on line 106 from the microprocessor 98. A one-shot multivibrator 112 outputs a firing pulse of predetermined duration as a third input to each of the NAND gates 108 and 110. Multivibrator 112 outputs its firing pulse in response to each square wave signal from amplifier 96, the firing pulse duration being smaller than one dot column square wave period from the amplifier.

The output of each NAND gate 108,110 is fed through a resistor R.sub.1 to the base of a respective transistor 114,120 whose emitter is connected to a voltage source V and whose emitter and base are coupled through a resistor R.sub.2. The collector of each transistor 114,120 is connected respectively to the base of another transistor 116,122 through a resistor R.sub.3. The collector of transistor 116,122 is coupled to the voltage source V through the coil 26 for each of the electromagnets 24 and the emitter is coupled to ground through a resistor R.sub.4. A resistor R.sub.5 is connected across the base and emitter of each transistor 116,122, and diode 118,124 is connected across the coil 26 as shown.

In operation, assume that the first embodiment, which is preferred, is to be used in which the columns A and B of print wires are spaced apart a distance equal to two dot column spaces. Consequently, the microprocessor 98 will be programmed to output alternately an odd column fire select pulse on line 104 upon receipt of one waveform from amplifier 96 followed by an even column firing select pulse on line 106 when the microprocessor 98 receives the next waveform from amplifier 96.

Each time the carriage is returned to the leftmost dot column of a line to be printed, a reset pulse is generated in a well-known manner (not shown) to reset the microprocessor 98 generating the column fire select pulses. Then, as the carriage and printhead move across the first or odd dot column, amplifier 96 generates a first waveform which is acted on by the microprocessor 98 to provide an odd column fire select pulse on line 104, which is fed to the NAND gates 108. In addition, multivibrator 112 generates a firing pulse of predetermined duration to enable the NAND gates 108 for a corresponding period of time. At this time, buffer 100 provides output data on pins 1 - 9 depending upon which of the nine dots of the first odd dot column C.sub.1 are to be printed on the record member. Consequently, the NAND gates 108 receiving print dot data from pins 1 - 9 will gate this data for the duration of the firing period of the pulse from multivibrator 112 to turn on transistors 114. With transistors 114 turned on, transistors 116 will be turned on so that current will flow through coils 26 to energize the corresponding wires 18 in odd side assembly 12 to print the first odd column.

When the carriage next moves the printhead one dot column space, column A is across even numbered dot column C.sub.2, but column B is not yet across dot column C.sub.1. Therefore, microprocessor 98 will produce an even column fire select pulse on line 106 which is fed to the NAND gates 110 and multivibrator 112 sends a firing period pulse to NAND gates 110 to enable them. However, at this time column B will not be over any dot column in the line and, therefore, buffer 102 will output logic data indicating that the nine print wires 18 in column B are not to be activated. When the carriage next moves one dot column space so that column A is across dot column C.sub.3 and column B is across dot column C.sub.1, the transistors 116 again will be closed in the manner indicated above so that dots will be printed on the odd dot column C.sub.3 in accordance with the data from buffer 100 for this dot column. When the carriage next moves a fourth dot column space, column A will be across even dot column C.sub.4 and column B will be opposite even dot column C.sub.2. At this time, microprocessor 98 will generate the even column fire select pulse on line 106 which, together with the firing period pulse from multivibrator 112, will enable NAND gates 110. Consequently, the even column data from buffer 102 for this column C.sub.2 will be gated through gates 110 to turn on transistors 120 which turn on transistors 122. Therefore, coils 26 in the even side assembly 14 will be energized in accordance with this data for column C.sub.2, thereby printing the appropriate dots on the record member. The above operation continues for the entire length of the line to be printed and then the carriage is returned to the leftmost position where the circuits in microprocessor 98 are reset to print another line of characters.

If it is desired to print characters in accordance with the second embodiment described above, then the microprocessor 98 is programmed to output simultaneously an odd column firing select pulse on line 104 and an even column firing select pulse on line 106 for each waveform generated by amplifier 96. When column A is opposite the first or leftmost dot column C.sub.1, all the NAND gates 108 and 110 will be enabled. However, at this time only buffer 100 will output data causing selected solenoids 26 in odd assembly 12 to be activated, while the data from buffer 102 will be such as to not energize coils 26 in the even side assembly 14. Then, when column A is opposite the second column C.sub.2 and column B is opposite the first column C.sub.1, gates 108 and 110 will again be enabled; however, buffers 100 and 102 will output data which will not cause any of the coils 26 to be energized. Upon the next movement of the carriage, column A will be across dot column C.sub.3 and column B will be across dot column C.sub.2. Consequently, all the NAND gates 108 and 110 again will be enabled and buffer 100 will output data to energize coils 26 to print the odd column while buffer 102 will output data to energize coils 26 for printing dots on even column C.sub.2. It can therefore be appreciated from the foregoing that as amplifier 96 outputs a waveform for each dot column, the print wires in columns A and B will either both be activated or both not be activated as the carriage moves across a line.

A more detailed description of the structure and operation of microprocessor 98 or other similar logic unit will now be given. The output of the amplifier 96, the position signal, is used as an interrupt signal to the microprocessor 98. The microprocessor 98 can be preprogrammed in such a fashion that each time a position signal arrives, it will execute a specified segment of a program previously stored in a memory device such as a Read Only Memory. The character fonts are stored also in such a memory device in a series of data words, each data words corresponding to a dot column and the presence or absence of a bit in a word denoting the presence or absence of a dot in a dot column. Typically, five such words will be necessary to store a 7 .times. 5 matrix character. To print a particular character, the microprocessor 98 will be programmed to present the data words to its output ports in a specified sequence. The data signals at the microprocessor output ports are normally buffered in buffers 100 and 102 to provide a steady continuous signal for the duration of the energization of coils 26.

To print a 7 .times. 5 dot matrix character, for instance, the program is so structured that the data word for the first column (column 1) is presented to the odd column information buffer 100 while the data word for the even column (column 0) is stored on the even column buffer 102, which in this first instance happens to be a blank word. Then, in the embodiment that energizes the odd and even column coils 26 alternately, a signal is presented to a third output port which enables only the odd column coils 26 to be energized for the duration controlled by the one-shot multivibrator 112. The next position signal from amplifier 96 would cause a signal to be presented to the third output port which would enable only the even column coils 26 to be energized for the duration controlled by the one-shot multivibrator 112. In the second embodiment, the odd and even fire-enable signals are presented to the third output port simultaneously, causing both odd and even column coils 26 to be energized simultaneously. Such signal will then need to be presented to the third output port only every other position signal arrival. The process then repeats for both embodiments with the data word for columns 3 and 2 presented to the odd and even column information buffer respectively and then the data words for 5 and 4 will likewise be presented. A microprocessor 98 which can be easily programmed by one skilled in the art to carry out the above operation is the Intel 8080 manufactured by the Intel Corp., Santa Clara, Calif.

Thus far, the description of the invention has been given in relation to the printing of what is known in the art as full dot column matrix printing. However, the invention can also be employed to print at half spaces in-between the full dot column positions, such half-space printing also being known in the art. This half-space printing is used since it allows for a more eye-pleasing structuring of the printed characters.

FIG. 7 illustrates the character D printed with dots at the half-spaces; it will be apparent that the matrix format shown in FIG. 7 is restructured in relation to the matrix format of FIG. 5.

To carry out the present invention, the character structuring of FIG. 7 is predetermined such that no printing on two consecutive half dot spaces is required of a particular coil 26; thus, the frequency requirement on the printhead 10 is no different from the two embodiments already described where full dot columns only are printed. The information stored in the Read Only Memory of microprocessor 98 of each 7 .times. 5 character would require four extra data words for the four half dot spaces (C.sub.2A, C.sub.3A, C.sub.4A, C.sub.5A) in between the five full dot positions (C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5), making a total of nine data words necessary to store the pattern for a character. Such character structure is commonly known as the 7 .times. 9 or 9 .times. 9 font style depending on the number of wires 18 used. The sequencing of the wires is quite obvious with the half-spaces labelled C.sub.2A, C.sub.3A, C.sub.4A, C.sub.5A as shown in FIG. 7, with C.sub.2A being the half-space between C.sub.1 and C.sub.2, C.sub.3A being the half-space between C.sub.2 and C.sub.3, etc., and defining C.sub.2A and C.sub.4A as being even and C.sub.3A and C.sub.5A as being odd. As before, the program stored in the Read Only Memory of microprocessor 98 will be so structured that the column A of wires will print only the odd-labelled dot columns (i.e., columns 1, 3A, 3, 5A, 5) and the column B of wires will print only the even-labelled dot columns (2A, 2, 4A, 4). It is to be noted that in this half-space dot matrix printing embodiment, the spacing d between columns A and B would still be the same as in the two embodiments described above in printing full dot columns.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.