CA1234508A - High quality printing method - Google Patents

Abstract

Abstract High quality printing method, by means of a matrix serial printer, comprising a printing head provided with needles arranged in a vertical column. The method consists in print-ing a printing line through two printing passes and in advancing the platen, between the first and the second pass, by a quantity equal to one time and a half times the vertical pitch, centre to centre, between two adjacent needles.

Description

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The presen-t invention relates to a method of hiyh quality printing using a do-t matrix serial printer and to a printer for carrying out the method.

In data processing systems and communica-tion systems, fast dot matrix printers are widely used. In such a printer a printing head provided with a plurality of printing needles, arranged in a vertical column, slides transversely with respect to a printing support. The selective and timed actuation of the needles during the movement of the printing head allows the printing of characters through the printing of pre-established matrix dots. Generally printing heads with seven and nine needles are used to generate characters constituted by dots arranged in seven or nine lines per five or nine column matrixes. The quality of the characters so obtained is limited by the number of discrete dots which can be printed, but the printing speed obtainable is very high in relation to the printer cost.

It is desirable to obtain from such printers alphanumeric symbols which symbols are similar in quality to symbols obtained by solid font printing, although the printing of better quality s~mbols may be detrimental to the printing speed. Several solutions have been adopted to this purpose.

Among -the latest and effective solutions the one disclosed in United States Patent No. 4,159,882 may be mentioned. The criteria disclosed in the patent to perform high quality printing with a matrix head are as follows. A conventional head is used having 7 or 9 needles which are vertically arranged with a pitch p between the centre of two contiguous needles, (the needle diameter being slightly lower than p).
The character printing in a printing line is performed with several printing passes in order to increase the vertical resolution and to obtain vertical lines where the printed dots overlap. Between one printing pass and the next the printing support is advanced vertically by a submultiple of pass p, . - .

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tha-t is l/2 or l/4. In this way characters constituted by dots arranged according to a 28-line matrix can be obtained with a four pass printing by using a 7-needle head.

The horizontal resolution of the matrix is increased by means of electronic circui-ts which enable the actuation of the needles in correspondence with subsequent printing positions at very little distance, less than the diameter of the needles and of the corresponding dots. The proposed solution is effective, but it can be carried out only by means of complicated and expensive mechanical devices.

The present invention is addressed only to the problems related to a high vertical resolution.

The pitch p between adjacent needles is 0.35 mm. and the needles have a slightly smaller diameter 0.33 mm. The character printing by several passes to obtain a better printing quality requires at each pass, according to the above-mentioned U.S. Patent No. 4,159,882, the advancement of the printing support by a half or a quarter of the pitch 0.35 mm, that is 0.175 mm. or 0.0875 mm. However, shifting the printing support by such small amounts necessitates the use of very precise and expensive operating devices. In practice a stepping motor with a high number of poles is used. The stepping motor must be directly coupled to a printing support feed roller. The feed roller must be of the friction type to ensure that there is an adequate contact area between the feeding device and fed support. Further it has to ensure that there is a suitable coefficient of friction. It has been shown experimentally that the use of different types of feeding mechanisms is not suitable for providing such small shifting of the support.

The use of pin tractors must be excluded. In the case of such little shifting of the support elastic deformation of ..

the support edges takes place, according to the perforation engaged with the pins, without causiny any shift of the part of the support at which the printing occurs. This is due to the friction between the fixed part of the printer and the printing support; indeed the local shift of the support according to the pin tractors does not cause sufficient local tension in the support as to overcome the starting friction.

The use of an economical stepping motor, that is one having a small number of poles and therefore with a large angle of rotation between each stable position and the next must also ~e exluded. Assuming that a typical pass angle for such a motor is 7.5, direct coupling of the motor to the feed roller would require the use of a feed roller having a diameter of only a few millimeters. Therefore the use of reduction gears with a reduction ratio of about 1/15 * 1/30 would become lmavoidable. Such a ratio may be obtained with several reduction gears which unavoidably introduce play or backlash as much as the desired shift. Alternatively, such a ratio may be obtained by means of one reduction gear having a special toothing, but that is expensive.

The consequent speed limitation in shifting the printing support, due to the use of a stepping motor having a large number of poles or to the use of reduction gears, must be taken into consideration. Generally matrix serial printers are bidirectional, that is, they are able to print through passes from left to right as well as from right to left equally well. One pass generally requires less than l second to be executed. The advance of the support between one pass and the next must therefore be executed in the shortest possible time. For instance, the execution of a line feed in 100 msec time affects the printer throughput by 10%. The line spacing currently used has a width of 4.25 mm. or 3.18 mm. If the support is advanced by 0.0875 mm. at each pass ~ 23~L5~

of the stepping motor, -the line feed adv~ncement of 4.25 mm.
requires 48 motor passes. Similarly a lead of 3.18 mm.
requires 36 motor passes.

It is ~nown that the stepping motors have a well limited range of speed, of n passes per second, within which they may operate with an almost constant torque. Beyond a certain speed the torque rapidly decreases. It is therefore prefer-able that the line feed be performed with the smaller number of motor passes in order not to penalize the execution time Of the line feeds.

These disadvantages are overcome by the high quality printing method of the present invention, which may be used with fast and economical dot matrix printers where character printing is performed according to a conventional 9x5 or 9x9 dot matrix.

The method allows quality character printing according to a dot matrix of 18 lines per an arbitrary column number.

According to the invention, the high quality printing is obtained with a first printing pass followed by a printing support advance equal to 1.5 times the vertical pitch between two needle axes, and with a subsequent pass. In this way each element of the advance of the printing support is about 0.53 mm., that is, such as can be performed without the previously mentioned inconveniences of a pin tractor eeding device. The feeding devices may be operated by low cost stepping motor with low reduction ratios (1/5) which can be easily obtained with one toothed gear and therefore involves little play. In addition, the line feed operations can be executed with a smaller number of motor steps because the individual motor step can be made to agree with the elementary steps of the printing support by 0.53 mm.; fast line feed is therefore obtained.

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Other advantages are also obtained. Thus a printer having a 9-needle head is preferably used in such a method. The use of a 9-needle head enables the character printing by dots arranged in a matrix of 18 lines to an arbitrary column number. The line distribution is not uniform; a central field of the matrix, having 16 lines uniformly spaced by a pitch equal to half of the pitch, centre -to centre, between two adjacent need].es, is obtained. Above the first line of the central field a printing line is available at a distance equal to the pitch, centre to centre, between two needles. Below the last line of the central field a printing line is available at a distance equal to the pitch, centre to centre, between two needles. In practice, the central field may be used for high quality printing according to a matrix of 16 x N, where N is an arbitrary column number.
The upper line can be used for accent marks, "umlaut" and overscoring. The lower line can be used for underscoring.
With such a 16xN matrix the best utilization of the character-generators is obtained, that is, the best use of the memories used to describe the characters to be printed which, as known, are byte arranged, that is with a parallelism of 8 bits. The underscorings, overscorings and stressings are not generally included in the character description but they are obtained by sum of the letter description with additional information, A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows the needle ends of a printing head preferably used in carrying out the method of the invention;

Figures 2 and 3 show horizontal marks obtainable with the needles of one such head depending on the head speed, the needle actuat;on frequency being the same;

Fi~ure 4 shows the vertical printing posi-tions obtainable;

Fi~ure 5 shows one example of quality characters obtainable by the method;

Figure 6 shows a preferred embodiment of the printiny support feeding mechanism used in carrying out the method of the invention; and Figure 7 shows the architecture of an electronic control system of a prlnter for carrying out the method of the invention.

Figure l shows the ends 1, 2, ...9 of the printing needles of a known type of dot matrix printing head. The needle ends are arranged in a column. The needles, of circular section, have a diameter D of about 0.33 mm. and their ends are arranged at a pitch p, the distance between their centres, f 0.352 mm.

Each of the needles can be selectively actuated in a perpen-dicular direction, normal to the plane of the drawing, in order to place a circular mark of the same diameter as the needle on a printing support coinciding with the drawing plane. The simultaneous operation of all the needles allows the marking of a vertical segment consisting of nine dots on the printing support, which is referred to below as "paper".
The segment "granulation" is immediately visible. The printing head slides on the paper in the hori~ontal direction shown by the arrow F. For instance a head transfer speed F
of 25.4 cm~sec is commonly used. A needle operation requires a certain time, and so there is an upper limit to the actuation frequency. This frequency depends on the kind of the head and may range from 500 Hz, for low cost heads, to 200Q Hz, for high performance heads.

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The printing of a horizontal line, constituted by dots having a pitch Pl, centre to centre, equal to 0.508 mm. is obtained (Fig. 2), if the same needle 1 of ~iy. 1 is actuated in succession at the frequency of 500Hz, and the transfer speed of 25.4 cm/sec. The l'granulation"of the line so obtained is immediately visible. It is, however, clear that by reduc-ing the head speed, for instance to 12.7 cm/sec, the marking of a horizontal line constituted by dots having a pitch P2, centre to centre, equal to 0.254 mm. is obtained. In this case the printed dots, of diameter of 0.33 mm., partially overlap forming a continuous line (Fig. 3). Good printing quality is therefore obtained, for the horizontal segments, by suitably reducing the speed of the head depending on the maximum frequency allowed for the needle actuation.

According to the method of the invention quality printing for vertical or inclined segments is obtained with two printing passes and with the advance of the printing support between one pass and the next one, by an amount H=1.5p, that is 0.53 mm. In this way, the horizontal printing positions obtained with the first pass add to the horizontal printing positions obtained with the second pass as shown in Fig. 4 by lA, 2A, ... 9A. The complete set of the horizontal printing positions obtainable together with the transverse shifting of the head defines a matrix of possible printing positions in which the column number N varies according to the head speed and to the instant of actuation of the needles according to the head position~

The horizontal resolution is not a concern of the present invention.

The number of the matrix printing lines so distributed is equal to 18. A matrix central field consists of 16 printing lines unlformly spaced by a vertical distance equal to 0.175 mm. In such field, the vertical marking of all the ,.

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possible printing positions constitutes a continuous segment in which the printed dots, each of a diameter of 0.33 mm., widely overlap with contiguous dots. In -the ~ase of inclined segments also the partial overlapping of contiguous dots is still obtained for a wide range of inclination from 0 to more than 45 to the ver~ical. Dot-composed characters having an excellent printing quality are therefore obtained and letters A and p are shown in Fig~ 5 as examples of such characters.

Above the central field a dot printing line is available at a distance of 0.352 mm. centre to centre, from the contiguous one. These printing positions can be used for overscoring the letters composed in the central field of the matrix. The overscoring printing does not interfere with and does not overlap the printing in the field below. Similarly, a line of dot printing positions is available below the central field at a distance 0.352 mm. from the contiguous one. Such printing positions can be used for the underscoring of letters composed in the central field of the matrix. The underscoring printing does not interfere with and does not overlap the impressions in the upper field.

Fig. 5 provides examples of overscoring and an underscoring.
The printing support advance between each printing pass and the subsequent one is suitably obtained with one step of a stepping motor 12 (Fig. 6). The stepping motor of an economical type may have a pass angle equal to 7.5. The printing support can be advanced with pin wheels 16 having a feeding circumference equal to 12.7 cm corresponding to a diameter D of about 4 cm. This is a typical size for feed wheels used in the printers. A paper feed of 0.53 mm. corres-ponds to an angular rotation of 1.5 for wheels of the size mentioned. This angular rotation can be obtained by coupling the stepping motor to the pin wheels through a reduction toothed pair Gl, G2 having a ratio of 1:5. Such a pair may ~,;, -, ~3~

be easily achieved with conventional -toothiny of involute profile and involving little play. The use of such a feed mechanism permits standard line eed passes of 4.25 mm~ and 3.18 mm. through 8 and 6 motor steps respectively. High feeding speed for the paper can therefore be obtained.

The invention method also provides advantages in terms of electronic control architecture in the character description matrices or character generators.

Figure 6 shows, in schematic form, the architecture of an electronic control system of a printer adapted to carry out the method of the invention. Th~ system comprises a micro-processor 10, a programmable I/O gate 11, a register 13, a control memory 14 and a programmable communication interface 15. All such devices are available as integrate circuits.
For instance the devices mentioned are marketed by INl'EL
under the following codes:
microprocessor 10: code 8085 programmable gate 11: code 8155 control memory 14: code 8316 A
programmable communication interface: code 8250 Register 13, which may be an 8-bit register, is marketed by several integrated circuit manufacturers (Texas, Fairchild, Motorola~ under the code LS 373. For detailed information about such devices reference is made to manufacturers' manuals. Only the information necessary for an understanding of the operation of the disclosed architecture is given herein.

Microprocessor 10 is supplied with 8 input/output pins, for data and addresses, which are connected to 8 leads which constitute bus AD07. Microprocessor 10 is provided with 8 address output pins which are connected to 8 leads, which constitute address bus A8-15. Microprocessor 10 is further provided with:
- an output CK for sending a cyclical timing signal `:

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to other system components, - an output ALE for sending an address "strobe"
signal, - two interrupt inputs RST and TRAP for receiving 5two separate program interrupt signals.
Programmable gate 11 is provided with input/output pins su~P
connected to BUS ADO-7 and with three groups of input/output gates A, B, C having 8, 8, and 6 pins respectively. The gate is programmable as the transfer direction of the several input/output groups can be pre-established by means of suit-able control signals. Gate 11 includes a set of 256 8-bit registers which may be used as an auxiliary memory and are indicated as BUFFER 24 in Fig. 7, a timing counter 25 and a support register AA, BB, CC for outputs A, B, C respectively.
The counter can be pre-set at a prefixed binary value~ Gate 11 is further provided with:
- an input terminal T.I. for receiving timing pulses, - an output terminal T.O. for delivering a timing pulse.
The pulses received by terminal T.I. cause the timing counter 25 to decrement. When the counter, owing to the decrement, is at binary status O (zero~ a timing signal is available at terminal T.O. - an input CE for receiving a selection/enable signal of the gate.

Register 13 is provided with a set of 8 inputs IN connected to bus ADO-7, with a strobe input IEN for input signals strobing connected, through lead 17, to the microprocessor output ALE, and with a set of 8 outputs OUT. Control memory 14, having an 8~bit parallelism and a 2K byte size, is provided with 11 address inpu~s. Eight of these are connected, through a channel 18 to the outputs of register 13. The remaining three inputs are connected to suitable leads of channel ~8-15, through channel 19.

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Memory 14 is provided with ~ data outputs connected to bus ADO-7 through channel 19, and with an input CE2 con-trolling the outputs. When CE2 is at logical level 1 -the outputs are enabled. Otherwise the outputs are virtually isolated. The programmable communication interface 15 serially receives some input data DATA IN from a modem. As soon as a character is completely received, interface 15 delivers an interrupting signal on output INT.

~en interface 15 receives an enabling signal on input CE3 the received character is transferred to an 8-o~tput set connected to bus ADO-7. The inter~ace operations are timed by a timing signal received at an input CKl connected to output CK of microprocessor 10 through lead 20. The enabling inputs CEl, CE2, CE3, are respectively connected to the higher weight leads A13, A14, A15 of bus A8-15.

Input TI of gate 11 is connected to output CK of processor 10 through lead 21. Outpu-t TO of gate 11 is connected to input TRAP of processor 10 through lead 22. Likewise the output INT of interface 15 is connected to input RST of processor 10 through lead 23. Output set A of gate 11 has two outputs respectively connected to two leads Nl, N2~ Actuation signals are sent, through these two leads, to needles 1 and 2 respectively of the printing head, that is, to two upper needles of the printing column. Output set B of gate 11 has seven outputs connected to leads N3, N9 respectively.

Actuation signals are sent through these leads to needles 3, ...9, respectively, of the printing head.

The eighth output of set B is connected to lead STR. A

STROBE signal is sent through the lead STR, which, ANDed with the signal present on leads Nl, ... N9, causes the selective and contemporaneous actuation of a certain number of needles. Output set C supplies control signals to the motor effecting the printing carriage movement and the printing support advance.

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Memory 14 stores suitable control programs for processor 10, consisting of 8-bit instructions as well as character description tables. An alphanumeric character which has to be composed in a matrix of g vertical dots per 7 horizontal dots is implemented by a 7~byte table. A high quality type ~hich has to be composed in a matrix of 16 vertical dots per N horizontal dots and whic~ is actually printed with two passes, is implemented by two N-byte tables (TABLE 1 and TABLE 2 of Fig. 7).

The operation of the system is very simple. When interface 15 reeeives a character it sends an interruption signal to the output INT, both when the character is of a type to be printed and when it is a command (space, lead, carriage return, etc.). Such INT signal received by processor 10 starts an interruption handllng program. In other words processor 10 cyclieally puts suitable address information on buses ADO-7 and A8-15 and fetehes program instruc-tions from memory 14. Owing to the execution of such program, processor 10 reeeives, through bus ADO-7, the character available at the output of interface 15 and handles it. If the received character is a command, the processor 10 eontinues by executing it. In particular, the received charaeter can indicate that the types to be printed must be of a high q~ality. In this ease the proeessor stores the information into a suitable internal register or into the buffer 24 of gate 11.

As soon as a code corresponding to a letter to be printed is reeeived, it is loaded into a "line buffer" zone. It is further used as an addressing code for memory 14 where a first eharaeter deseription table is identified. The first byte of this table is read out by proeessor 10 and, with suitable shift operations, it is partially loaded into register A of gate 11 and the remaining part is loaded into register B. In other words the eight bits, whose logic level , . , , :.
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l or o indicates whether the corresponding ~eedle has to be energized or not, are coupled -to suitable needles. Duriny a first printing pass eight bits are coupled to needles

2 ... 9. An information corresponding to a pxinting command is also loaded into the eighth bit of register B and the selective actuation of the needles therefore takes place.
From this instant the processor lO is free to supervise control operation differen-t from the printing command.

Further, the timing counter is present, which then decrements for the timing pulses received from input TI.

The read-out memory operations, the read-out byte processing operations, and the A, B, register loading operations occur in a few cycles of processor 10 and therefore in a few micro-seconds time which is negligible compared with the enabling period of the printing operations. The timing counter of gate 11 defines, by its setting to zero, the time interval between a printing operation and the subsequent one. Typically this interval may be of about 2 msec. After such 2 msec intervals the zero setting of gate 11 counter generates an interruption signal at output TO which, received by processor 10, causes the fetch from memory of a second byte of the character description table.

The operations already mentioned are repeated and the printing of a second dot column is commanded. In this way the printing of the letter corresponding to the first pass is performed.

Meanwhile, if other characters to be printed have been sent from interface 15 to the system, these had been s-tored in gate 11 memory buffer~ The printing corresponding to the first pass takes place for each of these characters fetched in an orderly manner by processor 10. As soon as the print-ing of a line has been completed, processor 10 acts on the feeding devices of the printing support which is advanced by 1.5p.

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The list of the characters -to be printed, stored in gate 11 buffer, is now used to inversely address the second description tables contained in memory 14. During this second pass the bytes read out from memory are coupled to needles 1 ... 8 instead of to needles 2, ... 9, by means of shift operations. In this way the high quality printing is completed Clearly the second printing pass, described above as being performed in reverse order, that is, with the printing head return movement, can also be executed in direct order if preceded by a return operation of the printing head.

The foregoing description refers only to features essential to an understanding of the invention and completely omits those features which are not important to the present context, such as the motor control for moving the printing head or the printing support. Essentially the description points out that each bit, included in the character description in memory 14, constitutes an information concerning the actuation or the non-actuation of a corresponding needle and that the correlation bit/needle is arranged according to the printing pass. This selective correlation according to a printing pass is essential for the control of a 9-needle head (generally M needles) with an 8-binary code (generally M-l) and is characteristic of the present invention. Obviously, such selective correlation is not essential if a head having a number of needles equal to or less than the number of bits constituting each column of the character description table, is used for quality printing.

In practice, as commercially available memories have an 8-bit parallelism the problem does not occur for the control of 7 or 8 needle heads which pérmit composition of quality letters according to a matrix of 12 or 14 lines per N
columns.

In the foregoing description it has been shown that the , ~Z3~

several character description bytes to be printed, once fetched from the table, are processed by processor 10 and loaded into register A, B. In practice it is also possible to process such bytes and load them into a zone of sUFFER
24 in order to have them ready as soon as their use is required. In this way the time interval can be further reduced between the instant when gate 11 generates the interruption signal, corresponding to a printing timing, and the instant when the information is available in registers AA, BB. In this case processor 10 has only to control the information transfer from buffer 24 to registers AA, BB with a reduced number of cycles. Although the needles referred to in the description may be regarded as impression elements, the invention is not limited to the field of matrix printers using impression needles, nor is it limited to impact printers.

The method of the invention is suitable for all serial matrix printers in which character printing is obtained by dot composition and the dots are impressed on the printing support by a column of printing elements through impact, electrical discharge, thermal transfer of ink or the like.
Likewise the invention is not restricted only to impression elements vertically arranged with respect to the printing line but also envisages elements arranged in directions which are inclined to the printing line.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A high quality printing method for printing dot composed alphanumeric characters using a matrix serial printer provided with N printing elements arranged in column with a vertical pitch P centre to centre, the print-ing elements being shifted transversely with respect to said column along a printing support line and being selectively actuated during said transverse movement, the printer com-prising means for advancing said printing support in the direction of said element column by a discrete multiple of an elementary advance, the method comprising:
- execution of a first line printing operation through a first transverse pass of said needles and the selective actuation of the lower N-1 of said printing elements, - advancing said printing support by a discrete amount equal to 1.5 times the vertical pitch P
of said needles centre to centre, and - execution of a second printing operation of said line with a second transverse pass of said needles and the selective actuation of the upper N-1 of said printing elements.

2. A method according to claim 1, wherein the needle actuation is effected by command of a first and a second succession of binary codes of N-1 bits, contained in a character description memory and read out one at a time from said memory, wherein the first printing operation with a first pass is performed by relating each of the N-1 binary code bits of said first succession to the control of the lower N-1 elements in said N elements column and wherein the second operation is performed by relating each of the N-1 binary code bits of said second succession to the control of the upper N-1 elements in said N elements column.

3. A matrix serial printer, having a printing support with N printing elements arranged in column with a vertical pitch P centre to centre, the printing elements being shifted transversely with respect to said column along a printing support line and being selectively actuated during such trans-verse movement, the printer further comprising:
- means for executing a first line printing operation, through a first transverse pass of said printing elements, and the selective actuation of the lower N-1 of said printing elements, - means for advancing the printing support by a discrete amount equal to 1.5 times the vertical pitch P of said printing elements, centre to centre, and - means for executing a second printing operation of said line with a second trans-verse pass of said printing elements and the selective actuation of the upper N-1 of said printing elements.

4. A matrix serial printer according to claim 3, wherein said advancing means comprises a step motor.

5. A matrix serial printer, as claimed in claim 2, and having nine printing elements, comprising central processor with 8-bit parallelism, a programmable gate having a plurality of output registers, loadable by said processor and having a total parallelism greater than eight bits, a control memory having an 8-bit parallelism and storing programs and character description tables, wherein said control memory stores first character description tables for the selective control of the lower set of eight of said nine elements during said first line printing operation and second character description tables for the selective control of the upper set of eight of said nine elements during said second line printing operation.

6. A high quality printing method utilizing a matrix serial printer provided with N printing elements in a column arranged with a vertical pitch P centre to centre, the printig elements being shifted transversely to said column along a printing support line and being selectively actuated during said transverse movement, the printer com-prising means for advancing said printing support in the direction of said column of elements for a discrete multiple quantity of an elementary advance equal to 1.5 times the vertical pitch of said elements comprising:
- executing a first line printing operation through a first transverse pass of said elements;
- advancing of said printing support in the direction of said elements in said column;
- executing a second printing operation of said line with a second transverse pass of said elements;
- said element actuation being effected by a command of a first and second succession of binary codes of N-1 bits, each succession being contained in a different table of a character description memory and read out one at a time from said memory; the first printing operation with a first pass being performed in response to each of the N-1 binary code bits of said first succession with each of the lower N-1 elements in said N elements column and the second printing operation being performed in response to each of the N-1 binary code bits of said second succession with each of the upper N-1 elements in said N elements in the column.