Method of improving the printing speed of an impact dot printer printing in a near letter quality mode

A method of operating an impact printer having a printhead normally intended to create letter quality characters during one pass across the printing medium at a carriage speed at least three times slower than the carriage speed at which the printer creates draft quality characters wherein the method of operation produces letter quality characters at least 50% faster than the prior art method of operation. To accomplish the throughput increase, the printer is operated in the same faster draft speed for the printing of both draft and letter quality characters. Letter quality characters, however, are created in two passes and the printing medium is not advanced to a next row position until after the two passes. During the first pass, the carriage is moved across the printing medium while printing one of each pair of horizontally close adjacent dots includes in the vertical strokes of the characters comprising a row of characters and a portion of other dots included in the row of characters. During the second pass, the other of each pair of horizontally close adjacent dots included in the vertical strokes of the characters comprising the said row of characters and any remaining unprinted other dots are printed.

BACKGROUND OF THE INVENTION 
This invention relates to impact dot printers employed for printing 
alphanumeric characters on a printing medium and, more particularly, to 
printing logic for use in an impact printer having a carriage moveable 
across a printing medium in passes from side to side, motor drive means 
for selectably moving the carriage, a printhead including a plurality of 
vertically oriented printwires disposed for printing a row of draft 
quality characters in one pass at a first carriage speed or a row of 
letter quality characters including vertical strokes of a width requiring 
pairs of horizontally close adjacent dots in one pass at a second carriage 
speed which is about three times slower than the first carriage speed, 
printing medium advancing means for advancing the printing medium from one 
printing row position to a next printing row position, and printing logic 
connected to the motor drive means, for moving the carriage across the 
printing medium, the printhead, and the printing medium advancing means 
for causing the printhead to print dots on the printing medium with the 
printwires, and for advancing the printing medium from one printing row 
position to a next printing row position, the logic of the invention being 
adapted for increasing the throughput of the printer when printing letter 
quality characters and comprising logic for performing the steps of: 
(a) deciding if the printer is to print draft quality or letter quality 
characters and if it is to print draft quality characters going to step 
(b), otherwise continuing to step (e); 
(b) moving the carriage across the printing medium at the faster first 
carriage speed while printing all the dots of the characters comprising a 
row of characters; 
(c) advancing the printing medium to a position for the printing of a next 
row of characters; and, 
(d) if the printer is finished printing an indicated number of rows of 
characters to be printed, then exiting, otherwise returning to step (b); 
(e) moving the carriage across the printing medium in a first pass at the 
faster first carriage speed while printing one of each pair of 
horizontally close adjacent dots included in the vertical strokes of the 
characters comprising a row of characters and a portion of other dots 
included in the row of characters; 
(f) moving the carriage across the printing medium in a second pass at the 
same faster first carriage speed while printing the other of each pair of 
horizontally close adjacent dots included in the vertical strokes of the 
characters comprising the row of characters and any remaining unprinted 
other dots included in the row of characters; 
(g) advancing the printing medium to a position for the printing of a next 
row of characters only after the first and second passes; and, 
(h) if the printer is finished printing an indicated number of rows of 
characters to be printed, then exiting, otherwise returning to step (e). 
As described in the parent application of which this is a 
continuation-in-part, U.S. Pat. No. 4,159,882 by Sanders et al. describes 
the first proposed multipass printing which, in some form, most of today's 
9-pin printhead impact dot printers utilize. That patent teaches that by 
making two, three, or four passes with the printhead and with appropriate 
vertical motion between the printhead and the paper being printed on of, 
respectively, 1/2 pin spacing, 1/3 pin spacing or 1/4 pin spacing between 
passes, increased vertical resolution in the printed character could be 
obtained. This allowed the first Near Letter Quality (NLQ) and Letter 
Quality (LQ) characters to be printed by impact dot matrix printers. In 
such a system, there is no serious problem relative to increasing the 
horizontal resolution. As is readily known and appreciated by those 
skilled in the art, horizontal resolution (and character quality, 
including such features as bold typeface) can be created by simply doing 
more passes and/or partial vertical paper movement with a printhead having 
a relatively small number of pins. 
As with most things involving computing, mechanical dot printing (involving 
for, example, impact, electrostatic, ink jet, etc. printheads) is a 
tradeoff between cost of materials (i.e. apparatus), print quality, and 
speed of throughput. In general, the impact type printhead have design 
limiting considerations with respect to speed of dot creation; that is, of 
the above-listed examples of dot creating printers, only the impact 
printer create their dots with a mechanical driving mechanism. In such 
printers, each dot is created by one end of a fine printwire being struck 
or pushed by a solenoid type actuator. The other end of the printwire is 
then pushed out of a printhead and strikes the printing medium through an 
inked ribbon which causes the ribbon at that point to create a dot on the 
printing medium. The limiting factor is the so-called "refire" rate, i.e. 
the time it takes for the mechanical printwire/actuator combination to 
strike one dot and then recycle to "fire" and create the next dot. 
In an effort to improve the speed and quality of impact type printers, 
single pass printheads were introduced. Such a prior art printhead is 
depicted in simplified form in FIG. 1 where it is generally indicated as 
10. The printhead 10 has a body containing two staggered rows of 
printwires 14. Other types of printwire arrangements are available; 
however, the principle of operation is the same as in the representative 
example of FIG. 1. The printheads are referred to by the number of 
printwires 14 (or "pins") they contain. Most often, such printheads 
contain eighteen or twenty-four pins. The 18-pin printheads are typically 
made with 11, 12 and 14 mil pins (i.e., printwires 14). The majority of 
24-pin printheads are made with 8 mil pins as this type of printer was 
originally developed for Asian characters such as Kanji or Hanguel. These 
characters need the fine 10 mil strokes made by 8 mil pins because of 
their much greater character complexity. The 8 mil pins create a 10 mil 
dot on the print medium because of the intervening ribbon through which 
the printwire contacts the print medium, as described above. 
When the above-described printheads are incorporated into printers intended 
for printing non-Asian characters as employed in the Western world, the 
stroke width must be increased. This, in turn, leads to serious print 
speed problems. Currently, 24-pin printers, such as the Toshiba model 
321SL, print Letter Quality with one pass of the printhead at 6 inches per 
second (IPS) carriage speed, i.e. the transit speed at which the printhead 
10 is moved laterally across the printing medium. The printer can print 
very good quality characters at 10 characters per inch (CPI), producing 60 
characters per second (CPS). The dots are printed at 180 dots per inch 
(DPI) at this speed, although the horizontal resolution is 1/360". These 
printers are able to generate vertical strokes of 10, 15.5 or 21 mils 
width. The same quality letters could be printed at 120 DPI, which would 
increase the speed 50%, except that it would make the vertical strokes 10, 
18.3 or 26.6 mils, i.e. either too thin or too thick for non-Asian 
characters. 
The problem is depicted in FIGS. 3-5. As the printhead moves across the 
printing medium from, for example left to right as depicted in FIG. 3, the 
printwires 14 can only be fired after the refire time has elapsed since 
the last firing thereof. As depicted in FIG. 2, the vertical rows of 8 mil 
printwires 14 must be spaced from one another by integer multiples of the 
horizonal resolution. Thus, for example, the vertical rows of printwires 
14 may be on 4/360" centers as depicted in the figure. This aspect is not 
critical to the problem being described, however. It is the refire time 
vis-a-vis the carriage speed that causes the problem. As depicted in the 
left portion of FIG. 5, the adjacent vertical rows of printwires 14 may be 
fired at slightly different times (depending on the horizontal spacing 
between the vertical rows) so as to fire along a common vertical line 16 
and thereby create a vertical row 18 of over lapped dots 20 as depicted in 
the center portion of FIG. 5. As graphed in FIG. 4, given a fixed refire 
rate, the faster the printhead 10 traverses the printing medium, the 
greater the distance between the adjacent vertical lines 16 along which 
the dots 20 can be created. At a typical "draft speed", the dots 20 are 
spaced apart 1/16" between centers and 6.6 mils between dot edges as 
depicted in the center portion of FIG. 5. To get the LQ or NLQ character 
definition in one pass, the carriage speed must be drastically reduced to 
1/3rd the draft speed giving a spacing between centers of 1/180" with a 
dot overlap of 2 mils as indicated in the graph of FIG. 4. This speed 
reduction is typically a factor of three, or even four; that is, in the 
NLQ or LQ mode, the printer operates at one-fourth to one-third the 
carriage speed that it employs in the draft mode. 
Wherefore, it is the object of the present invention to provide a novel 
method of operating a 24-pin printhead, or the like, which will provide a 
substantial increase in the print speed of such printheads when printing 
in the NLQ or LQ mode. 
Other objects and benefits of this invention will become apparent from the 
description which follows hereinafter when taken in conjunction with the 
drawing figures which accompany it. 
SUMMARY 
The foregoing object has been achieved in an impact printer having a 
carriage movable across a printing medium in passes from side to side, 
motor drive means for selectably moving the carriage, a printhead 
including a plurality of vertically oriented printwires disposed for 
printing a row of draft quality characters in one pass at a first carriage 
speed or a row of letter quality characters including vertical strokes of 
a width requiring pairs of horizontally close adjacent dots in one pass at 
a second carriage speed which is around three times slower than the first 
carriage speed, printing medium advancing means for advancing the printing 
medium from one printing row position to the next printing row position, 
and printing logic connected to the motor drive means, for moving the 
carriage across the printing medium, the printhead, and the printing 
medium advancing means for causing the printhead to print dots on the 
printing medium with the printwires, and for advancing the printing medium 
from one printing row position to a next printing row position. This 
describes physically what exists in just about all 24 pin printers. The 
improved method of operation of the present invention for increasing the 
throughput of the printer when printing letter quality characters 
comprises the steps of: 
(a) moving the carriage across the printing medium in a first pass at the 
faster first carriage speed while printing one of each pair of 
horizontally close adjacent dots included in the vertical strokes of the 
characters comprising a row of characters and a portion of other dots 
included in the row of characters; 
(b) moving the carriage across the printing medium in a second pass at the 
same faster first carriage speed while printing the other of each pair of 
horizontally close adjacent dots included in the vertical strokes of the 
characters comprising the row of characters and any remaining unprinted 
other dots included in the row of characters; and, 
(c) advancing the printing medium to a position for the printing of a next, 
row of characters only after the first and second passes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The object of the invention is attained by operating, for example, a 24-pin 
printhead in a manner exactly opposite its intended manner of operation as 
instructed by the manufacturers thereof. By so doing, the inventor herein 
was able to surprisingly and unexpectedly attain a 50% (or even greater) 
increase in the printing speed of NLQ and LQ characters by such a 
printhead. 
Consider the typical prior art impact printer 22 of FIG. 6 including 
carriage 24 having a printhead 10 such as the printhead of FIG. 1 with a 
row 18 of vertically oriented printwires 14 capable of printing dots of 
diameter "d" and having driving assembly for moving the carriage 24 back 
and forth across the printing medium 32 including a drive motor 26 
connected to the carriage 24 through a drive belt 28 and a printing medium 
advance mechanism 34 wherein the drive motor 26, printhead 10, and 
printing medium advance mechanism 34 are connected to printing logic 30 to 
be controlled thereby. The typical prior art impact printer 22 is driven 
by the logic 30 in two modes--draft mode and LQ (or NLQ) mode. If the 
refire rate is "r" milliseconds and it is desired to create vertical 
strokes having a width of 2d in LQ characters, the carriage speed "lqs" in 
the LQ mode must be such that r.lqs=d; that is, the printhead 10 will only 
move from the vertical position of one row of dots it has created to a 
position one dot width, i.e. "d", away when it can next refire to create 
the adjacent row of dots for a total dot width of 2d. Since the quality of 
draft mode characters is less important and speed is the prime factor of 
importance, the carriage speed in the draft mode may be several times the 
LQ speed, e.g., the draft quality speed "dqs" could equal 31 qs. 
From the introduction of such printer, the above-described process has been 
the accepted mode of operation; that is, the manufacturers state that such 
printers have two speeds, draft speed and LQ speed, and one is stuck with 
a LQ speed which will create the desired vertical stroke width at the 
refire rate of the printhead incorporated into the printer. This well 
accepted prior art printing method is shown in the flowchart of FIG. 7. 
Note that once the decision is made as to whether the printer 22 is in the 
draft or LQ mode, the logic is basically the same, each row of characters 
is printed in one pass at the proper carriage speed to affect the desired 
vertical stroke width. In this regard, it is important to note that the 
prior art method of driving a printer 10 treats aspects such as 
underlining, bold typeface, and the like, as a second row of characters on 
the same line; that is, the printing medium 32 is not advanced and the row 
of special characters needed to create the desired effect on the 
previously printed row is printed without moving the printing medium 32 
vertically. The present invention is based on violating the 
above-described prime directive of the printer manufacturers by employing 
the novel driving method shown in the flowchart of FIG. 8. As can be seen, 
in the method of this invention, only the draft quality carriage speed is 
employed for both the draft and letter quality modes of operation. Thus, 
not only is there a substantial increase in printing throughput in the LQ 
mode; but, additionally, a cheaper, single speed drive motor 26 can be 
employed with an attendant savings in cost of manufacture as well. 
To implement the novel method of the present invention, the logic of FIG. 8 
is employed in the printing logic 30 driving the drive motor 26, the 
printhead 10, and the printing medium advancing means 34 of a printer 22 
such as that of FIG. 6. If desired, a pre-existing printer can be 
retrofitted to practice the present invention and derive the benefits 
thereof by retaining the mechanical components and merely changing the 
logic within the printing logic 30. As can be seen from FIG. 8, the logic 
makes the same first decision as in the prior art flowchart of FIG. 7; 
that is, the logic first determines if it is in the draft or LQ mode. If 
it is in the draft mode, it prints the rows of characters at draft speed 
just as in the logic of FIG. 7. If, however, it is in the LQ mode, a major 
deviation from the prior art approach of FIG. 7 takes place. Referring to 
FIG. 9, operating at draft speed (instead of the substantially slower LQ 
speed of the prior art method), the printing logic 30 causes the printhead 
10 to print a row of half characters (where the term "half character" 
refers to the vertical strokes of a line of characters and every other dot 
in the horizontal lines 9a and 9b). In other words, the logic 30 prints 
half the dots in all the vertical strokes in the row on a first transition 
of the carriage 24 across the printing medium 32 as shown in FIG. 9a and 
9b. The dots comprising the other aspects of the row of characters can be 
done at this time or during the second pass to follow, as desired. In a 
second pass across the printing medium 32 (again at the faster draft 
speed), the logic 30 causes the printhead 10 to produce the balance of the 
half characters (i.e. the second row of adjacent dots 20 to create the 
desired width of vertical strokes) as well as any remaining portions of 
the characters of the row not printed on the first pass as shown in FIG. 
9b and 9c. The resultant character is shown in FIG. 9e and 9f. Then, and 
only then, does the logic 30 cause the printing medium 32 to advance to 
the next row to be printed. As in the prior art of FIG. 7 and as described 
above, if character enhancements are required to the now printed row of 
characters (such as bold or underlining) this is accomplished by printing 
another row of "characters" on the same vertical position employing a 
third (or even fourth) pass as necessary to create the desired effect. 
At the printing speed of contemporary impact printers, the facts upon which 
this invention is based are not readily obvious or apparent. All that one 
watching the operation of such a printer is aware of is that there is a 
difference between the draft and letter quality speeds and that the 
carriage of the printer is moving quite rapidly. Typically, such printers 
work bidirectionally; that is, they print in both directions and there is 
no dead time during "carriage return" as in most typewriters and single 
direction printers. The above-described novel method of operation only 
became apparent to the inventor herein when he was taking detailed timing 
measurements related to the operation of impact printers with electronic 
instrumentation for other purposes and he suddenly recognized and realized 
that with the improved turnaround times being achieved (i.e. the time for 
the carriage 24 to reverse direction), the time for the printhead 10 to 
make two printing passes over the printing medium 32 at draft speed was 
still less than the time for the printhead 10 to make one pass at LQ 
speed. Quite surprisingly (and completely unexpectedly as confirmed by 
subsequent conversations on the subject with printer manufacturers) while 
the single pass at LQ speed took time "3t", for example, each of the two 
passes at three times the speed took time t, or a total of 2t for the two 
passes. Thus, the inventor herein realized that the printing of the 
characters in LQ mode could be affected at draft speed in two-thirds the 
elapsed time of the prior art method--a 50% increase in speed--by printing 
them in two passes instead of the tradition one pass that those skilled in 
the art assumed was the fastest approach. Obviously, where the difference 
between the LQ and draft speed is greater, the time savings (and, 
therefore, attendant percentage improvement in printer performance) is 
also greater. 
Thus, it can be seen from the foregoing description that by deviating from 
standard and accepted practice and adopting the novel two pass method of 
operation of the present invention, a substantial increase in printer 
performance in impact printers employing printheads designed for single 
pass use can be realized.