U.S. patent number 3,624,661 [Application Number 04/824,419] was granted by the patent office on 1971-11-30 for electrographic printing system with plural staggered electrode rows.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Ronald F. Borelli, Michael S. Shebanow.
United States Patent |
3,624,661 |
Shebanow , et al. |
November 30, 1971 |
ELECTROGRAPHIC PRINTING SYSTEM WITH PLURAL STAGGERED ELECTRODE
ROWS
Abstract
In an electrographic printing system, a multiple row electrode
structure wherein successive rows are mutually spaced from each
other, each row including mutually spaced electrodes, the
electrodes of successive rows being positioned in a staggered
manner with respect to each other. The system further comprises
improved electrode drive circuitry, including a plurality of
high-voltage drivers and a selection matrix wherein a plurality of
passive elements are coupled to the drivers. A plurality of output
lines couple the matrix to the electrodes so as to selectively
apply a high voltage to the electrodes in order to produce a latent
image on a dielectric medium. Toning means subsequently make the
latent image visible.
Inventors: |
Shebanow; Michael S. (Medfield,
MA), Borelli; Ronald F. (Medfield, MA) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
25241365 |
Appl.
No.: |
04/824,419 |
Filed: |
May 14, 1969 |
Current U.S.
Class: |
347/141;
101/DIG.37; 346/139C; 347/142; 178/30 |
Current CPC
Class: |
B41J
2/395 (20130101); G03G 15/325 (20130101); G06K
15/14 (20130101); Y10S 101/37 (20130101) |
Current International
Class: |
B41J
2/395 (20060101); B41J 2/39 (20060101); G06K
15/14 (20060101); G06K 15/02 (20060101); G03G
15/00 (20060101); G03G 15/32 (20060101); G03g
015/00 () |
Field of
Search: |
;346/74ES |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Military Standardization Handbook Mil-HDBK-215, 15 June 1960 page
2-30. Copy in 346/74.
|
Primary Examiner: Konick; Bernard
Assistant Examiner: Britton; Howard W.
Claims
Having now described the invention, what is claimed as new and
novel and for which it is desired to secure Letters Patent is:
1. An electrographic printing system of the kind wherein a
recording medium is moved along a path to have latent images formed
thereon by the application of a high potential across the medium
and wherein a toner is subsequently applied to the medium to make
the latent image visible, said electrographic printing system
comprising:
a. an electrode structure adjacent said path including
1. a plurality of mutually spaced rows of electrodes, successive
electrodes within each row being spaced from each other, the
electrodes of successive rows being positioned in a staggered
manner with respect to each other,
2. a single steady-state potential means disposed adjacent the
opposite side of said path and extending substantially for an
entire electrode row width for imparting a continuous potential
across the medium over the total medium area covered by said
electrode rows, and
b. electrode drive circuitry for selectively energizing each
electrode individually including
1. a character generator
2. first and second groups of high-voltage drivers connected to be
energized by said character generator,
3. a selection matrix including a first plurality of passive
elements coupled to the output of each of said first group of
high-voltage drivers, a second plurality of passive elements
coupled to the output of each of said second group of high-voltage
drivers, each of said elements coupled to one of said first group
of drivers being connected to form a common node with a separate
element coupled to one of said second group of drivers, an output
line connecting each of said nodes to one of said electrodes, each
of said output lines being selectively adapted to apply a high
voltage to its corresponding electrode in dependence upon the
output of said character generator.
2. An electrographic printing system as defined in claim 1 wherein
the spacing between successive rows of electrodes is substantially
equal to the width of each of said electrodes in a direction
transverse to said rows.
3. An electrographic printing system as defined in claim 1 wherein
each of said passive elements includes a resistor connected between
the output of a high-voltage driver and said common node.
4. An electrographic printing system as defined in claim 1 wherein
each of said passive elements includes a diode connected between
the output of said high-voltage driver and said common node, said
common node being resistively coupled to a reference potential.
5. An electrographic printing system as defined in claim 1 wherein
each of said high-voltage drivers has an input terminal and an
output terminal and further includes
a. a first transistor having base, emitter and collector
electrodes, said emitter being tied to ground and said base being
coupled to said input terminal; and
b. a second transistor having base, emitter and collector
electrodes, said last-recited emitter being connected to the
collector electrode of said first transistor, said last-recited
base being coupled to a reference potential, and said last-recited
collector being coupled to said output terminal.
6. The printing system of claim 1 wherein said steady-state
potential means is a roller employed to feed said medium along the
prescribed path.
7. The printing system of claim 1 wherein each of said electrodes
terminates in a common surface and said common surface is
convex.
8. The printing system of claim 1 wherein said steady-state
potential means is biased at a voltage of less magnitude than said
high voltage applied to said electrodes when energized and greater
than the voltage applied to said electrodes when not energized.
9. An electrographic printing system as defined in claim 1 wherein
said electrodes terminate in a common surface, each electrode
defining a working surface within said common surface, said working
surfaces being aligned with respective spaces between the
electrodes of successive rows and being dimensioned to
substantially fill said spaces.
10. An electrographic printing system as defined in claim 9 wherein
each of said working surfaces has a square shape.
11. In an electrographic printing system of the kind wherein a
recording medium is moved along a path to have latent images formed
thereon by the application of a high potential across the medium
and wherein a toner is subsequently applied to the medium to make
the latent image visible, an electrode structure adjacent said path
including, a plurality of mutually spaced rows of electrodes,
successive electrodes within each row being spaced from each other,
the electrodes of successive rows being positioned in a staggered
manner with respect to each other, means for selectively energizing
individually, and a simple steady-state potential means disposed
adjacent the opposite side of said path and extending substantially
for an entire electrode row width for imparting a continuous
potential across the medium over the width of an electrode row,
wherein said single steady-state potential means is effective to
provide a continuous potential to the medium for each successive
row of said plurality of rows. 12The apparatus as defined in claim
11, and further including electrode drive circuitry adapted to
selectively energize said
electrodes by applying a high voltage thereto. 13. The printing
system of claim 11 wherein said steady-state potential means is a
roller employed to
feed said medium along a prescribed path. 14. The apparatus as
defined in claim 11 wherein said electrode structure consists of a
pair of spaced rows of electrodes, each electrode of a row being
aligned with the space defined between the electrodes of the other
row and having substantially
the same dimension in a direction along said rows. 15. The
apparatus of claim 14 wherein each of said electrodes terminates in
a square working surface lying within a common surface, the spacing
between said rows and between adjacent electrodes within a row
being substantially equal to the
side of one said square working surfaces. 16. The apparatus of
claim 15
wherein said common surface defines a plane. 17. The apparatus of
claim 15 wherein said common surface is convex.
Description
BACKGROUND AND OBJECTS OF THE INVENTION
The present invention pertains generally to the field of image
reproduction and relates more particularly to an electrographic
printing system wherein the medium to be recorded on includes a
conductive base substrate and a dielectric layer.
There presently exist various specific techniques of image
reproduction most of which are concerned with the electrostatic
transfer of charges. Generally speaking in electrographic printing
systems, including one employing the principles of the present
invention, a latent image is formed on a dielectric medium by
placing the medium in the field established between two electrodes.
These two opposed electrodes which can assume various shapes
(round, square, character-shaped, etc.) have a high electrical
potential difference applied across them, thereby establishing the
necessary field. The latent charged image formed is in the shape of
the electrode that faces the dielectric surface of the medium.
Most electrographic printing systems can generally be categorized
into two areas with respect to electrode configurations; systems
employing character-shaped electrodes and systems employing
pin-shaped electrodes. In systems of the former type, for example,
a print drum is rotated at high speed and selected electrodes are
pulsed when the desired character is facing the dielectric surface,
causing the formation of a latent image on the medium at the area
where the character electrode was located. Associated with such
systems, however, are certain disadvantages. For instance, it has
been found necessary to provide a drum consisting of individual,
electrically insulated segments and to provide means for
commutating to each segment of the rotating character drum. Thus
there are mechanical problems associated with the rotating drum. In
addition, in order to operate at a reasonable printing speed
without causing character smear, the duration of the selection
pulse must be short and the paper has to remain stationary during
printing. This results in a printing speed limitation with a system
of this type.
In printing systems where an array of pin electrodes is employed,
various patterns, such as alphanumeric characters, can be
reproduced by selecting predetermined electrodes as the recording
medium passes these electrodes. Associated with such a system,
however, are certain problems that result in a poor quality
printout. Two of the more significant problems derive from the poor
contrast density and resolution of the finally printed characters.
The contrast (shade) density may be defined as the degree of
darkness as represented on the gray scale, while resolution may be
defined as the capability of forming perfectly shaped characters.
Theoretically, 100 percent contrast provides printed characters
which are perfectly black. The pin electrodes have to be separated
a minimum distance from each other, so that they can be selectively
activated. If this is not the case, poor density and poor
resolution of the printed character result.
Another problem associated with a pin electrode system derives from
the fabrication of the electrode array itself. Usually the
electrodes are very small in cross section and are located close
together. The structure, therefore is prone to damage and is
generally difficult to fabricate.
The cost of presently available electrographic printing systems is
relatively high. One of the important factors contributing to the
high cost of such systems is the necessity for providing one driver
per electrode. In systems requiring relatively high resolution and
shade density, the total number of electrodes may be of the order
of 200 electrodes per inch. Where a separate high-voltage driver is
required for each of these electrodes, the cost of the system
becomes inordinately high.
It is an object of the present invention to provide an
electrographic printing system which is not subject to the
foregoing disadvantages.
It is another object of the present invention to provide an
electrographic printing system which provides a printout of
improved quality with respect to resolution and shade density.
It is a further object of the present invention to provide an
electrographic printing system which is capable of operating at
relatively high speeds.
It is still another object of the present invention to provide a
novel electrode structure for such a printing system which can be
easily and accurately fabricated at relatively low cost.
It is a further object of the present invention to provide an
economical electrographic printing system which has improved
reliability.
SUMMARY OF THE INVENTION
The foregoing objects are satisfied in the present invention by
providing an electrographic printing system of the kind wherein a
recording medium has latent images formed thereon by the
application of a high potential across the medium and wherein a
toner is subsequently applied to the medium to make the latent
images visible. The printing system further comprises a multiple
row electrode structure, a character generator, and a selection
matrix which permits the use of a smaller number of high-speed
drivers than the total number of electrodes in the electrode
structure.
The printing system which constitutes the subject matter of the
present invention permits printing with improved resolution by
providing the heretofore unattainable capability of printing
between adjacent electrode areas. A staggered electrode structure
is employed in the present invention which provides the capability
of printing at a far higher shade density (up to 100 percent), than
was heretofore possible. Further advantages of the invention derive
from its ability to be inexpensively fabricated.
These and other objects of the invention as well as the features
and advantages thereof will become apparent from the following
detailed specification, when read in connection with the drawings,
in which:
FIG. 1 is a perspective drawing (partially in block form) of a
portion of a preferred embodiment of the present invention.
FIG. 2A is an end view of the electrode structure shown in FIG.
1.
FIG. 2B discloses a segment of the recording medium showing the
latent charge pattern for the letter E.
FIGS. 3A through 3E show the various stages of fabrication of a
dual electrode structure.
FIG. 4 is a block diagram of the electrode structure and its
associated drive circuitry.
FIG. 5 is a circuit diagram of one embodiment of the electrode
drive circuitry.
FIG. 6 is a circuit diagram of the driver shown in FIGS. 5 and
7.
FIG. 7 is a circuit diagram of a second embodiment of the electrode
drive circuitry.
ELECTRODE STRUCTURE
FIG. 1 is a perspective view of a portion of a preferred printing
system in accordance with the present invention, showing an
electrode structure 10 having a section thereof cut away thereby
exposing electrodes 12. For the illustrative embodiment shown, each
electrode has a substantially square cross section and terminates
in a working surface 14, which lies within a common surface in
close proximity to a recording medium 20. In one practical
embodiment, surface 14 is 0.005 inch square and the electrode rows,
as well as adjacent electrodes within a row, are separated by a
distance (d) of 0.005 inch.
In the preferred embodiment of the invention illustrated, two rows
of electrodes 15 and 17 are used. Each working surface 14 (except
those at the end of the rows), is aligned with the space between a
pair of working surfaces in the adjacent row. The spacing between
the two rows of electrodes is a function of paper speed and the
time required to print one scan line.
More specifically, a scan line can be represented as an imaginary
line on the medium having the width of an electrode, as shown in
FIG. 2B. Given an upper limit on the speed with which a row of
electrodes can be energized to print a latent image on the scan
line above it, the minimum spacing between electrode rows is
determined by the paper speed and the completion of printing of a
single scan line. Such printing must be completed before that scan
line moves above the second row of electrodes.
Rollers 16 and 18 show a means of propelling medium 20 past
electrode structure 10. Various other propelling means can be used
all of which fall within the spirit and scope of the present
invention. Medium 20 can include a conductive base substrate, such
as treated paper having affixed thereto a dielectric layer of a
prescribed thickness, usually thinner in dimension than the base
material. The dielectric side of medium 20 faces roller 16, whereas
the conductive side faces roller 18.
As previously mentioned, the generation (printing) of a latent
image occurs when a high potential is applied across the recording
medium at a predetermined place on the medium. Over this
predetermined area (the area above the electrode surface 14), an
electrostatic charge transfer takes place and the dielectric
retains this charge pattern for a sufficient period so that a toner
can be applied and fused to the medium in areas where the charge is
present. This toning step makes the latent image visible.
With the system shown in FIG. 1, a source 19 provides the high
voltage necessary for printing. This high voltage is coupled to a
roller 18 by way of a commutating brush 21, which is adapted to
apply the high voltage to roller 18, and is then applied to the
conductive side of medium 20. As the medium moves in the direction
indicated by arrow 23 at a speed of, for example, 10 inches/second,
selected electrodes are pulsed to ground by electrode drive
circuitry 24 thereby creating a charge on the dielectric surface of
medium 20 in a particular pattern. A character generator 26 is
connected to electrode drive circuitry 24 and determines the
configuration or printing pattern.
Generator 26 receives suitable electrical waveforms (not shown),
e.g. from a computer, representative of pictorial, alphanumeric, or
other information to be recorded and converts these waveforms into
timed and distributed electrical pulses which are applied to
circuitry 24. Configuration generator 26 may be a typical function
generator, such as disclosed in U.S. Pat. No. 3,289,030 to Lewis et
al.
By way of illustration, let it be assumed that printing of a latent
image is to take place in section 21 of medium 20. The portion of
section 21 that has either passed electrode structure 10, or is
above it, is shown shaded so as to indicate where the latent image
has been formed. Actually the dielectric surface of medium 20
retains the charge. For the purposes of illustration, however, the
side of medium 20 visible in FIG. 1 is shaded. Directly above
electrode structure 10, the latent image is tooth-shaped, as
shown.
At one point in time in the operation of the system, the first
electrode row 15 has been energized and the second electrode row 17
has not yet been energized. Had only one electrode row been used
for printing, the required minimum spacing between the adjacent
electrodes in a row would have yielded a printing density no higher
than approximately 60 percent. With the staggered, dual electrode
structure shown in FIG. 1, however, a printing density of up to to
100 percent can be attained. The use of this particular structure
also allows for increased resolution, particularly when printing
curved alphanumeric characters. This improved resolution is due
primarily to the fact that twice as many scans exist in the
direction of paper motion as a result of the staggered electrode
arrangement, than is the case where a single electrode row is used.
As a consequence, better character definition is provided by the
present invention.
PRINTING EXAMPLE
As an aid to a better understanding of the printing system of the
present invention, reference is directed to FIGS. 2A and 2B. FIG.
2A shows an end view of the staggered dual electrode structure
while FIG. 2B shows a segment of medium 20 on which a character has
been printed. The segment is shown dissected into imaginary cells
(elements) 30 dimensioned identically to the electrode surface 14
(0.005 inch square, for example). The entire character segment
consists of a group of cells arranged in a 16.times.25 matrix. To
provide intercharacter spacing, a 13.times.16 array of cells
defines the particular character, an E-shaped character in the
example of FIG. 2B.
The electrodes shown in FIG. 2A have been designated a through h
for electrode row 15 and a' through h' for electrode row 17. Like
designations are shown along scan line 1 of FIG. 2B. Assuming that
the electrode structure 10 remains stationary and that the segment
of medium 20 shown in FIG. 2B is about to pass over the electrodes
in the direction of the arrow showing paper motion, the following
occurs. As determined by character generator 26, for printing the
character E, when scan line 1 is positioned above first electrode
row 15 electrodes a through g are excited, but electrode h is not.
The cells designated a through g are electrostatically charged, but
cell h is not. When scan line 2 is above the first electrode row
15, the same electrodes are excited (a through g). Simultaneously,
scan line 1 is located over an interelectrode surface 25, which is
part of the aforesaid common surface. Electrode row 17 has not yet
been excited at this time.
At a later time, when scan line 3 has moved directly above the
first electrode row 15, only electrode a is excited.
Simultaneously, scan line 1 has moved above the second electrode
row 17. Electrodes a' through f' are now excited but electrodes g'
and h' are not. This procedure repeats in a manner to permit the
printing of latent images in the shaded areas of FIG. 2B. The
character generator 26 programs the excitation sequence of each
electrode row. Thus, it is possible to print alphanumeric
characters, special characters, and virtually any other desired
patterns.
Although only a segment of medium 20 is shown in the drawing, in a
practical character-printing system 132 characters may be printed
in a horizontal direction. With a possible 25 scans being used to
complete a character, 132 characters (a character line) may be
printed for every 25 scans. The number of electrodes needed to
accomplish this is determined as follows. With 132 character
positions and 16 cells per character position, there are a total of
2,112 electrodes per scan line (1,056 electrodes per electrode
row). If it is desired to print 5,000 lines of characters per
minute, with the paper moving at approximately 10 inches/second
(maximum character height of one-eighth inch), it takes 12
milliseconds to print a character line (60 sec./min. .div. 5,000
character lines/min.). If 25 scan lines per line of characters are
used, a scan line is printed in 0.48 milliseconds (12 milliseconds
.div. 25 scan lines).
In order to obtain a print of good quality, a print pulse width of
between 40 and 50 microseconds is needed. The pulse width is
determined primarily by the RC time constant of the electrographic
medium. Using a minimum pulse width of 40 microseconds, one could
use 12 (0.48 milliseconds .div. 40 microseconds) print intervals to
print a scan line. Using a pulse width of 50 microseconds, only 9.6
print intervals per scan line are required. If one chooses an
intermediate value of 11 intervals, one can use 11 intervals each
of 12 character positions. The printing pulse for this particular
example would be approximately 43.5 microseconds.
Thus, in a practical embodiment of the invention, one scan line may
include 132 character positions, and 25 scan lines will complete
the printing of the entire line of characters. The 132 character
positions are printed in 11 intervals. During each such interval,
12 character positions are printed, i.e. 192 (12.times.16) cells
are printed (corresponding to 192 electrodes). Each print interval
thus takes 43.5 microseconds and a total of 12 milliseconds is
needed to print an entire character line. With the dual electrode
row operation explained with particular reference to FIG. 2B, the
192 electrodes that are excited at one time to print the 12
character positions, physically constitute two rows each having 96
electrodes which are staggered as shown in FIG. 2A.
Electrode Fabrication
The electrode structure of the present invention can be fabricated
in various ways. The particular electrode cross section need not be
square in shape, but can instead be rectangular, circular or have
various other shapes. FIG. 3A shows a portion of a printed circuit
board 40 having two conductive copper layers 42 separated by an
insulative layer 44 of glass epoxy or like material. These three
layers are affixed together by glueing or other suitable means.
One particular way of fabricating the electrode structure is to
start with an etched printed circuit board having copper conductors
on both sides which, in a preferred embodiment, are 0.005 inch in
width and spaced 0.005 inch apart (see FIG. 3B). The techniques
used in making this etched board are similar to those used in
manufacturing conventional printed circuits. However, due to the
small width of the conductors (electrodes) and the small
interconductor spacing certain process control is required.
The first step in fabricating the electrode structure is to deposit
a photoresist on both sides of the copper-glass epoxy-copper
laminate. After suitable cleaning and drying of the board, a
negative is aligned with the coated board (one on each side) and it
is exposed to ultraviolet light. The board is then developed in a
conventional developer. The final step in obtaining the structure
of FIG. 3B is a chemical etching step. This is accomplished by
immersing the board in a warm 40 percent solution of a mild acid
such as ferric chloride. After washing to remove all traces of the
acid and drying, the board is ready for the next fabrication
step.
Subsequently the gaps between strips 52 must be filled with epoxy.
Epoxy sheets 50 are shown abutting against strips 52 in FIG.
3C.
Externally, glass epoxy sheets 46 are placed against sheets 50.
Spacers 48 determine the extent to which sheets 46 can be pressed
against sheets 50. When the assembly is heated and pressure is
applied, as shown by arrows 54, the epoxy sheets 50 melt and fill
the cavities between strips 52. After the structure has cooled, a
grinding and polishing operation takes place to obtain the final
electrode structure 57 shown in FIG. 3D. An end view of the FIG. 3D
structure is shown in FIG. 3E along with rollers 56 and medium
58.
ELECTRODE DRIVE CIRCUITRY
As previously mentioned, prior art electrographic printing systems
use a driver circuit for each electrode that is to be excited.
Because of the high voltage levels required for printing (in the
vicinity of 750 volts), sharing of the high-voltage driver circuits
proved unsuccessful heretofore.
FIG. 4 is a block diagram showing electrode drive circuitry 24 and
electrode structure 10. A preferred implementation of circuitry
that can be used as electrode drive circuitry 24 is shown in FIGS.
5, 6 and 7. Electrode drive circuitry 24 has position inputs 70 and
data inputs 72. For the practical embodiment of the invention
discussed hereinabove, the position inputs number 11, corresponding
to the 11 printing intervals, while the data inputs number 192,
corresponding to the 192 electrodes excited to print 12 character
positions. (Refer to printing example above). Output lines 74
connect from electrode drive circuitry 24 to electrode structure
10, 11 such connections being shown. However, in reality each
connection includes 192 lines capable to excitation of their
corresponding segments of electrode structure 10.
FIG. 5 disclosed one embodiment of a part of electrode drive
circuitry 24, including drivers 76, data inputs 72, position inputs
70, outputs 74 and a resistor matrix. Resistor pairs 81a, 81b
through 89a, 89b connect individually in series with their common
joining node being referred to as nodes 81 through 89 respectively.
Nodes 81 through 89 then connect externally to output line 74. The
other terminals of resistors 81a, 84a and 87a respectively connect
in common to a a driver 76, while the terminal of resistor trios
82a, 85a, 88a; 83a, 86a, 89a; 81b, 82b, 83b; 84b, 85b, 86b; and
87b, 88b, 89b each connect in common to the other drivers 76 of
FIG. 5. In this embodiment, all resistors are of approximately the
same value. The drivers that receive the data line inputs 72 are
continuously switching with each new data scan line presented. More
than one data driver can be and in most cases is, active at one
time. The drivers that receive the position inputs 70 on the other
hand, are active, one at a time.
The printing scheme of FIG. 1 used with the resistive matrix of
FIG. 5 requires that the electrode (roller 18) on the conductive
side of the medium be biased at a high voltage of, for example, 700
volts. The pin electrodes which face the dielectric side of the
medium, when switched to ground, provide the necessary high voltage
for printing.
In presently available electrographic printing systems, there is
insufficient charge established on the dielectric surface of the
medium to attract and hold the toner when the applied voltage
across the dielectric is of the order of one-half of the usual
700-volt potential. Thus, the 350-volt difference can be considered
to be a threshold value below which successful printing will not
occur.
This fact is taken advantage of in the present invention, as shown
below. The drivers shown in FIGS. 5, 6 and 7 have a binary output
of 900 volts for nonselect operation and 0 volts for select
operation. The data drivers, therefore, have either 900 volts or 0
volts at their outputs. Since only one position driver of a total
of 11 drivers is on at any one time, this driver will have an
output of 0 volts while the remainder are at 900 volts. It will be
understood that the position inputs 70 for all the position drivers
are sequentially energized by character generator 26.
Referring to FIG. 5 in particular, assume that the left position
driver 76 is selected along with the uppermost data driver 76. The
outputs from these two drivers would therefore be at ground
potential and the voltage at node 81 would be essentially ground.
In a practical embodiment, the electrode on the other side of the
medium is biased to 700 volts. The electrode associated with node
81 then prints. The nodes 85, 86, 88 and 89 are then at 900 volts
and no printing occurs. (There is actually a reverse 200-volt
potential difference across the recording medium.) The remaining
nodes, 82, 83, 84, 87 are at one-half of 900 volts or 450 volts.
The potential difference across the medium in that case is 250
volts, (i.e. 700 volts, - 450 volts) which is well below the
threshold voltage of 350 volts. Node 81 is the only one, therefore,
that has the correct potential applied thereto to facilitate
printing.
It will be understood that position inputs 70 are sequentially
energized for the drivers 76 as a result of the action of character
generator 26. As a result, the action described above will occur in
sequence, i.e. the nodes will be selected sequentially in groups of
threes, i.e. nodes 81-82-83; 84-85-86; and 87-88-89. Similarly, the
outputs 74 will be selected sequentially in accordance with the
above sequence and with the selected data input.
FIG. 6 shows a preferred circuit configuration for the driver 76.
The input at terminal 110 is a 0-volt or +15-volt signal. The input
signal is normally at ground and goes to the +15-volt level for
selection (output 128 goes toward ground for selection). A diode
112 has its cathode connected to an input terminal 110 and its
anode connected in common to the anode of a diode 116. A resistor
114 connects from the anodes of diodes 112 and 116 to a power
supply +V.sub.1. The parallel combination of resistor 118 and
capacitor 119 connect between the cathode of diode 116 and the
anode of a diode 120. The cathode of diode 120 connects to the base
of transistor 124 while resistor 122 is coupled from the base of
transistor 124 to power supply -V.sub.1. Transistors 124 and 126
connect in series with the emitter of transistor 124 grounded, the
collector of transistor 124 coupled to the emitter of transistor
126 and the collector of transistor 126 connect via resistor 130 to
high-potential supply +V.sub.2. Output terminal 128 is connected to
the collector of transistor 126. A resistor 132 ties from the base
of transistor 126 to a high-potential supply +V.sub.2, a resistor
134 connects from the base of transistor 126 to ground and a
capacitor 133 connects from the base of transistor 126 to
ground.
In operation, when driver 76 is not selected, input terminal 110 is
held at ground potential and a forward current of approximately 3.5
ma. flows through diode 112 and resistor 114. Little or no current
flows in diodes 116 or 120 and the slight negative bias on the base
of transistor 124 determined primarily by resistor 122, maintains
transistor 124 turned off. Transistor 126, which is rendered
capable of conduction by the positive bias on the base of
transistor 126 (resistors 132 and 134 in part provide the positive
bias), is maintained in its off condition because there is no path
to ground, i.e. transistor 124 is nonconductive.
When driver 76 is to be selected, the voltage applied to input
terminal 110 goes to approximately +15 volts. Diode 112 becomes
back biased, while diodes 116 and 120 conduct. Current flows from
source +V.sub.1, through resistor 114, diode 116,
resistor-capacitor pair 118, 119, diode 120 and resistor 122 to
source -V.sub.1. Due to the preselected values of resistors 114,
118 and 122 (the resistance of resistors 122 is greater than the
resistance of resistor 114 plus resistor 118), the base voltage of
transistor 124 becomes positive, thereby turning transistor 124 on.
This action is speeded up by bridging resistor 118 and by capacitor
119.
When the input signal goes positive therefore, capacitor 119
instantaneously shorts resistor 118 and transistor 124 is rapidly
saturated. This action causes transistor 126 to conduct due to the
positive base voltage established by resistors 132 and 134 and
capacitor 133. The voltage output at terminal 128 which was at
approximately +V.sub.2 (+900 volts, for example) now assumes a
value of approximately 0 volts (slightly positive). This voltage is
supplied by way of output resistor 130.
In FIG. 7 there is disclosed another embodiment of electrode drive
circuitry, corresponding reference numerals having been retained.
As shown, drive circuitry 24 includes drivers 76, data inputs 72,
position inputs 70, sequential outputs 74 and a diode-resistor
matrix. Diode pairs 91a, 91b through 99a, 99b connect individually
in series with their cathodes being connected to nodes 91 through
99, respectively. Nodes 91 through 99 then connect externally to
sequential output lines 74 and also, respectively to one side of
resistors 91c through 99c. The other terminals of resistors 91c
through 99c connect to ground potential. The anodes of the diodes
91a, 94a and 97a connect in common to a driver 76 while the anodes
of diode trios 92a, 95a, 98a; 93a, 96a, 99a; 91b, 92b, 93b; 94b,
95b, 96b; and 97b, 98b, 99b each connect in common to the other
drivers 76 of FIG. 7. The drivers that receive the data line inputs
72 are continuously switching with each new data scan line
presented. More than one data driver can be, and in most cases is,
active at one time. The drivers that receive the position inputs 70
on the other hand, are active one at a time.
Referring to FIG. 7, assume that the left position driver 76 is
selected along with the uppermost data driver 76. The outputs from
these two drivers (refer to FIG. 6) are therefore at ground
potential and the voltage at node 91 is essentially at ground,
(diodes 91a and 91b are reverse biased). With the electrode on the
other side of the medium at 700 volts, the electrode associated
with node 91 prints. The nodes 95, 96, 98 and 99 are therefore at
900 volts and no printing occurs. In practice, there is actually a
reverse 200-volt potential difference across the medium. The
remaining nodes, 92, 93, 94, 97 are at one-half of 900 volts or 450
volts. The potential difference across the medium in that case
would 250 volts (700-450 volts) which is well below the threshold
voltage of 350 volts. Node 91 is the only one, therefore, that has
the correct potential applied thereto to facilitate printing.
The present invention has been described with reference to certain
illustrative embodiments. It should be understood, however, that
modifications may be made in the apparatus described which lie well
within the scope of the present invention. For example, a structure
using more than a pair of electrode rows could be used
advantageously. If three rows were used, for instance, the
individual electrodes in each row could be spaced somewhat further
apart. Also, the voltage levels and polarities need not be as set
forth in the illustrative example. A potential difference of 700
volts may be needed for printing. However, the roller could be kept
at ground and the pin electrodes may be selectively pulsed to the
high voltage, either positive or negative. Further, the common
surface of the electrode structure can assume various shapes.
From the foregoing it becomes apparent that the apparatus of the
present invention provides an improved electrographic printing
system. The staggered multiple row electrode structure provides for
improved resolution and for the possibility of obtaining 100
percent shade density. This is particularly advantageous when
printing alphanumeric characters. The electrode drive circuitry
also furnishes additional advantages in that fewer drivers are
needed than in presently available systems, with an attendant cost
savings. Improved reliability and cost savings is also a feature of
the present invention, particularly with reference to the
above-illustrated fabrication techniques of the electrode
structure.
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