GB2249203A - Laser printer scanning device with inhibition of laser emission to avoid printing outside nominal print area - Google Patents

Abstract

A laser scanning device has a counter 28 for counting clock pulses in response to which drive signal is applied to generate a laser beam for scanning a photoconductive drum according to a batch of dot data read out from a memory 22 for each line of image, to form a latent image on the drum, and further includes an arrangement 29, 25 for inhibiting generation of the laser beam, during scanning outside the nominal print area, in response to the count. A horizontal synchronization signal is generated 27 to reset the counter 28. <IMAGE>

Description

LASER SCANNING DEVICE FOR LASER PRINTER, HAVING MEANS FOR INHIBITING LASER EMISSION TO AVOID PRINTING OUTSIDE NOMINAL PRINT AREA The present invention relates generally to a laser scanning device for a laser printer, and more particularly to a technique for avoiding the formation of electrostatic latent image outside the nominal recording area on a photoconductive drum.

For printing characters such as letters and symbols on a recording medium, there have recently been available a laser printer and a telecopier or facsimile equipment, which uses a read-only me;nory or other non-volatile storage medium for storing outline data representative of the outlines of the characters, and a data converting device adapted to process the outline data according to a selected character size and convert the thus processed outline data into corresponding dot data representative of image dots, which collectively define the characters having the selected size. The dot data are used to operate a laser scanning device for scanning a photoconductive drum so as to image-wise expose local spots on the drum, to laser beams which correspond to the image dots represented by the dot data, whereby electrostatic latent image is formed on the photoconductive drum.The thus formed latent image is developed by a developing device into visible image on a recording medium.

A laser scanning device to which the present invention is applicable is shown in Fig. 1, wherein a laser beam B produced by a semiconductor laser 3 is incident upon a polygon mirror in the form of a hexagon mirror 5, through a collimator lens 4 disposed between the laser 3 and the mirror 5. The hexagon mirror 5 has six reflecting faces by which the incident laser beam B is reflected toward a photoconductive drum 8. Since the polygon mirror 5 is rotated at a predetermined constant speed, the laser beam B reflected by each reflecting face of the mirror 5 is deflected over a predetermined angular range as indicated by arrow E in Fig. 1, whereby the surface of the drum 8 is scanned in the axial direction as indicated by arrow A in Fig. 1, with the laser beam B, which is incident upon the drum surface through a focusing lens 6 and a cylindrical lens 7.Thus, the photoconductive drum 8 is scanned repeatedly by deflection of the laser beams B reflected by the different reflecting faces of the mirror 5, while the drum 8 is rotated at a constant speed in the direction indicated by arrow F. With the local spots on the drum 8 selectively irradiated with the laser beam B according to the dot data, an electrostatic latent image is formed on the drum.Within the angular scan range E of deflection of the laser beam B, there is disposed a small-sized reflecting mirror 9 such that the mirror 9 is aligned with scan position PO on the drum 8, so that the laser beam B reflected by the mirror 9 is incident upon a photosensitive element 10, which in turn generates an electric signal for triggering a suitable circuit to produce a horizontal synchronization signal to be applied to a control device, which includes a video control circuit and a DC control circuit necessary to control the laser scanning device.

When the control device receives each horizontal synchronization signal, a batch of dot data representative of image dots for one line is transferred from a page buffer into a serial line buffer (scan buffer), during a data transfer period between the moment corresponding to scan position Pl slightly spaced from the above-indicated scan position PO in the scanning direction A, and the moment corresponding to scan position P2 from which the image-wise scanning or exposure of the drum 8 is started.The local spots between the above-indicated scan position P2 and scan position P3 (which defines the end of the nominal recording length P2-P3) are selectively exposed to the laser beam B according to the batch of dot data, which are sequentially read out from the serial buffer one bit after another, each bit corresponding to a local spot on the drum, beginning with the first bit stored at the first address of the serial buffer, in response to clock pulses generated by an oscillator circuit. The batch of dot data is applied to a laser driver circuit, which generates a laser control signal according to the bits of the received dot data, and the semiconductor laser 3 is selectively turned on and off according to the laser control signal, so that the local spots on the drum 8 are selectively irradiated with the laser beam B.As a result, electrostatic latent image corresponding to one line of dots is formed on the photoconductive drum 8. The above operation is repeated according to a batch of dot data for one page of characters.

A predetermined number of horizontal scanning operations corresponds to one line of characters.

For the horizontal synchronization signal to be generated after the formation of electrostatic latent image for each line on the drum 8, the control device generates a compulsory laser emission signal when the hexagon mirror 5 has rotated to a position corresponding to scan position P4 which is aligned with the end of the angular scan range E.

This compulsory laser emission signal remains active until the mirror 5 has rotated to a position corresponding to the scan position Pl indicated above, so that the laser beam B is incident upon the reflecting mirror 9, to assure the generation of the horizontal synchronization signal before each scanning operation in the direction A. It is noted that the serial buffer has a memory capacity corresponding to the number of the local spots within the nominal recording range P2-P3.

As described above, the electrostatic latent image for one line is formed on the photoconductive drum 8 over the nominal recording range between the scan positions P2 and P3, according to a batch of dot data currently stored in the serial buffer. However, the stored dot data which have been used for the formation of the latent image are again read out from the serial buffer, starting with the dot data stored in the first address, during a time period between the moment corresponding to the scan position P3 and the moment corresponding to the scan position P2 at which a batch of dot data for the next line has been stored in the serial buffer.Namely, the pointer to designate the address of the serial buffer designates the first address of the buffer immediately after the dot data bit stored in the last address of the buffer has been read out, that is, immediately after the laser beam B has reached the scan position P3. Consequently, the portion of the drum 8 between the scan positions P3 and P4 and the portion of the drum between the scan position Pl (determined by the right end of the mirror 9) and the scan position P2 are image-wise exposed according to the dot data already used for the image-wise exposure of the nominal recording area of the drum 8, whereby electrostatic latent image is unnecessarily and undesirably formed on those portions of the drum 8. This latent image is also developed by the developing device into visible image on the recording medium, in the left margin area between the scan position Pl (corresponding to the left edge of the medium) and the scan position P2, and in the right margin area between the scan position P3 and the scan position P4 (corresponding to the right edge of the medium).

This visible image in the margin areas appears as stains. If the serial buffer has a large memory capacity enough to store a batch of dot data corresponding to the number of local spots between the scan positions P2 and P4 (right edge of the recording medium P), however, the bits of the dot data for the right margin area between P3 and P4 are set to "0" indicative of the absence of image dots, i.e., a blank space, and the reading of the dot data corresponding to the right margin area after the laser beam B has reached the scan position P3 will not cause image dots to be printed in the right margin area. In this case, only the left margin area is undesirably stained by image dots based on the dot data bits stored in the leading portion of the serial buffer.

It is therefore an aim of the present invention to provide a laser scanning device for a laser printer, which is adapted to avoid the formation of electrostatic latent image outside the nominal recording area on a photoconductive drum of the printer.

According to the present invention, there is provided a laser scanning device comprising: (a) laser generating means for generating a laser beam; (b) scanning means for deflecting said laser beam over a predetermined angular range, to repeat a scanning operation for image-wise exposing a photoconductive drum in a scanning direction parallel to a length of the drum; (c) oscillating means for generating clock pulses; (d) memory means for storing a batch of dot d'ata for one line of image at a time; (e) laser-emission control means, operable for each scanning operation by said scanning means, for reading out said batch of dot data from said memory means in response to said clock pulses, and selectively turning on and off said laser generating means according to said batch of dot data; (f) photosensitive means for generating a horizontal synchronization signal upon incidence of said laser beam thereupon; (g) counting means for counting the number of said clock pulses and producing an output indicative of a count of said number, said counting means being reset upon reception of said horizontal synchronization signal; and (h) inhibiting means, response to said output of said counting means, for inhibiting said laser generating means from being turned on to generate said laser beam, during a time period between a first moment at which said count of said counting means is equal to a first value indicative of a first scan position spaced in said scanning direction from a position at which said horizontal synchronization signal is generated, and a second moment at which said count is equal to a second value indicative of a second scan position at which the formation of said latent image is started.

In the laser scanning device for a laser printer of the present invention constructed as described above, the photosensitive means generates the horizontal synchronization signal upon reception of the laser beam, during each scanning operation in which the laser beam generated by the laser generating means is deflected by the scanning means to image-wise expose the photoconductive drum for forming a latent image corresponding to the batch of dot data stored in the memory means. When the laser-emission control means receives the horizontal synchronization signal the laser-emission control means stores a batch of dot data for the next line of image into the memory means.

Upon completion of storage of this batch of dot data, the laser-emission control means starts controlling the laser generating means according to the batch of dot data, for selectively turning on or off the laser generating means data, in response to the clock pulses generated from the oscillating means.

After the bit of the dot data batch stored at the last address of the memory means has been read out at the end of a scanning operation according to this batch of dot data to form a latent image for one line over the nominal recording area, the laser-emission control means reads out, from the first and subsequent addresses of the memory means, the corresponding bits of the dot data which have been used for the last scanning operation. Namely, the dot data stored in the memory means is continuously read out as long as the clock pulses are generated. Consequently, a latent image is formed also in the left margin area outside the nominal recording area, that is, between the left end of the photoconductive drum, and the left end of the nominal recording area at which the normal image-wise exposure of the drum is started.The latent image formed in the left margin area on the drum causes undesirable printing of image dots as stains in the corresponding left margin area of a recording medium when the latent image on the drum is developed into a visible image on the recording medium.

To avoid the above drawback, the present laser scanning device is equipped with the counting means and the inhibiting means. The counting means counts the number of the clock pulses generated from the oscillating means, and the count is reset when the horizontal synchronization signal is generated. The inhibiting means operates in response to the output or current count of the counting means, so that the laser-emission control means is inhibited from turning on the laser generating means, during the time period between the moments at which the count becomes equal to the first and second values, i.e., between the first moment immediately after the horizontal synchronization signal is generated, and the second moment at which the normal image-wise exposure of the drum is started.

Accordingly, the latent image will not be formed in the left margin area of the photoconductive drum which precedes the nominal recording area. Although a latent image is formed in an area near the left end of the photoconductive drum which is to the left of the position corresponding to the first value of the count of the counting means, the width of the recording medium does not cover this left end area of the drum, whereby the recording medium is not undesirably stained. That is, the area of the drum corresponding to the left margin of the recording medium is not undesirably image-wise exposed based on the dot data bits which have been used for the last line.

The present laser scanning device is adapted to inhibit the laser-emission control means from turning on the laser generating means to generate the laser beam, from the moment corresponding to the first count value of the counting means, to the moment corresponding to the second count value, whereby the latent image is formed according to the batch of dot data stored in the memory means, over the nominal recording length of the photoconductive drum, which corresponds to the nominal recording area of the recording medium. Thus, the present scanning device does not suffer from the conventionally experienced staining of the margin area of the medium after the latent image on the image-wise exposed drum is developed into a visible image.

The present invention will further be described hereinafter with reference to the following description of exemplary embodiments of the invention and the accompanying drawings, in which: Fig. 1 is a schematic plan view of a laser printer, to which the present invention is applicable; Fig. 2 is a schematic view including a block diagram showing a control arrangement according to one embodiment of a laser scanning device of the present invention; Fig. 3 is a time chart indicating various signals generated in the laser scanning device of Fig. 2; and Fig. 4A and 4B are a flow chart schematically illustrating a laser scanning control routine executed in another embodiment of the laser scanning device of this invention.

Referring first to Figs. 1 and 2, there is shown a laser scanning device constructed according to one embodiment of the present invention, which is indicated generally at 1 in Fig. 1. This laser scanning device 1 employs an optical system similar to that used in the known arrangement as described above. Briefly, the laser scanning device 1 has a base plate 2 on which are disposed: the semiconductor laser 3 for generating the laser beam B; the collimator lens 4; a scanner motor 23 (Fig. 2) for rotating the hexagon mirror 5 in the predetermined direction F at a predetermined constant speed; the focusing lens 6 disposed between the mirror 5 and the photoconductive drum 8; and the cylindrical lens 7 disposed between the focusing lens 6 and the drum 8.The photoconductive drum 8 is disposed on the rear side of the cylindrical lens 7, extending in parallel with the length of the cylindrical lens 7. This drum 8 is rotated at a predetermined constant speed, by a cartridge-contained drum drive motor 24. The collimator lens 4 functions to adjust the laser beam B into parallel rays, while the focusing lens functions to focus the laser beam B reflected from the mirror 5 to be focused on the cylindrical surface of the photoconductive drum 8, irrespective of the varying length of the optical path from the mirror 5 to the different local spots on the drum 8.The cylindrical lens 7 functions to make necessary compensation for a possible variation in the relative angle of the different faces of the hexagon mirror 5, so that the laser beams B reflected by any faces of the mirror 5 irradiate the same circumferential position of the drum 8 (same position as viewed in the direction perpendicular to the plane of the base plate 2 and the axis of the drum).

In the optical system described above, the laser beam B produced by the semiconductor laser 4 is incident upon the mirror 5 through the collimator lens 4, for reflection toward the photoconductive drum 8. The reflected laser beam B is incident upon the surface of the drum 8 through the focusing lens 6 and cylindrical lens 7. With the hexagon mirror 5 rotated, the laser beam B is deflected over the predetermined angular range E, so that the drum 8 is repeated scanned in the predetermined scanning direction A.

The semiconductor laser 3 is selectively turned on and off according to the dot data, so that the local spots on the drum 8 are selectively exposed to the laser beam B, so as to form electrostatic latent image represented by the dot data.

The scanning range E covers the nominal recording area P2-P3 on the drum 8.

On the base plate 2, the reflecting mirror 9 is disposed within the angular scanning range E, such that the mirror 9 lies on a straight line connecting the hexagcn mirror 5 and the scan position PO on the photoconductive drum 8, which position PO is adjacent to the left end of the drum 8. Also disposed on the base plate 2 is the photoconductive element 10 adapted to receive the laser beam B which is reflected by the reflecting mirror 9. With the beam B incident upon the element 10, a horizontal synchronization signal HSYN having a high level is applied to a controller 20 of the laser scanning device, as described below.

There will next be described a control system for the laser scanning device, which includes the controller 20 as shown in Fig.2.

The controller 20 is similar to a controller used for the known laser scanner, which includes a video controller for generating a video signal and a DC control circuit for controlling the scanner motor 23 and the drum drive motor 24. The controller 20 includes an oscillator circuit 21 which generates a timing clock signal CL having a predetermined frequency (about 10MHz, for example), and a serial line buffer 22 adapted to store a batch of dot data for one line at a time. The memory capacity of the serial buffer 22 is determined to store multiple bits of dot data corresponding to the local spots on the drum 8, within the nominal recording length defined by the scan positions P2 and P3, at which the image-wise exposure of the drum 8 is started and ended.The controller 20 successively reads out from the serial buffer 22 the individual bits of dot data one after another in response to the clock signal CL generated by the oscillator circuit 21. As a result, a video signal Sv indicative of the presence or absence of an image dot is generated from the controller 20, for each bit of dot data read out from the serial buffer 22.

The video signal Sv is applied to a laser driver circuit 26 through an AND gate 25, so that the laser driver circuit 26 applies to the semiconductor laser 3 a laser control signal Sc for selectively turning on or off the semiconductor laser 3 according to the level of the video signal Sv, whereby the laser beam B is generated or not generated according to the level (high or low) of the laser control signal Sv. When the angular position of the hexagon mirror 5 corresponds to the scan position PO on the drum 8, the laser beam B generated by the laser 3 and reflected by the mirror 5 is reflected by the reflecting mirror 9 and is received by the photosensitive element 10. As a result, the element 10 applies an electric signal to a synchronization signal generator 27, which in turn generates the horizontal synchronization signal HSYN having a high level.This signal HSYN is applied to the controller 20, and to a reset terminal of an increment counter 28. This counter 28 is adapted to count the number of the pulses of the clock signal CL. Upon generation of the horizontal synchronization signal HSYN, the counter 28 is cleared. Namely, the count of the counter 28 is reset to zero.

The control system for the laser scanning device 1 also includes a discrimination circuit 29, which consists of an AND gate or flip flop circuit adapted to operate depending upon the count of the counter 28. Described in detail, the discrimination circuit 29 applies a emission-inhibit signal Sj having a low level to the AND gate 25, during a time period between the moments at which the count of the counter 28 is equal to N1 and N2, respectively, and a time period between the moments at which the count of the counter 28 is equal to N3 and N4, respectively. The count N1 indicates the scan position P1 slightly to the right of the scan position PO which corresponds to the moment at which the level of the horizontal synchronization signal HSYN from the generator 27 is changed to high (H).In other words, the count N1 indicates the angle of deflection of the laser beam B which corresponds to the right end of the reflecting mirror 9. The count N2 indicates the scan position P2 at which the image-wise exposure of the drum 8 is started according to the dot data already stored in the serial buffer 22. The count N3 indicates the scan position P3 at which the image-wise exposure of the drum 8 is terminated. The count N4 indicates the scan position P4 which corresponds to the right end of the angular scanning range E, i.e., the right end of the scanning operation in the direction A.

During the above-indicated periods for which the level of the emission-inhibit signal Sj is held low (L), the level of the output of the AND gate 25 is low, even if the the level of the video signal Sv is high (H) representing the presence of an image dot. Accordingly, the laser control signal Sc generated by the laser driver circuit 26 is held low (L), thereby inhibiting the laser 3 from emitting the laser beam B.

The discrimination circuit 29 is connected directly to the laser driver circuit 26, so that the circuit 29 applies to the circuit 26 a compulsory laser emission signal Ss having a high level, for a time period from the moment at which the count of the counter 28 is equal to N4, to the moment at which the count is equal to N1. The high level of the compulsory laser emission signal Ss causes the level of the laser control signal Sc to be high (H) for holding the laser 3 in the on state, to thereby continuously emit the laser beam B by the time corresponding to the scan position P1 (count N1 of the counter 28). This arrangement assures the generation of the horizontal synchronization signal HSYN (high level) in response to the laser beam B incident upon the photosensitive element 10.

Referring to the time chart of Fig. 3 in which the scan positions PO, P1, P2, P3 and P4 are indicated in the order of description in the scanning direction A, there will be described a laser scanning operation performed by the present laser scanning device 1. For the convenience' sake, the operation will be described, beginning with the generation of the horizontal synchronization signal HSYN (high level) from the synchronization signal generator 27.

As the pulses of the clock signal CL are generated from the oscillator circuit 21, the multiple bits of dot data are successively read out from the respective addresses of the serial buffer 22, as the addresses are designated by the pointer which is incremented in synchronization with the clock pulses. It is noted that the the number of pulses of the clock signal CL corresponding to the nominal recording area as shown in Fig. 3 is lower than the actual number of pulses, for easier understanding.

When the angle of deflection of the laser beam B by the hexagon mirror 5 corresponds to the scan position PO, the controller 20 and the counter 28 receive the horizontal synchronization signal HSYN (high level) from the synchronization signal generator 27. Consequently, the counter 28 is cleared, and the controller 20 operates to store a batch of dot data bits for the next line in the serial buffer 22, during the time period between the moments corresponding to the scan positions PO and P2. When the count of the counter 28 has reached N1 corresponding to the scan position P1, the discrimination circuit 29 changes the level of the compulsory laser emission signal Ss (to be applied to the laser driver circuit 26), from the high level (H) to the low level (L) level.At the same time changes the level of the emission-inhibit signal Sj (to be applied to the AND gate 25), from the high level (H) to the low level (L), whereby the output of the AND gate 25 is made low (L) until the count of the counter 28 becomes equal to N2. That is, the level of the signal Sj is held low by the time corresponding to the scan position P2. Therefore, the level of the laser control signal Sc generated by the laser driver circuit 26 is held low, inhibiting the image-wise exposure of the drum 8 according to the video signal Sv from the serial buffer 22, during the time period between the moments corresponding the scan positions P1 and P2.

When the count of the counter 28 becomes equal to N2, the level of the emission-inhibit signal Sj is changed to high (H), while at the same time the pointer to designate the addresses of the serial buffer 22 is cleared to designate the first address of the buffer 22. Consequently, the bits of dot data stored in the buffer 22 are successively sequentially read out, in response to the pulses of the clock signal CL, whereby the video signal Sv whose level corresponds to each bit of the dot data is applied to the AND gate 25, so that the laser control signal Sc whose level corresponds to that of the video signal Sv is applied to the semiconductor laser 3, for forming electrostatic latent image on the drum 8 over the nominal recording length P2-P3.

When the count of the counter 28 becomes equal to N3 corresponding to the scan position P3, the level of the discrimination signal Sj is changed to low (L). The level of this signal Sj is held low until the counter of the counter 28 becomes equal to N4 corresponding to the scan position P4. During this time period, the level of the laser control signal Sc is held low, inhibiting the image-wise exposure of the drum 8 between the scan positions P3 and P4 according to the level of the video signal Sv even if the bits of the stored dot data are read out from the serial buffer 22.

The relative position between the scan positions P2 and P3 on the drum 8 and a recording medium P is indicated in Fig. 1. As indicated in Fig. 3, the nominal recording length on the recording medium P is defined by the scan positions P2 and P3. According to the present laser scanning device 1 operated as described above, electrostatic latent image is formed only in the area of the drum 8 which is between the scan positions P2 and P3, and therefore the visible image is formed over the predetermined recording length of the medium P which corresponds to the area P2-P3 of the drum 8. Accordingly, the left and right margin areas of the recording medium P will not be stained due to the deflection of the laser beam B over the angular range E which covers the right and left margin areas of the medium P.

The control system for the laser scanning device 1 which includes the controller 20 and the discrete electronic circuit elements 25-29 as shown in Fig. 2 may be constituted by a microcomputer which operates according to a laser scanning control program stored in a read-only memory, so as to control the semiconductor laser 3 depending upon the signals CL, Sv, Sj and Ss and the count C which represents the number of pulses of the clock signal CL. An example of a laser scanning routine according to the control program is illustrated in the flow chart of Figs. 4A and 4B. The read-only memory of the microcomputer also stores data representative of the count values N1, N2, N3 and N4 which correspond to the scan positions P1, P2, P3 and P4 on the drum 8, respectively.

The laser scanning control routine of Figs. 4A and 4B is initiated when a print start command is received by the microcomputer. The control routine is started with step S10 (Fig. 4A) in which the scanner motor 23 and the drum drive motor 24 are activated by the DC controller circuit.

Step S10 is followed by step S11 in which the horizontal synchronization signal HSYN is read in. Step S12 is then executed to determine whether the level of this signal HSYN is high (H) or not. If an affirmative decision (Yes) is obtained in step S12, the count C is reset to zero in step S13. Step S13 is followed by step S14 to determine whether the count C is zero or not. If an affirmative decision (Yes) is obtained in step S14, step S15 is executed to perform an operation to store a batch of dot data for the next line into the serial buffer 22. This operation is effected based on the clock signal CL. After execution of step S15, the control flow goes back to step S11.

While the level of the horizontal synchronization signal HSYN is low (L), the control flow goes to steps S16 and Si7 to determine the level of the clock signal CL. If the level of the clock signal CL is high (H), the count C is incremented in step S18. When the count Cl is equal to N1 after the execution of step S18, namely, when an affirmative decision (Yes) is obtained in the next step S19, the control flow goes to step S20 in which the level of the compulsory laser emission signal Ss is changed from the high level (H) to the low level (L). As described above, the high level (H) of this signal Ss causes the level of the laser control signal Sc to be held high to continuously turn on the laser 3.

When the count C is equal to or larger than N1 and is smaller than N2, an affirmative decision (Yes) is obtained in step S21 (Fig. 4B), which is executed following step S20 or when a negative decision (NO) is obtained in step S19. Consequently, step S21 is followed by step S22 in which the level of the emission-inhibit signal Sj is changed to the low level (L) to maintain the laser control signal Sc at the low level (L), thereby inhibiting the laser 3 from generating the laser beam B, irrespective of the level of the video signal Sv. Step 522 is followed by step S28 in which the appropriate set of dot data is read out from the serial buffer 22 in response to the clock signal CL.Step 528 is followed by step S29 to determine whether a printing termination command indicative of the end of a printing operation is received or not. If the printing termination command is not present, the control flow goes back to step S11.

When the count C is equal to or larger than N2 and is smaller than N3, an affirmative decision (Yes) is obtained in step S23, and step S23 is followed by step S24 in which the level of the emission-inhibit signal Sj is changed to the high level (H) to thereby permit the level of the laser control signal Sc to change depending upon the level of the video signal Sv, namely, permitting the semiconductor laser 3 to generate the laser beam B depending upon the level of the video signal Sv. Then, the control flow goes back to step S11 through steps S28 and S29, unless the printing termination command is present.When the count C is equal to or larger than N3 and is smaller than N4, an affirmative decision (Yes) is obtained in step S25, and step S25 is followed by step S26 in which the level of the emission-inhibit signal Sj is changed to the low level (L), to maintain the laser control signal Sc at the low level (L), thereby inhibiting the laser 3 from generating the laser beam B, irrespective of the level of the video signal Sv. In this case, too, the control flow goes back to step S11 through steps S28 and S29, unless the printing termination command is present.

When the count C is equal to or larger than N4 or when the count C is equal to zero or larger and is smaller than N1, a negative decision (No) is obtained in steps S21, S23 and S25, and the control flow goes to step S27 in which the level of the compulsory laser emission signal Ss is changed to the high level (H), thereby maintaining the laser control signal Sc at the high level (H) to hold the laser 3 in the on state for continuously generating the laser beam B. The control flow goes back to step S11, unless the printing termination command is present.

While the present invention has been described in detail in its presently preferred embodiments, it is to be understood that the invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes, modifications and improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the following claims.

In the illustrated embodiments, the maximum recordable length of the photoconductive drum 8 ranges from the scan position P2 to the scan position P3. However, the maximum recordable length of the drum 8 may be extended such that the scan position P4 defines the right end of the recordable length. If the memory capacity of the serial buffer 22 is increased so as to store a batch of dot data for the extended maximum recordable length, the level of the emission-inhibit signal Sj is held high (H) during the time period between the moments corresponding to the scan positions P3 and P4, so that electrostatic image can be formed also in the area between the positions P3 and P4, or this normally right margin area can be made blank with the corresponding dot data bits set to represent the absence of image dots.

Claims (14)

CLAIMS:

1. A laser scanning device comprising: laser generating means for generating a laser beam; scanning means for deflecting said laser beam over a predetermined angular range, to repeat a scanning operation for image-wise exposing a photoconductive drum in a scanning direction parallel to a length of the drum; oscillating means for generating clock pulses; memory means for storing a batch of dot data for one line of image at a time; laser-emission control means, operable for each scanning operation by said scanning means, for reading out said batch of dot data from said memory means in response to said clock pulses, and selectively turning on and off said laser generating means according to said batch of dot data; photosensitive means for generating a horizontal synchronization signal upon incidence of said laser beam thereon;; counting means for counting the number of said clock pulses and producing an output indicative of a count of said number, said counting means being reset upon reception of said horizontal synchronization signal; and inhibiting means, responsive to said output of said counting means, for inhibiting said laser generating means from being turned on to generate said laser beam, during a time period between a first moment at which said count of said counting means is equal to a first value indicative of a first scan position spaced in said scanning direction from a position at which said horizontal synchronization signal is generated, and a second moment at which said count is equal to a second value indicative of a second scan position at which the formation of said latent image is started.

2. A laser scanning device according to claim 1, wherein said oscillating means continuously applies said clock pulses to said laser-emission control means at least during each scanning operation, and said laser-emission control means sequentially reads out from said memory means successive bits of said dot data in a predetermined order, in response to said clock pulses, said control means repeating sequential reading of said dot data as long as said clock pulses are generated.

3. A laser scanning device according to claim 1 or 2, wherein said laser-emission control means has a pointer to sequentially designate addresses of said memory means from which respective bits of said dot data are read out, said pointer being reset when said count of said counting means becomes equal to said second value.

4. A laser scanning device according to any one of claims 1-3, wherein said laser-emission control means includes drive means for activating said laser generating means to generate said laser beam, and drive control means for applying to said drive means a drive signal corresponding to said batch of dot data, said inhibiting means including means for inhibiting said drive control means from applying said drive signal to said drive means.

5. A laser scanning device according to any one of claims 1-4, wherein said inhibiting means applies to said emission-control means an emission-inhibit signal which holds said laser generating means in an off position, irrespective of values of bits of said dot data read from said memory means.

6. A laser scanning device according to any one of claims 1-5, wherein said inhibiting means inhibits said laser-emission control means from applying said drive signal to said drive means, also during a time period between a third moment at which said count of said counting means is equal to a third value indicative of a third scan position at which the formation of said latent image is terminated, and a fourth moment at which said count is equal to a fourth value indicative of a fourth scan position spaced a predetermined distance in said scanning direction from said third scan position.

7. A laser scanning device according to claim 6, further comprising means for applying to said drive means a compulsory laser emission signal for holding said laser generating means in an activated state to continuously generate said laser beam, during a time period between said fourth moment and said first moment, so that said laser beam is incident upon said photosensitive means to generate said horizontal synchronization signal.

8. A laser scanning device according to any one of claims 1-7, further comprising means for storing said batch of dot data for each line of image in said memory means, during said time period between said first and second moments.

9. A laser scanning device according to any one of claims 1-8, wherein said counting means and said inhibiting means are constituted by discrete electronic circuits.

10. A laser scanning device according to any one of claims 1-9, wherein said counting means and said inhibiting means are constituted by a computer which has a memory storing a control program for performing functions of said counting and inhibiting means.

11. A laser scanning device comprising: laser generating means for generating a laser beam; drive means for activating said laser generating means to generate said laser beam; scanning means for deflecting said laser beam over a predetermined angular range, to repeat a scanning operation for image-wise exposing a phodoconductive drum in a scanning direction parallel to a length of the drum; memory means for storing a batch of dot data for one line of image at a time; control means, operable for each scanning operation, for applying to said drive means a drive signal corresponding to said batch of dot data for each line of image, for activating said laser generating means to form on said photoconductive drum a latent image corresponding to said batch of dot data;; oscillating means for generating clock pulses so that said control means applies said drive signal to said drive means, in response to said clock pulses; photosensitive means for generating a horizontal synchronization signal upon incidence of said laser beam thereupon; counting means for counting the number of said clock pulses and producing an output indicative of a count of said number, said counting means being reset upon reception of said horizontal synchronization signal; and inhibiting means, responsive to said output of said counting means, for inhibiting said control means from applying said drive signal to said drive means, during a time period between a first moment at which said count of said counting means is equal to a first value indicative of a first scan position spaced in said scanning direction from a position at which said horizontal synchronization signal is generated, and a second moment at which said count is equal to a second value indicative of a second scan position at which the formation of said latent image is started.

12. A laser scanning device according to claim 11, wherein said inhibiting means includes an AND gate provided in a circuit through which said drive signal is applied from said control means to said drive means, and means for applying to said AND gate a signal having a low level, during the time period between said first and second moments.

13. A laser scanning device constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

14. A laser printer, telecopier or facsimile device incorporating a laser scanning device according to any one of the preceding claims.