US3854566A

US3854566A - Photoelectric tabulating apparatus

Info

Publication number: US3854566A
Application number: US00363918A
Authority: US
Inventors: G Ellis
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-05-25
Filing date: 1973-05-25
Publication date: 1974-12-17
Anticipated expiration: 1991-12-17
Also published as: GB1442450A; DE2418530A1; SE390273B; JPS5020814A; NL7403793A; BE815420A; CA982969A; IT1012805B; FR2230504A1

Abstract

Apparatus for implementing the tabulating function on a typewriter or communications printer terminal. An opaque cylinder is provided which rotates in synchronism with the relative movement between the printing mechanism and the recording medium. The cylinder wall is perforated with a series of holes along a line of rotation and sliding shutters in the cylinder wall may be selectively displaced to either block a hole or allow the passage of light therethrough. The holes correspond to character spaces, or print columns. A light source and a phototransducer are aligned so that when a hole is clear, the phototransducer produces a signal in response to light impinging thereon from the light source. The shutters are displaceable by two hammers, one on each side of the cylinder. One of the hammers displaces the shutters into the ''''tab clear'''' position and the other hammer displaces the shutters into the ''''tab set'''' position. Receipt of a ''''tab'''' signal causes the apparatus to search for a hole which has its associated shutter displaced to the ''''tab set'''' position.

Description

[11] 3,854,566 Dec. 17, 1974 1 PHOTOELECTRIC TABULATING APPARATUS George R. Ellis, Fairport, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: May 25, 1973 [21] Appl. No.: 363,918

[75] Inventor:

[58] Field of Search 197/16, 70, 82, 84 R, 84 A, 197/84 B, 114 R, 133 R,176, 177,178,179,

[56] References Cited UNlTED STATES PATENTS 1,054,875 3/1913 Smith 197/179 2,439,470 4/1948 Jackson 197/84 B X 2,879,876 3/1959 Palmer et a1 197/16 2,953,231 9/1960 Dersch 197/176 3,217,856 11/1965 Miles 197/114 R 3,315,776 4/1967 Barkdoll 197/176 X 3,429,415 2/1969 Hirata 1 197/70 X 3,511,353 5/1970 Harlow et al. 197/133 R 3,559,792 2/1971 Guldenpfennig 197/187 3,656,041 4/1972 Bonzano 197/133 R X 3,750,797 8/1973 Hunt et a1 197/176 Primary Examiner-Ernest T. Wright, Jr.

[5 7] ABSTRACT Apparatus for implementing the tabulating function on a typewriter or communications printer terminal. An opaque cylinder is provided which rotates in synchronism with the relative movement between the printing mechanism and the recording medium. The cylinder wall is perforated with a series of holes along a line of rotation and sliding shutters in the cylinder wall may be selectively displaced to either block a hole or allow the passage of light therethrough. The holes correspond to character spaces, or print columns. A light source and a phototransducer are aligned so that when a hole is clear, the phototransducer produces a signal in response to light impinging thereon from the light source. The shutters are displaceable by two hammers, one on each side of the cylinder. One of the hammers displaces the shutters into the tab clear position and the other hammer displaces the shutters into the tab set position. Receipt of a tab signal causes the apparatus to search for a hole which has its associated shutter displaced to the tab set position.

5 Claims, 5 Drawing Figures ;34 Control Local or 67m Remote /n,ou/

PAn-jmgg DEC] H914 v sum 10F 2 Local or Remote lnpuf Circuit PHOTOELECTRIC TABULATING APPARATUS BACKGROUND OF THE INVENTION This invention relates to printing apparatus and, more particularly, to an arrangement for tabulation control in a typewriter-like printing terminal.

In a typewriter-like printing terminal, it is often desirable to provide an arrangement for setting and utilizing tab stops in a manner such as found on certain typewriters and communications printer terminals. In a communications printer terminal which is heavily electronic in implementation, it has been found difficult to provide conventional mechanical arrangements for the implementation of tabbing functions such as horizontal tab, vertical tab, and form feed. These controls are the primary print head and paper movement controls for preparing tabular and/or formated printed material. It is desirable that these printing position controls should be settable, operable, and clearable from either a local (keyboard) or remote (another terminal or computer) source. In addition, the arrangement should be nonvolatile (not lost due to power loss), compact, simple, reliable, and settable in any desired position (e.g.; any of 80 columns for horizontal tab).

SUMMARY OF THE INVENTION In accordance with the principles of this invention, there is provided a short section of a cylinder which is affixed to a shaft or other drive device which rotates in synchronism with the printhead or paper movement in a printing terminal. One such cylinder is required for horizontal tab control and another is required for vertical tab and form feed control. The opaque cylinder shell is perforated with a series of small apertures along a line of rotation. Aligned with each aperture is a small sliding shutter having an opening therein. The shutter extends beyond the cylinder ends. In the tab set position a shutter opening aligns with a desired cylinder aperture whereas in the tab clear position the shutter blocks the cylinder aperture. The shutters are positioned by two hammers, one on each end of the cylinder and directly opposed to each other. The hammers are activated upon respective receipt of tab clear or tab set signals and each shutter remains in its set or clear position by spring detent or other means until relocated by a hammer. A fixed light source is introduced to one side of the cylinder aperture in line with a phototransducer on the other side of the aperture. During printing, detection of a ta signal will cause the printhead or paper to advance until the next open aperture in the cylinder is reached.

DESCRIPTION OF THE DRAWING The foregoing will become more readily apparent upon reading the following description in conjunction with the drawing in which:

FIG. 1 depicts a simplified schematic of a printer terminal in which the principles of this invention may be employed;

FIG. 2 depicts an enlarged view of a tab cylinder incorporating the principles of this invention and FIG. 2(a a) depicts a shutter which may be utilized with the mechanism of FIG. 2;

FIG. 3 depicts an electrical schematic diagram of an illustrative tab control system; and

FIG. 4 depicts an electrical schematic diagram of an illustrative circuit which may be utilized to generate sensing signals from the phototransducer.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT Turning to FIG. 1, depicted therein is a simplified schematic of a typical carriage drive mechanism utilized in a printer terminal. For a more detailed description of such a mechanism, the reader is referred to US. Pat. No. 3,686,678. Since an understanding of the carriage drive mechanism is only ancillary to an understanding of the present invention, only a brief description thereof is deemed necessary for the purpose of describing this invention.

Shown in FIG. 1 is a print head or printing mechanism 10 which is arranged to move laterally with respect to the recording medium 12. Print head 10 may illustratively be an electrode stylus such as that disclosed in the aforementioned patent wherein recording medium 12 is a dielectric recording material upon which is deposited electric charge for later development by an electroscopic marking material. Alternatively, print head 10 may be a golf ball or daisy wheel print head which moves across paper 12 and prints thereon by striking an inked ribbon interposed therebetween. It is also understood that printing mechanism 10 may be stationary and recording medium 12 may move laterally, as is the case in a standard typewriter. What is important for the purposes of this invention is that there is relative movement in spaced increments between printing mechanism 10 and recording medium 12.

Returning now to the embodiment depicted in FIG. 1, print head 10 is mounted on a slider bar, not shown, and is attached to cable 14. Cable 14 is wrapped around idler pulleys 16 and 18 and is wound around capstan 20. Capstan 20 is mounted for rotation on shaft 22 which is attached to print head drive 24. Drive 24 may be either a stepping motor or a continuously driven motor. Furthermore, the print head drive mechanism may be arranged so that drive 24 is bidirectional to return print head 10 to an initial starting point or the mechanism may have some other means, such as a return spring, for returning print head 10 to an initial point. Mounted for rotation on shaft 28 is tab cylinder 26. Shaft 28 is shown being drivingly connected to shaft 22 through worm drive 30, but it is understood that other methods of coupling the

shafts

22 and 28 are also contemplated for use with this invention. Cylinder 26 makes one or less revolutions each time print head 10 traverses recording medium 12 from an initial to a final position.

Tab cylinder 26, to be described in more detail hereinafter with respect to FIG. 2, is illustratively a hollow cylinder with a single string of small holes equally spaced around the circumference of its surface. Internal to cylinder 26 is light source 31 in alignment with optical detector, or phototransducer, 32. Cylinder 26 is arranged, as will become evident from the following discussion, so that each of the holes 100 may be selectively blocked or left open. Each hole 100 corresponds to a particular character space, or column, as print head 10 traverses recording medium 12. These selective blockings and opening may be utilized to set tabs. For the purpose of this description, an open hole 100 corresponds to a tab set." Thus, when phototransducer 32 produces a signal at its output, this indicates that the character space at which print head is presently positioned has a tab set condition. The entire operation of the mechanism depicted in FIG. 1 is controlled by control circuit 34, which illustratively may be of the type described in the aforementioned patent. Circuit 34 receives signals from a local or remote input to cause print head 10 to traverse recording medium 12 and record printed indicia thereon. The local input to control circuit 34 may be, for example, a typewriterlike keyboard and the remote input may be a computer vor similar device or another communications printer terminal. The input may include a tab instruction so that circuit 34 may control drive 24 to move print head 10 rapidly across recording medium 12 until a signal from phototransducer 32 indicates the presence of a tab set condition at a particular character space, at which time the motion of print head 10 may be arrested. It is to be understood that although the foregoing description has related to moving the print head 10 horizontally across the recording medium 12 in order to print a line of characters, it is also contemplated that a similar arrangement may be utilized for vertical movement of the recording medium 12 with respect to the print head 10. With such an arrangement, the rotation of the corresponding tab cylinder corresponds to a particular increment of vertical movement, for example, one rotation of the vertical tab cylinder for each page of the recording medium 12.

Referring now to FIG. 2, shown therein is an illustrative horizontal tab cylinder 26 embodying the principles of this invention. The shell of cylinder 26 is perforated with a single string of small holes 100 equally spaced around its circumference. For an 80 column print line, 80 such holes 100 will be provided. Each hole 100 thus corresponds to a print position, or character space. The arc length within which each hole 100 is placed, and the limits of the hole 100 dimensions, are determined from simple geometry. Let A arc length, R radius of cylinder 26, C circumference of cylinder 26, and H number of holes 100. Then:

If, for example, the cylinder 26 has a 3 inch diameter and there are 80 holes 100 corresponding to 80 character spaces per line, then A equals 0.118 inches. Hence, the diameter of the holes 100 must be less than 0.1 l8 inches. Alternatively, depending upon fabrication process, etc., small rectangular slits may be employed. Their center to center spacing must be 0.1 18 inches for the above example and their width must obviously be less than that figure.- it is clear then that in order to utilize a small diameter cylinder 26, the holes 100 and the subsequently described mechanism must be produced in a precision manner.

Within the shell wall of cylinder 26 and corresponding to each hole position is a shutter 102, slideable in a direction parallel to the axis, or shaft 28, of cylinder 26. FIG. 2(a) shows a detailed view of such a shutter '102 having an opening 103. Aligned with shutters 102 are two hammers, 104C and 1048, one on either side of cylinder 26. These hammers 104C and 1048 are electrically operated solenoids and are fixedly mounted on a surface, not shown. Hammer 1048 is the set tab" hammer which, when activated, taps a sliding shutter 102 with which it is aligned so that opening 103 exposes the hole in the cylinder wall. The shutter 102 remains in position through some detent method, such as a flat or wire compressing spring between it and the cylinder cavity, a coil spring loaded ball, friction fit, or other. Hammer 104C is the clear tab" hammer and when it is activated it strikes a shutter 102 so as to move the shutter 102 to cover the associated hole 100 so that no light can pass therethrough. Cylinder 26 may rotate continuously or in step with the printing which is occurring, but the hammers 104C and 1048 can only be activated when the cylinder 26 is stopped or is being stepped at a relatively slow speed.

A continuous light source is applied to one surface of the cylinder 26 in line with the holes 100. It is shown in FIG. 2 to be illustratively introduced from the inside surface from lamp 106 through light pipe 108. Light pipe 108 may be employed to simplify mounting and positioning of the light and may be a solid glass or plastic or a small non-coherent fiber optic bundle. One end of cylinder 26 is open to permit access of the light source. Opposite the light source there is fixedly mounted to asurface, not shown, a phototransducer 32 which may be a phototransistor. Phototransducer 32 is aligned with the holes 100 such that when a hole 100 is not obstructed by a shutter 102, the light from source 106 will pass through the hole 100 and cause a signal to appear at the output of the phototransducer 32. At this point it should also be noted that hammers 104C and 1048 are aligned with the shutter 102 which is associated with the hole 100 through which light from source 106 passes to phototransducer 32. Hammers 104C and 1048, phototransducer 32, and shaft 28 all lie in the same plane and the light from source 106 passes through the hole 100 in that plane.

The tab system operation will now be described with reference to FIG. 3. Communication interface logic and decoder 300 is connected via communication link 302 to a remote input source such as a computer, not shown. Tab commands are entered and decoded therein to control the operation of print head drive 24. Tab commands may also be generated from a local keyboard via tab" switch 304, tab set switch 306 and tab clear switch 308. To set a tab, the print head 10 is moved to the column of interest and a tab set signal is entered either from logic and decoder 300 or tab set key 306. This signal is transmitted through OR gate 310 to activate one-shot 312 to produce a pulse which causes driver 314 to operate set hammer 1048. The operation of set hammer 104$ moves the shutter 102 corresponding to that column of interest into a position where light may pass from source 106 through the corresponding hole 100 to phototransducer 32. in a similar manner, a tab clear" command can come from logic and decoder 300 or tab clear" key 308, through OR gate 316, and then a pulse is generated from one-shot 318 to cause driver 320 to operate clear hammer 104C. The operation of clear hammer 104C moves the corresponding shutter 102 so as to block light from source 106 through the corresponding hole 100. Thereafter, during a printing operation, a ta signal may be received either from logic and decoder 300 or tab key 304. This signal passes through OR gate 322 and through one-shot 324 to printer control logic 326. Printer control logic 326 then acts to cause driver 328 to operate print head drive 24 to rapidly move the print head 10 across the recording medium 12. Tab cylinder 26 rotates about shaft 28 in synchronism with the print head 10. Light source 106 is always supplying light and phototransducer 32 is responsive to any light which it may receive from source 106. The print head and cylinder 26 will advance until a cylinder hole 100 is encountered which permits light passage from source 106. This triggers phototransducer 32 to send a signal through inverter 330 to oneshot 332 which then sends a pulse to printer control logic 326. This pulse causes logic 326 to arrest the motion of drive 24. Printing may then take place from this tab position.

FIG. 4 depicts an illustrative circuit which may be used with a phototransistor 32 utilized as the phototransducer element. When light from source 106 passes through a. hole 100 in the wall of cylinder 26 and impinges upon the base of phototransistor 32, this transistor 32 is turned on. The subsequent flow of current through phototransistor 32 into the base of transistor 330 causes transistor 330 to turn on, thereby causing the collector of transistor 330 to be at a low potential, placing the same low potential at terminal 340. When no light from source 106 impinges upon the base of phototransistor 32 because a shutter 102 blocks the corresponding hole 100, phototransistor 32 is off, thereby keeping transistor 330 in the off state. The collector of transistor 330, and consequently terminal 340, are then at a relatively high potential.

At this point, some other considerations are in order. One system option can be the provision of a clear all tabs" signal (not shown) which may be employed to cause the print head 10 to step through all the columns and activate the clear hammer 104C as each shutter position is passed.

The mass of cylinder 26 should be small so as to minimize the effects of inertia, momentum, over-travel, etc., as repeated accurate positioning is essential. Depending upon printing and shaft rotational velocity, the

rotation of cylinder 26 may be bidirectional as the print.

head 10 advances during printing and then returns as each carriage return instruction is signaled. Alternatively, if high speeds and rapid reversal characterize the operation, rotation of cylinder 26 may be unidirectional. The shaft 28 and cylinder 26 may be clutched such that they continue rotating in the advance direction as a carriage return signal is entered until an index position is reached. The shaft 28, cylinder 26 and print head 10 will then be positioned at the column number 1 reference point. A cylinder index point may be achieved by a light and phototransistor technique similar to that previously described. A hole in the closed end of the cylinder 26, aligned with the light and phototransistor would serve for this purpose. If additional reference points are required for timing or positional check purposes, additional holes in the closed end may be employed or additional holes adjacent to the tab stop holes 100 which are never closed by the shutters 102 may be employed, or a conventional shaft encoder may be mounted on the cylinder drive shaft 28 for this purpose. A check circuit (for lamp, phototransistor, amplifiers, etc.) may be employed by, for example, reserving column number 1 for this purpose as it is seldom if ever utilized as a tab position. An always open hole in this position should cause a signal to be generated coincident with each carriage return signal. Failure to generate a signal upon a carriage return would set an alarm, interlock, operator indicator, or any similar purpose signal may be generated.

At this point, it should be noted that occasionally successive tab instructions may be received. If the cylinder 26 is at rest at a tab stop position, the circuitry must be aware of that fact and movement of the print head 10 has to be initiated if a second tab instruction is received so that the circuitry may search for the next such position. That is, it must not be stuck on the current position because the search is satisfied by the current position.

Accordingly, there has been shown apparatus which is simple, reliable, and economical, for performing the tabulating function in an environment such as a typewriter or communications printer terminal. It is understood that the above described arrangement is merely illustrative of the application of the principles of my invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of my invention. For example, an alternative embodiment for tab cylinder 26 would have reflective areas substituted for holes 100. Shutters 102 would then either cover or expose these reflective areas. With this embodiment, the phototransducer 32 and light source 106 would be positioned on the same side of the cylinder wall, either inside or outside the cylinder 26, and would be arranged so that light from the source 106 would be reflected from an exposed area to impinge upon the phototransducer 32.

What is claimed is:

1. Tabulator apparatus for use in a printer having relative movement in fixed increments between a printing mechanism and a recording medium, said apparatus comprising a cylindrical shell connected for rotation in synchronism with said relative movement, said shell having a plurality of apertures along a line of rotation of said shell, said apertures being spaced along said line to correspond to said fixed increments,

a plurality of shutters slidably mounted on said shell,

each of said shutters being associated with a respective one of said apertures and having an opening therein selectively alignable with a respective aperture,

alight source mounted to transmit light to said apertures, and

a phototransducer having an output terminal for providing a signal thereon when light from said light source impinges on said phototransducer, said phototransducer being mounted in spatial relation to said shell so that said apertures pass said phototransducer in correspondence with the relative movement of said printing mechanism and said recording medium, light from said light source impinging on said phototransducer only when a shutter opening is aligned with a respective aperture passing said phototransducer.

2. The apparatus of claim 1 further including a first selectively actuatable hammer mounted relative said shell so as to be adjacent a first end of the shutter associated with the aperture passing said phototransducer,

a second selectively actuatable hammer mounted relative said shell so as to be adjacent the second end of the shutter associated with the aperture passing said phototransducer,

first means responsive to a first signal for actuating said first hammer to position said adjacent shutter so as to align the opening of said adjacent shutter with said passing aperture, and

second means responsive to a second signal for actuating said second hammer to position said adjacent shutter so as to block said passing aperture.

3. The apparatus of claim 2 further including driving means energizable in response to a third signal for causing relative movement between said printing mechanism and said recording medium, and

means responsive to said third signal and said phototransducer signal for deenergizing said driving means.

4. In a printer wherein a printing mechanism and a recording medium move relative to each other in fixed increments along a printing line to define columns, tabulator apparatus comprising a cylinder arranged to rotate in synchronism with said relative movement, said cylinder having a plurality of apertures along a line of rotation thereof, each of said apertures corresponding to a respective one of said defined columns and being at a fixed point as the printing mechanism and recording medium define the respective column,

a plurality of shutters each movably mounted between a first position and a second position on said cylinder, each of said shutters being associated with a respective one of said apertures and having an opening therein aligned with a respective aperture when said shutter is in said first position and blocking said aperture when said shutter is in said second position,

a light source mounted to transmit light to the aperture at said fixed point,

an optical detector mounted adjacent said fixed point, said detector providing a signal when light from said light source impinges thereon, said light impinging on said detector only when the shutter associated with an aperture at said fixed point is in said first position,

first means mounted adjacent a first end of the shutter associated with the aperture at said fixed point, said first means being responsive to a first signal for moving said adjacent shutter to said first position,

second means mounted adjacent the second end of the shutter associated with the aperture at said fixed point, said second means being responsive to a second signal for moving said adjacent shutter to said second position,

I driving means energizable in response to a third sig- 5. in a printer wherein a printing mechanism and a recording medium move relative to each other in fixed 10 increments along a column to define printing lines, tabulator apparatus comprising a cylinder arranged to rotate in synchronism with said relative movement, said cylinder having a plurality of apertures along a line of rotation thereof, each of said apertures corresponding to a respective one of said defined printing lines and being at a fixed point as the printing mechanism and recording medium define the respective printing line,

a plurality of shutters each movably mounted between a first position and a second position on said cylinder, each of said shutters being associated with a respective one of said apertures and having an opening therein aligned with a respective aperture when said shutter is in said first position and blocking said aperture when said shutter is in said second position, I

a light source mounted to transmit light to the aperture at said fixed point,

driving means energizable in response to a third signal for causing relative movement between said printing mechanism and said recording medium along said column, and

means responsive to said third signal and said optical detector signal for deenergizing said driving means.

Claims

1. Tabulator apparatus for use in a printer having relative movement in fixed increments between a printing mechanism and a recording medium, said apparatus comprising a cylindrical shell connected for rotation in synchronism with said relative movement, said shell having a plurality of apertures along a line of rotation of said shell, said apertures being spaced along said line to correspond to said fixed increments, a plurality of shutters slidably mounted on said shell, each of said shutters being associated with a respective one of said apertures and having an opening therein selectively alignable with a respective aperture, a light source mounted to transmit light to said apertures, and a phototransducer having an output terminal for providing a signal thereon when light from said light source impinges on said phototransducer, said phototransducer being mounted in spatial relation to said shell so that said apertures pass said phototransducer in correspondence with the relative movement of said printing mechanism and said recording medium, light from said light source impinging on said phototransducer only when a shutter opening is aligned with a respective aperture passing said phototransducer.

2. The apparatus of claim 1 further including a first selectively actuatable hammer mounted relative said shell so as to be adjacent a first end of the shutter associated with the aperture passing said phototransducer, a second selectively actuatable hammer mounted relative said shell so as to be adjacent the second end of the shutter associated with the aperture passing said phototransducer, first means responsive to a first signal for actuating said first hammer to position said adjacent shutter so as to align the opening of said adjacent shutter with said passing aperture, and second means responsive to a second signal for actuating said second hammer to position said adjacent shutter so as to block said passing aperture.

3. The apparatus of claim 2 further including driving means energizable in response to a third signal for causing relative movement between said printing mechanism and said recording medium, and means responsive to said third signal and said phototransducer signal for deenergizing said driving means.

4. In a printer wherein a printing mechanism and a recording medium move relative to each other in fixed increments along a printing line to define columns, tabulator apparatus comprising a cylinder arranged to rotate in synchronism with said relative movement, said cylinder having a plurality of apertures along a line of rotation thereof, each of said apertures corresponding to a respective one of said defined columns and being at a fixed point as the printing mechanism and recording medium define the respective column, a plurality of shutters each movably mounted between a first position and a second position on said cylinder, each of said shutters being associated with a respective one of said apertures and having an opening therein aligned with a respective aperture when said shutter is in said first position and blocking said aperture when said shutter is in said second position, a light source mounted to transmit light to the aperture at said fixed point, an optical detector mounted adjacent said fixed point, said detector providing a signal when light from said light source impinges thereon, said light impinging on said detector only when the shutter associated with an aperture at said fixed point is in said first position, first means mounted adjacent a first end of the shutter associated with the aperture at said fixed point, said first means being responsive to a first signal for moving said adjacent shutter to said first position, second means mounted adjacent the second end of the shutter associated with the aperture at said fixed point, said second means being responsive to a second signal for moving said adjacent shutter to said second position, driving means energizable in response to a third signal for causing relative movement between said printing mechanism and said recording medium along said printing line, and means responsive to said third signal and said optical detector signal for deenergizing said driving means.

5. In a printer wherein a printing mechanism and a recording medium move relative to each other in fixed increments along a column to define printing lines, tabulator apparatus comprising a cylinder arranged to rotate in synchronism with said relative movement, said cylinder having a plurality of apertures along a line of rotation thereof, each of said apertures corresponding to a respective one of said defined printing lines and being at a fixed point as the printing mechanism and recording medium define the respective printing line, a plurality of shutters each movably mounted between a first position and a second position on said cylinder, each of said shutters being associated with a respective one of said apertures and having an opening therein aligned with a respective aperture when said shutter is in said first position and blocking said aperture when said shutter is in said second position, a light source mounted to transmit light to the aperture at said fixed point, an optical detector mounted adjacent said fixed point, said detector providing a signal when light from said light source impinges thereon, said light impinging on said detector only when the shutter associated with an aperture at said fixed point is in said first position, first means mounted adjacent a first end of the shutter associated with the aperture at said fixed point, said first means being responsive to a first signal for moving said adjacent shutter to said first position, second means mounted adjacent the second end of the shutter associated with the aperture at said fixed point, said second means being responsive to a second signal for moving said adjacent shutter to said second position, driving means energizable in response to a third signal for causing relative movement between said printing mechanism and said recording medium along said column, and means responsive to said third signal and said optical detector signal for deenergizing said driving means.