TPf-hnical Field

The invention relates to a printer for recording data on a record medium.

Background of the Invention

In record media processing machines in which different operations such as reading, sorting and encoding of documents are done on the same machine, the speed of the document being processed must often decrease at the printing station because conventional printers are not capable of matching processing speeds which can be attained by other modules of the machine. An additional transport mechanism may therefore be employed to adjust the document speed, first to the slower printer speed, and then to the normal speed of the machine. This speed variation limits the machine throughput and is a major cause of document jam and edge damage. Also, in order to produce a good quality, high resolution print, such as defined by ABA Standard for E-13B, impact printing with full face characters is normally used. This produces a high acoustical noise level.

A typical printer employed in a financial item processing machine may be an impact printer comprising two banks of hammers, placed on the same line. The hammers carry a complete set of full face characters. During a printing operation, a document moves at a constant speed in front of the hammer banks. A hammer carrying a selected character will be operated whenever the appropriate location for that character in the message, as determined beforehand, appears in the proper position with respect to the hammer. In other words, the proper time for the firing of each hammer in such an apparatus is a function of the location of each particular

character, as well as a function of the paper speed and position. This dependency makes the control of hammer firing more complex. Furthermore, the speed of the document through the printer is limited by the contact time between the hammer and the document, as determined by print character smearing, and also by the cycle time of the hammer in the worst-case situation in which a hammer must strike two or more times consecutively. All of these considerations affect the throughput speed of the machine.

A printing device which utilizes rolling pressure contact for printing, rather than impact, is known from U.S. Patent No. 3,537,563. This printing device includes a hollow cylindrical body rotatably mounted on a vertically extending shaft. The outer surface of the cylindrical body has a plurality of longitudinal slots in which a corresponding plurality of printing rods are slidably mounted, each rod carrying a serial array of characters. During a printing operation the said shaft is moved horizontally in a direction parallel to a printing line, and at the same time the cylindrical body is rotated on the shaft, thereby producing a rolling motion of the cylindrical body over the printing area. While the cylindrical body is rotating, each of the printing rods is moved upwardly in turn a selected distance by means of a spring so as to bring a selected character on the rod to the level o the printing line. Although this printing device has certain advantages over the impact printer referred to previously, it has the disadvantage that its speed of operation is limited due to the time required for moving each printing rod to its selected printing position.

Disclosure of the Invention

It is accordingly an object of the invention to provide a printing apparatus which utilizes rolling pressure contact for printing and which has the capability of operating at a high speed.

It is a further object of the invention to provide a printing apparatus whose speed of operation can be changed continuously if desired.

According to the invention there is provided a printer for recording data on a record medium including rotatable print character support means having a printing position on the surface thereof, first drive means for driving said support means in rotational movement, platen means positioned in cooperative relation to said support means to effect rolling contact pressure printing, a plurality of character sets mounted for movement with respect to the outer surface of said support means, each said character set comprising a plurality of character elements, and positioning means for moving each character set individually as said support means rotates to place a predetermined character at the printing position of said support means, characterized in that each character set is arranged to be moved by a respective flexible means which is driven in operation by a respective one of a plurality of rotatable members mounted for rotation coaxially with said support means, and further characterized by a plurality of second drive means for respectively driving said rotatable members, and control means for controlling the relative speeds of rotation of each of said rotatable members and said support means whereby a selected character element of each character set may be positioned in turn at said printing position. A printing apparatus constructed in accordance with the present invention is capable of printing non-interrupted lines, by printing one

character serially in line after another in the same order in which the characters appear in the printed message. Each print character is dynamically positioned at a printing line as the structure supporting the print character continues to rotate, and the character selection procedure is the same for all characters.

As printing is accomplished by contact pressure rather than by impact, a low noise level is achieved. A further advantage of a printing apparatus in accordance with the present invention is that its speed can be changed continuously and that the speed of the printing apparatus can be synchronized easily to the normal speed of the paper through the apparatus, thereby decreasing the tendency of the paper to jam and to incur damage. Further advantages are that the apparatus is capable of high throughput, and that the tolerance and spacing of the characters are defined by the geometry of the design and do not affect the printing speed.

Brief Description of the Drawings

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a printing apparatus in accordance with the present invention, also showing document drive means and ribbon supply and take-up means;

Fig. 2 is an enlarged perspective view of part of the printing apparatus showing the cooperation of a printing drum, various print character sets thereon, locating rings on the drum, and a pressure roller;

Fig. 3 is an elevational view, partially in section, of the printing drum and associated elements;

Fig. 4 is a sectional view taken along line 4-4 of Fig. 3;

Fig. 5 is a sectional view taken along line 5-5 of Fig. 3; Fig. 6 is a sectional view taken along line

6-6 of Fig. 3;

Fig. 7 is a sectional view taken along line 7-7 of Fig. 3;

Fig. 8 is a sectional view taken along line 8-8 of Fig. 3;

Fig. 9 is a perspective view of part of the printing apparatus, particularly showing a lower frame member which supports lower cable pulleys;

Fig. 10 is a perspective view showing the path of movement of a cable associated with a character set;

Fig. 11 is a plan view of a character element used in the printing apparatus;

Fig. 12 is an end view taken along line 12-12 of Fig. 11;

Fig. 13 is a sectional view taken along line 13-13 of Fig. 12;

Fig. 14 is a diagram showing the relative speeds required for movement of character elements into printing position;

Fig. 15 is a circuit diagram showing control logic circuitry for controlling a drive motor for positioning a print character set; and

Fig. 16 is a block diagram showing the relationship of the control logic circuitry for the various print character sets with a microprocessor and an associated disk file and keyboard.

Best Mode for Carrying Out the Invention

Referring now to Fig. 1 of the drawings, a printer 20 comprises a printing unit 22, a paper transport unit 24 and a ribbon drive unit 26. In the

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printer, a record medium 28, which may for example be a check upon which magnetic ink character recognition information is to be printed, is brought into the printing unit 22 by the paper transport unit 24. The printing unit 22 transfers ink from a ribbon 30 to the record medium 28. The ink may, for example, be in the form of a dry powder coated on said ribbon 30. The ribbon drive unit 26 ensures that fresh ribbon 30 is fed properly into the printing unit 22 and that the used portion of said ribbon is removed from said printing unit. Attached to the printer 20 is a hopper, not shown, which feeds the checks or other record media, one by one, to the transport unit 24-, in the direction represented by the arrow 25. This feeding is done in synchronization with the operation of the paper transport unit 24 and the printing unit 22.

The paper transport unit 24 comprises a number of belts 32 located both upstream and downstream of the record media flow, with respect to the printing unit 22. The check or other record medium 28 is gripped between each pair of belts 32 and is transported by said belts at a uniform speed in a direction represented by the arrow 27, continuing in the same direction as the arrow 25. In order to ensure that the record medium 28 has the same speed as the speed of the printing unit 22, the belts 32 are driven by a printer synchronous motor 34 via a system which includes pulleys 29 and 31 and a drive belt 33. The ribbon drive unit 26 includes a supply reel 36, two motor-driven capstans 38 and 40, two tape accumulator vacuum columns 42 and 44 having ribbon sensors 46 and 48, respectively, associated therewith, and finally a pick-up reel 50 on which the used ribbon is collected. Fresh ribbon is supplied by the reel 36 to the accumulator 42 by the action of the capstan 38

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driven by a motor 52. The accumulator 42 is provided with apertures around the peripheral walls to cause the ribbon 30 to be pulled into the cavity of said accumulator 42 by a controlled vacuum. This action, combined with the supply of ribbon provided by the capstan 38, causes the ribbon loop 56 to expand within the accumulator 42 _* This loop expansion continues until the loop covers the location of the position sensor 46. At that time, under control of the sensor 46, the motor 52 is switched off so that the supply ribbon 30 to the accumulator 42 is terminated for the time being.

During the printing operation, pinch rollers 54, under control of an activator (not shown), such as a solenoid, are opened to release the ribbon 30 to be drawn into the printing unit 22 for usage. At any other time, the pinch rollers 54 are clamped to fix the forward end of the loop 56 and thereby prevent such loop from being pulled back by the vacuum. As printing progresses, the size of the ribbon loop 56 decreases until it uncovers the position sensor 46. This causes the motor 52 to be activated once more so that new ribbon is supplied to the accumulator 42. The supply reel 36 is equipped with a friction pad (not shown) in order to maintain tension on the ribbon 30 at all times.

The collection of used ribbon 30, by winding on the reel 50, is similar to that of the feeding operation, except that it occurs in reverse. Used ribbon 30 exits from the printing unit 22 and is fed into the accumulator 44. Said accumulator is equipped with the position sensor 48 and apertures around the periphery thereof, similar to the accumulator 42, so that vacuum is created in the accumulator. The position sensor 48 is coupled to a motor 49 to drive the capstan 40. This provides a control for the ribbon loop size in the cavity of the accumulator 44.

The ribbon is fed from the accumulator 44 to the pick¬ up reel 50, which is driven through a frictional engagement.

In the printing unit 22, the amount field on the check 28, for example, is printed with E-13B characters by pressing and rolling the ribbon 30 and the record medium 28 between a plurality of character beads, or elements, 58, in character sets 60 which are supported by a printing drum 62, and a pressure roller 64.

One printing cycle of the apparatus is required for printing each check or other record medium, and a complete rotation of the drum 62 corresponds to a printing cycle. Thus the diameter D of the printing drum 62 at its largest cross-section, where printing takes place, is given by the equation

^{D L} maχ/V* where Lmax is the maximum length of the check or other record medium. The printing drum 62 is fixed to a shaft 55 and is driven by the synchronous motor 34 acting through a pulley 57, a belt 59 and a pulley 61 fixed to the shaft 55.

An optical encoder 35 is fixed on the shaft 55 and provides information concerning angular position and rotational speed of the motor 34 and the shaft 55.

The drum 62 cooperates with the pressure roller 64 to move the check or other record medium 28 at a speed which is the same as the paper transport speed provided by the interacting belts 32. The shaft 55 is mounted in the machine framework by two bearings 51 and 53 (Fig. 3). The record medium 28 (Fig. 1) may pass through the printing unit 22 in one of two modes: a printing mode or a non-printing mode. If the printing mode is selected, a document sensor 66 senses the presence of a record medium 28 in order to activate a double acting activator 68, which may take the form of a solenoid, for example, and which

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functions to straighten a pair of toggle links 70 which have one pivoted end 72 connected to the frame of the printer 20, with the other end of said pivoted link set connected to a lever 74. The lever 74 supports the pressure roller 64, and is connected at its other end to the frame by shaft 76. The straightening of the links 70 causes the pressure roller 64 to press against the printing drum 62 with the record medium 28 and ribbon 30 captured and squeezed therebetween, the roller 64 rotating in operation in the opposite direction to the drum 62. If the non-printing mode is selected, the activator 68 is not energized and the toggle links 70 remain in an unactivated position, so that the pressure roller 64 is positioned to leave an.open space between itself and the printing drum 62 so that record media 28 may pass between said drum 62 and said roller 64 without printing taking place thereon.

As is also shown in Figs. 2-7 inclusive, the printing drum 62 has an interior opening 82 extending therethrough. An upper cap 84 is fixed to the top of the drum 62, and to the shaft 55. The cap 84 provides mounting means for a plurality of sets of pulleys 86, 88 corresponding to the number of columns of information to be printed on a record medium by the printing apparatus, as will subsequently be described in greater detail. Similarly, a lower cap 85 is fixed to the shaft 55 adjacent to the bottom of the drum 62 and provides the mounting means for a plurality of lower guide pulleys 87, one for each column of information to be printed on a record medium.

The printing drum 62 carries a plurality of "bead chains" or character sets 60 thereon, each set being made up of a plurality of individual character beads or elements 58, which are shown in greater detail in Figs. 11, 12 and 13, and which are strung on a cable 92. Each element 58 of a character set 60

includes a type face 94 thereon, and is provided with a bore 96 therethrough to receive the cable 92 on which the individual elements 58 are fixed immovably in a serial fashion, by crimping or other suitable means. The number of beads or elements 58 included in a given character set 60 will vary in accordance with the application for which the printer is employed, and typically may include the numerals 0-9 inclusive, together with any symbols necessary, such as those used for encoding MICH information on checks.

A plurality of character sets 60 are mounted on the drum 62, so as to extend generally parallel to each other and to the axis of rotation 98 of the drum 62, as shown, for example, in Fig. 2. Rings 100 and 102, which are positioned on the drum to provide surfaces for engaging the roller 64, are interrupted along their periphery to receive the character sets 60, which are positioned in a recessed portion 104 of the drum 62. As best shown in Fig. 8, the ends of each cable 92 are anchored in a respective member 106 which is positioned on the shaft 55 and is rotatable with respect thereto _* One member 106 is provided for each character set 60. Anchoring of the ends of the cable 92 may be by any suitable means, such as by a plastic retainer 108 which is located in a bore 110 of the member 106. A set screw 107 or other suitable means may be employed to crimp the retainer 108 and secure the cable ends against movement. The circumference of the member 106 is grooved in a double groove configuration 111 (Fig. 3) to receive the cable 92. A respective pulley 112 is secured to each member 106 to rotate therewith about the shaft 55 and is connected by a belt 114 and a further pulley 116 to a respective driving motor 118 (Fig. 1) to be rotated thereby. For the sake of clarity, only two members 106 and associated elements are shown in Figs _* 1 and 3.

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An optical encoder 119 is coupled to each motor 118 and provides information concerning the ' angular position and rotational speed of the motor 118 and the associated member 106. A cut-out portion 120 in each member 106 (Fig. 8) as well as second cut¬ out portions 122 in each pulley 112 are provided to enable the cables 92 for the various character sets to pass therethrough without interference. As best shown in Figs. 3, 5, 7 and 9, a lower frame member 124 is fixed to the shaft 55, by any suitable means such as a key 125, adjacent to each member 106. One lower frame member 124 is provided for each character set 60. Each lower frame member 124 rotatably mounts a set of lower guide pulleys 126, 128, 130, 132 and 134, around which the cable 92 for the corresponding character set 60 is arranged to cause movement of said character set in a predetermined manner, as will be subsequently described in greater detail. A runaway prevention pin 121 extends downward from each frame member 124 and coacts with a slot 123 in the corresponding member 106 to limit movement of the member 106 in the event of drift of the member 106 in the event of malfunction of its motor 118 (Fig. 1) . This prevents possible damage to the cable 92 and the elements 58 of the character set 60.

Each frame member 124 includes two elements 136 and 138, bolted together by fasteners 140 and 142. An additional element 144 is pivotally mounted by shaft 146 on the element 136 and provides a bearing means for the pulley 134. The element 144 is urged in a clockwise direction as viewed in Fig. 7 by a spring 148 which is positioned in a recess 150. This arrangement functions to maintain the desired degree of tension in the cable 92, which may be adjusted by an adjusting nut 152. A surface 154 on the element 144 cooperates with a detector represented schematically by the circle 156 to provide an indication of breakage of a cable 92 if the element

144 swings far enough in a clockwise direction under urging of the spring 148 to interact with the detector 156 _r which detector may be of any suitable type, such as an electrical switch or a light beam which can be interrupted by the element 144.

As best shown in Fig. 10, from one end which is secured in the retainer 108 of the member 106, the cable 92 for each character set 60 extends around the pulleys 134, 132 and 126 on the frame member 124 (Figs. 9 and 10), upward through the interior aperture 82 in the drum 62, around the upper pulleys 86 and 88, through the various elements 58 (to each of which said cable is secured) of the character set 60, around the lower pulley 87, around the pulleys 128 and 130 on the frame member 124, around the member 106 in the grooves 111 thereof, and back to the retainer 108 to which the other end of said cable is secured.

Relative movement of the character set 60 with respect to the drum 62 for positioning of a predetermined character element 58 in printing position is accomplished by rotational movement of the member 106 for that character set 60 with respect to the drum 62, which causes movement of the cable 92, thereby altering the location of the character set 60 on the drum 62. This is normally done while both the drum 62 and the member 106 are rotating, and is accomplished by imparting a differential rotational velocity (either positive or negative) to the member 106. It will be recalled that each member 106 is secured to a pulley 112 which is rotated through the belt 114 by the respective driving motor 118.

The manner in which the relative speeds of the drum 62 and a member 106 are combined to produce the desired movement of the respective character set 60 is graphically shown in Fig. 14, in which the main shaft angular position is represented on the horizontal axis, and the angular speeds of the drum 62

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and the member 106 are represented on the vertical axis. The normal rotational speed of the drum 62 is represented by the line 160. This speed is constant, and will be termed synchronous speed. When the rotational speed of the member 106 is also at synchronous speed, there is no relative movement between the character set 60 and the drum 62.

It may thus be seen that in order to shift the relative position of the character set 60 with respect to the drum 62, in order to position a different character element 58 at the printing position, it is necessary to change the relative rotational speed of the member 106 with respect to the rotational speed of the drum 62. Fig. 14 illustrates this. For example, in order to shift the character set 60 the equivalent of five printing positions in a first direction, with respect to the drum 62, the member 106 is accelerated from the synchronous speed at which it is rotating, by accelerating the respective motor 118, at a rate represented by the line 162 in Fig. 14. At a predetermined time, the member 106 is then decelerated, as may be accomplished by altering the input to the motor 118, at a rate represented by the line 164, also designated "a5", until the member 106 returns to synchronous speed as represented by line 160. The net result of this acceleration and deceleration is to move the character set 60 five positions with respect to the drum 62. Movement of five positions in the opposite direction may be achieved by first decelerating the member 160 and then accelerating it, as represented by lines 166 and 168, also designated ^w c5", in Fig. 14. It will be noted that for a large incremental movement of the character set 60 comprising six to nine positions in the first direction, the member 106 is first accelerated as represented by line 162 to a predetermined velocity, then maintained at that

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constant velocity, as represented by the line 170, for a predetermined length of time, and then decelerated to synchronous speed, as represented by a line 172 (also designated ⁿ a9 ^w ) for a nine-position movement. A significant advantage of the present invention over prior art structures is that these printing position movements of the various character sets are accomplished smoothly and dynamically during rotation of the drum 62, with the member 106 associated with a given character set 60 being at synchronous speed as the given character element is rotated into printing position, thereby making a high printing speed possible.

A circuit 180 for controlling the operation of the D.C. motor 118 which drives each of the members 106 is shown in Fig _* 15. One such circuit is provided for each motor 118. In broad terms, the circuit 180 may be considered to include two feedback loops: a velocity loop 182 and a position loop 184 _* The velocity loop 182 is used to effect large changes in rotational velocity of the motor 118, for example, on the initial acceleration or deceleration required for movement of a character set 60, while the position loop 184 is used for small changes in position of the character set 60, as for example when said character set has completed the greater portion of its predetermined movement for a given printing operation, and a final minor adjustment in position is required. The velocity loop- 182 includes a tachometer 186, which may, for example, comprise an 290 tachometer chip, manufactured by SGS-ATES, Milano, Italy, and a filter and amplifier circuit 188, which may, for example, comprise a low pass RC (resistor- capacitor) filter in combination with a low DC offset operational amplifier such as a National F411A manufactured by National Semiconductor Corporation, Santa Clara, California, and which produces a signal

which is derived from the optical encoder 119, and which is proportional to the speed of the motor 118.

Each of the optical encoders 35 and 119 may be a Renco model R2000, manufactured by Electrocraft Corporation, Minneapolis, Minnesota. Each optical encoder 35 and 119 provides three outputs: a TTL index, comprising one pulse per revolution, and two sine wave quadrature output signals, an A phase signal and a B phase signal. Signals from the two optical encoders 35 and

119 are shaped by the Schmitt triggers 234 and 236, which may be of type 74LS14 manufactured by National Semiconductor Corporation _* These signals are transmitted over conductors 237 and 240, respectively, to a comparator logic circuit 238, which may comprise two cascaded 4-bit counters, such as 74LS193 counters, manufactured by National Semiconductor Corporation. This circuit is connected so that A phase pulses from the optical encoder 35 squared by Schmitt trigger 234 cause the circuit 238 to count in one direction, while A phase pulses from the optical encoder 119 squared by Schmitt trigger 236 cause it to count in the opposite direction. A microprocessor 202, to which the comparator logic circuit 238 is connected by a conductor 239 which is applied to a port 241 of the microprocessor 202, utilizes the net count information from the comparator logic circuit 238 to alter a speed word which is provided to a digital to analog converter 222, so that the DC motor 118 will again change its speed, by reducing speed if it had previously increased speed, and vice versa, to return to its synchronous speed, once the desired phase of the motor 118 with respect to the motor 34 is attained. A summing circuit 190, which customarily takes the form of a junction of resistors on each joining conductor, compares the signal produced by the

tachometer 186 with a reference signal established by the setting of a speed set circuit 192, which comprises a variable resistor, having a maximum resistance of, for example, 5000 ohms, having one end connected to a base reference potential, shown here as ground, and having the other end connected to a source of positive potential of appropriate magnitude, such as +12 volts, to produce an error signal. This error signal is processed by an error amplifier 194, which may take the form of an operational amplifier such as a National F411A manufactured by the National Semiconductor Corporation, and by a power amplifier 196, and is used to drive the DC motor 118. The power amplifier 196 may be any suitable amplifier, such as the FA9000 manufactured by Electrocraft Corporation, Minneapolis, Minnesota, capable of supplying the necessary current to the motor employed and having appropriate feedbacks to supply a constant current to the motor regardless of speed for speeds which are less than 4000 rpm.

The setting of the speed set circuit 192 determines the synchronous speed of the motor 118. This synchronous speed differs from the synchronous, speed of the synchronous motor 34 which is used to drive the drum 62, by a constant multiplier which is equal to the drive ratio of the belt drive system which includes the belt 114 and the pulley 116.

Once the speed of the DC motor 118 comes within 5% of the synchronous speed of the motor 34, a lock signal, such as a "logic 1" output, is generated by a logic lock circuit 198, which may comprise a pair of operational amplifiers such as LN311 amplifiers manufactured by National Semiconductor Corporation. This signal is sent by conductor 200 to a port 201 of the microprocessor 202, which may be of type 8051 manufactured by Intel Corporation, Santa Clara _r California.

The microprocessor 202 controls the circuit 180, and in this instance it recognizes the signal from the logic lock 198 and sends a control signal through a port 204 via a conductor 206 to a switch 208, which may be of type TL191 manufactured by Texas Instruments Incorporated, to cause said switch to close, thereby phase locking motors 34 and 118 and causing the position loop 184 to be rendered effective in controlling the speed of the motor 118. At this point, the bead chain 60 is stationary, but is not in a proper position for printing.

A phase difference between the A phase output pulses of the two optical encoders 35 and 119, associated with the motors 34 and 118 respectively, produces an output signal at the summing junction 190 which forces the motor 118 to adjust its speed. A phase detector 210 and a filter and amplifier circuit 212 in the position loop 184 process the signals taken from the encoders 35 and 119 on conductors 207 and 209 _* The resulting signal is then applied through the switch 208 and a conductor 214 to the summing junction 190. The phase detector 210 produces an output signal which is proportional to the phase difference between the motors 34 and 118, and may be of type No. AD532JD, manufactured by Analog Devices, Norwood,

Massachusetts; and the filter and amplifier circuit 212 may comprise a low pass RC filter and a low DC offset operational amplifier such as Model LF411A, manufactured by National Semiconductor Corporation. Synchronization of the motor 118 with the motor 34 results in the bead chain 60 being held stationary with respect to the printing drum 62.

After initial power up or reset of the system, once the motors 34 and 118 are locked in phase, the bead chain 60 must first move to a predetermined home position with respect to the printing drum 62. The TTL index pulse from both

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optical encoders 35 and 119 must be coincident in order for the character "zero* to be in home or initial position. In order to make these index pulses coincident, the number of A phase pulses or counts from the encoder 119 occurring between the TT pulses from the encoders 35 and 119 is first counted by the comparator logic circuit 238. Because of the mechanical constraint which includes the pin 121 and the slot 123, this number will be less than 30, and the phase of the index pulse of encoder 35 will lag the phase of the index pulse of encoder 119. The motor 34 must therefore be speeded up and return to synchronous speed to move the necessary number of A phase counts to cause its index pulse from encoder 35 to be coincident with the index pulse from encoder 119.

From home position, a desired character 58 in the chain 60 is positioned for printing by moving the chain so that the desired character is on the printing line. This is accomplished by loading a number, representative of the character to be positioned, into the microprocessor 202 via data bus 244. The microprocessor 202 by reference to a look-up table in memory (RAM) , correlates this number with the number of counts required to place the new character in the correct printing position.

A 5-bit code, representative of the required motor speed, hereinafter designated as the speed word, is placed on data lines 218 which are connected to port 220 of the microprocessor 202. This signal is converted from digital to analog form by the digital to analog circuit 222, which may be of type DAC0831, manufactured by National Semiconductor Corporation _* The resulting analog signal is transmitted through a switch 224, which may be of type TL191 manufactured by Texas Instruments Incorporated. The polarity of the signal is controlled by the microprocessor 202 through

a conductor 226 which causes the signal to be transmitted over either a first branch 228, or a second branch 230 containing an inverter 232 which may be of type 74LS04 manufactured by National Semiconductor Corporation, to the summing junction 190. From the junction 190, the signal is applied through the error amplifier 194 and the power amplifier 196 to the motor 118 to produce the desired alteration in its speed. Polarity selection by the switch 224 enables the speed of the motor 118 to be either increased or decreased, depending upon the positioning movement desired for the corresponding bead chain 60.

It may be seen that in order to move the character bead chain 60 from one printing position to another, the microprocessor 202 generates a different speed word in response to an external command applied over the data bus 244, so as to cause the chain 60 to move a selected distance in a selected one of two ^" opposite directions generally parallel to the axis of rotation of the drum 62. In the illustrated embodiment, it is contemplated that each character of the bead chain 60 is eighteen optical encoder counts from the next, so that, for example, to move the bead chain eight characters, 144 counts would be required.

When the shaft of the motor 118 has advanced to a predetermined position which is a certain number of counts, say 50, away from its new position, the speed word is progressively reduced, by use of a previously established look-up table associated with the microprocessor 202, so that the motor speed linearly decreases from high speed back to synchronous speed. Switch 208 is then closed and the movement is complete. If the step to be undertaken is less than 50 counts, the look-up table is entered directly, and the step is automatically performed. This routine is employed to position the character zero to its

printing position, which is considered to be home, and is also utilized for positioning of all other characters.

Referring now to Fig. 16 _r it will be seen that the system circuitry 252 includes a plurality of circuits 180, which are provided in the printing apparatus, one for each bead chain 60 _* A microprocessor 242, which may be of type 68000, manufactured by Motorola, Inc., Phoenix, Arizona, exercises overall system control, and is connected by the data bus 244 and control lines 246 to the various individual circuits 180, each control line 246 being connected to the microprocessor 202 of the respective circuit 180 via a port 245 (Fig _* 15) . A suitable memory unit 248, which may take the form of a floppy disk manufactured by Shugart Corporation, Sunnyvale, California, is coupled to the microprocessor 242, as is a keyboard 250 for providing operator input. The control system of Fig. 16 provides input data to the circuits 180 for positioning their bead chains 60; controls the sequence of movement of the chains; initiates the necessary start-up functions; and performs such other control operations as may be required in particular printing applications.