CA1138710A - Microcomputer control of ribbon drive for printers - Google Patents

Abstract

Abstract A microcomputer controls a ribbon drive assembly in a high speed wire matrix printer to eliminate rib-bon slack, to insure proper ribbon positioning and to conduct diagnostics in conjunction with turning on the printer such as start-up time each day and after re-placement of the ribbon with a new ribbon, the diag-nostics checking to be sure that a ribbon is actually in the proper position, that it is threaded across the print line and also checking for proper operation of the ribbon drive, logic, and electronics. Tests are also made for proper ribbon drive during normal rib-bon feeding operations.

Description

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; 1 MICROCOMPUTER CONTROL OF RIBBON DRIVE FOR PRINTERS

- Background of the Invention This invention relates to ribbon drive assem-; blies, but more particularly to control systems for eliminating ribbon slack in conjunction with automat-: ic diagnostics. High speed printers, including wire -- matrix printers, that utilize inked ribbons many times encounter difficulties in controlling the rib-bon motion and maintaining proper positioning and ribbon tension.

Summary of the Invention In accordance with the present invention, control means is provided in conjunction with a high speed wire matrix printer to insure removal of ribbon slack, to insure proper ribbon positioning and to conduct diagnostics in conjunction with turning on the printer such as at start-up time each day and after replacement of the ribbon with a new ribbon, the diagnostics checking to be sure that a ribbon is actually in the proper position, that it is threaded across the print line and also checking for proper operation of the ribbon drive, logic, and electronics.

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1 Cross-Reference to Related Patent Applications , 1 The present patent application is one of a group of ~`v copending patent applications which describe the same overall printer subsystem configuration but which ~ individually claim different inventive concepts embodied `; in such overall printer subsystem configuration. These ` related patent applications were filed on the same date, ~ namely, October 19, 1979. One that is of particular *,~ interest is noted below:
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Application Serial Number (IBM Docket No. BC9-79-020) entitled "Printer Subsystem with Microprocessor Control", the inventors being Messrs. W.W. Boynton et al.
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For a better understanding of the present invention, together with other and further advantages and features thereof, reference is made to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

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` 3 Brief ~escription of the Drawings ', -:, ~;~ Referring to the drawings:
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;; Fig. 1 is a simplified system diagram of the printer subsystem in which the invention is in-- 5 corporated.

Fig. 2 illustrates the printer console having a ~ number of printer components including forms feeding.
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Fig. 3 is a frontal view of the printer.

Fig. 4 is an exploded view of various printer assemblies including the forms feed assembly, the print assembly, and the ribbon drive assembly.

Figs. 5 and 6 illustrate a ribbon shield that is useful in the printer.

Fig. 7 is an overhead view of the printer slightly from the rear of the unit showing the forms feed assembly open.

Fig. 8 shows a number of electrical components and control blocks for the printer subsystem.

Fig. 9 illustrates a ribbon drive control circuit for ~he printer, the circuit utilizing a microcomputer.

Fig. 10 shows some representative waveforms derived during operation ol the ribbon drive motors.
"' . 3~3r~ o Fig 11 illustrates a routine useEul for ribbon slack removal and diagnostic error detection.
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Fig. 12 is a routine used during normal ribbon . drive operations.

~; 5 Description of Printer Subsystem and Printer ~lechanisms .
In order to best illustrate the utility of the present invention, it is described in conjunction : ~ with a high speed matrix printer, typically capable of printing in a high range of lines per minute on continuous forms.

Fig. 1 illustrates a representative system con-figuration including a host system 1 and the printer subsystem 2 which includes a printer control unit 3 and printer electronics 4. Command and data signals are provided from the host system by way of interface 5, and command and control signals are provided from ; printer control unit 3 to the printer electronics 4 way of bus 6. Status signals are supplied by printer - control unit 3 to host system 1 by way of interface 5.
Typically, the host system 1 generates information including commands and data, and monitors status.
Printer control unit 3 receives the commands and data, decodes the commands, checks for errors and generates status information, controls printing and spacing, and conducts printer diagnostics. Printer electronics 4 executes decoded control unit commands, monitors all printer operations, activates print wires, drives motors, senses printer emitters, and controls operator panel lights and switching cir-; 30 cuitry. It controls the tractor/platen mechanism, :' :1~315~10 .i ~
` ; 5 the ribbon drive, the print head (i.e., ~ctuator group) carrier, the operator panel, and the printer sensors.

The elements of the system, such as the printer ~ .
` 5 control unit and printer electronics, incorporate one or more microprocessors or microcomputers to analyze commands and data and to control operations.

Figs. 2 and 3 illustrate various components of the printer all of which are housed in the console 10. Various access panels or covers such as those designated 11, 12, and 13 are provided. Top cover 11 has a window 14 that enables an operator to observe forms movement during operation of the printer and when the top cover is closed. Forms (documents) 15 are provided from a stack 16 and can be fed in one embodiment upwardly or downwardly as viewed in Figs.

2 and 3 by means of a forms feed assembly 20 which includes one or more sets of forms tractors such as the upper set comprising tractors 90 and 91. A forms guide 28 guides the forms after printing to a takeup stack, not shown, but positioned below the printing - mechanism and to the rear of the printer console.
The printer incorporates a print assembly 30 that is positioned generally in a horizontal relationship with respect to forms 15 at a print station 32.
Print assembly 30 is more clearly visible in other : views. This is also true of the printer ribbon drive assembly 40 which is located in closer proximity to the front of the printer. Printer control unit 3 and its associated microprocessors are generally located behind the side cover 13.

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1~1l31~ ~10 A ribbon 41 is provided on one of the spools 42 or 43, which are disposable. Each box of ribbons would preferably contain a disposable ribbon shield 46 that fits between print assembly 30 and forms 15 to keep ribbon 41 in proper alignment and to mini-mize ink smudging on form 15. Two motors drive rib-bon 41 back and forth between spools 42 and 43. The printer control unit detects ribbon jams and end-of-ribbon (EOR) conditions. A ribbon jam turns on an error indicator and stops printing. An ~OR condition reverses the ribbon drive direction.

The printer includes an operator panel 26 tshown and described in greater detail in the Boynton et al application) that consists of several operator control keys, two indicator lights, a power - on/off switch, and an operator panel display.

A 16-position mode switch 65 (shown and described in the Boynton et al application) has an on-line position that permits printing to be controlled by the using system. All other positions ` are off-line and do not allow printing to be initi-ated from the using system.

Overview of Printer Mechanisms ,,,~
Figs. 4-7 show details of construction of the ;~ 25 forms feed assembly 20, print assembly 30, and rib-; bon drive assembly 40.
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, Forms ~eed assembly 20 has end platcs ~sid~
castings) 21 and 22 which support the various forms feed mechanisms including a drive motor 23 to drive tractors 90-93 through timing bel~ 109 and a platen 29 located behind the forms and against which the print wires 33 are actuated during printing. Motor 23 has a forms feed emitter assembly 24 and there is a separate end of forms and jam detector emitter 25.

The print assembly includes a base casting 75 supporting various mechanisms including print motor 76, shown in phantom in order that other ele-ments may be seen more easily, and connected to drive a print head carrier 31 with actuator block assembly 77 in a reciprocal fashion horizontally to effect printing on an inserted form. The print assembly also drives a print emitter 70 having an emitter glass 71 and an optical sensor assembly 72.

The ribbon drive assembly 40 includes a support casting 44, a cover 45, and drive motors 49 and S0.

; 20 Forms Feed Assembly In order to load paper in the printer the forms feed assembly 20 pivots away from the base casting 75 at pivot points 80 (80') and 81 (81') to allow access to thread the paper into position. Latches 83 and 84 are raised by the operator so that extremities 83a and 84a disengage eccentric pins 85 and 86 on the forms feed tractor. The forms feed assembly then pivots away from the operator as viewed in Figs. 3 and 4. This allows access to the tractors 90-93 so that the operator may load paper.

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:' .3~7:10 The forms feed assembly is then reclosed and re-latched by latches 83 and 84 for normal machine opera-tion. During the time that the forms feed asse~bly is pivoted back for service, a switch 94 prevents machine operation. This switch is actuated by a tang 95 on the forms feed assembly when it is closed.

Referring to Figs. 4 and 7, the forms feed assembly includes means for adjusting for forms - thickness. As mentioned, the entire forms feed assembly pivots back from the rest of the printer about pivot points 80 and 81. In the closed position the forms feed assembly is in such a position that a spiral cam 96 engages a pin 97 on the main carrier shaft 98 of the print assembly 30. Adjustment of the spiral cam and knob assembly 96 is such that it rotates the main carrier shaft 98. Assembly 96 is retained in position by a spring loaded detent assem-bly. This has a spring loaded pin which engages notches in the knob so that it is held in the posi-; 20 tion set by the operator. Associated with shaft 98 '~ are eccentrics such as portion 98a on the left end of shaft 99 with tenon 100 onto which latch 83 is mounted. Rotation of shaft 98 thus moves latches 83 and 84 which changes the distance between assemblies 20 and 30 and thus the distance between the ends ofprint wires 33 and platen 29. This adjustment en-ables the printer to accommodate forms of various thicknesses. The printer can handle forms from one part to six parts thickness.

The paper feeding is accomplished by the four sets of tractors 90-93 two above the print line and two below the print line. The individual tractors ~ ~38 10 include drive chains to which pins are attach~d ~t the proper distance to engage the holes in the form.
As an example, tractor 90 has drive chain 101 with pins 102. Chain 101 is driven by a sprocket 103 attached to a shaft 104 which also drives the sprocket and chains for tractor 91. ~ractors 92 and 93 are driven from shaft 105. Because the tractors are above and below the print line, the printer is able to move the paper in either direction. Thq normal direction of forms drive is upwardly in Figs. 3 and 4. However, it is possible to move the paper downwardly, as well.
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Rotation of shafts 104 and 105 and forms feed-ing is accomplished by appropriate drive of motor 23 in the proper direction which in turn drives pulleys 106 and 107 (to which shafts 104 and 105 are connected) from motor pulley 108 by means of drive-timing belt 109. Cover 110 covers belt 109 and pulleys 106-108 during rotation. The forms feed emitter assembly 24 includes an emitter wheel 47 with marks to indicate rotation and a light emitting ` diode assembly 48 that serve to indicate extent of rotation of motor 23 in either direction and as a consequence, the extent of movement of the forms as they are driven by motor 23.

The capability of the printer to feed paper in both directions offers some advantages. For example, in order to improve print visibility at the time the Stop button is pushed by the operator, the paper may be moved up one or two inches above where it normal-ly resides so that it can be easily read and can be :

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10, easily adjusted ~ox registration. ~hen the Start key is depressed, the paper is returned to its normal ~ printin~ position back out of view of the operator.
- The printer may also be used in those applications where plotting is a requirem~nt. In this case a plot may be generated by calculating one point at time and moving the paper up and down much li~e a plotter rather than calculating the entire curve and printing it out from top to bottom in a raster ` 10 mode.

End of forms and jam detection is accomplished in this assembly by a sprocket 112 just above the lower left tractor. The teeth in this sprocket pro-,' ~ trude through a slot 113a in the flip cover 113.
This sprocket is not driven by any mechanism but simply is supported by a bearing. The sprocket en-gages the feed holes in the paper as it is pulled past by the tractor assemblies. On the other end of the shaft 114 from the sprocket is a small optical emitter disc 115. The marks in this disc are sensed by an LED phototransistor assembly 116 and supplied to the electronics of the subsystem. The electronics verifies that marks have passed the phototransistor at some preselected frequency when the paper is being fed. If the mark is not sensed during that time, the machine is shut down as either the end of forms has occurred or a paper jam has occurred.
:~; ' The castings 88 and 89 supporting the tractors ~' 90-93 are adjustable left or right in a coarse ad-justment in order to adjust for the paper size used in a particular application. After they are proper-ly positioned they are locked in place on shaft 67 by locking screws such as locking screw 87.

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-- ~3~10 ~ 11 tractors are driven by the two shafts 104 and 105 from motor 23 as previously described. The motor adjusts in the side casting 21 in slots 1~0 in order to prove the correct tension for belt 109.

Besides the coarse adjustment, there is also a fine adjustment which is used to finally position in very small increments laterally the location of the printing on the forms. This is done by a threaded knob 66 which engages shaft 67 to which both tractor castings clamp. This shaft floats between side - castings 21 and 22 laterally. The threads in knob 15 engage threads on the right end of shaft 67. The know is held in a solid position by a fork 68.
Therefore knob 66 stays stationary and the threads driving through the shaft force shaft 67 laterally left or right, depending upon the direction in which knob 66 is rotated. Shaft 67 is always biased in one direction to take out play by a spring 69 on the left end of the shaft. As the paper leaves the top of the tractors, it is guided up and toward the back of the machine and down by wire guide 28.

In order to insure that the distance between the pins in the upper tractors is in correct re-lationship to the pins in the lower tractors an adjustment is performed. This adjustment is made : by inserting a gauge or piece of paper in the tractor assembly which locates the bottom pins in the correct relationship to the top pins. This is done by loosening a clamp 121 on the end of shaft 104. Once this position is obtained, then clamp 121 is tightened and in effect phases the "~
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' ' ~3~ 0 top set of tr~ctors to the bottom set so that holes in the paper will engage both sets of tractors cor-rectly. Forms may be moved throu~h the tractor forms feed mechanism manually by rotating knob 12- This know simply engages the top dri~e shaft 104 of the upper tractor set and through the timing belt 109 provides rotational action to the lower tractor set, as well.

Print Assembly .
In Figs. 4 and 7, a carrier 31 comprising an actuator block 77 and support 78 accommodate all the print heads with their wire actuators 35 and print wires 33. This assembly is structured to hold from two up to eight or nine print head groups of eight actuators each. Thus, a printer with eight print head groups, as shown in Fig. 4, has sixt~-four print wire actuators and sixty-four associated print wires. Only two actuators 35 are shown positioned in place in Fig. 4. The other sixty-two actuators would be located in apertures 133, only a few of which are depicted. To insure long life of the print wires, lubricating assemblies 134 containing oil wicks are positioned in proximlty to the print , wires. The print wire actuators fire the wires to ` 25 print dots to form characters. Carrier 31 is shuttled back and forth by a lead screw 36 driven by motor 76. Lead screw 36 drives the carrier back and forth through nuts which are attached to the carrier. When carrier 31 is located at the extreme left as viewed in Fig. 4 (to the right as viewed in Fig. 7), this is called the "home" position.
When the carrier is moved to the home position, a '' . .

38 ,!~,0 cam 37 attachcd to the carrier engages a pin 38, the ~` pin being attached to the main carrier shaft 98.
If the machine has not been printing for some period of time, in the neighborhood of a few seconds, the printer control unit signals the carrier to move all the way to the left, in which case cam 37 en-gages pin 38 to rotate the main carrier shaft 98 approximately 15 degrees. On each end of the shaft are the eccentrically located tenons, such as tenon 100, previously describcd. These tenons engage the latches 83 and 84 labeled "7" so that the distance between the print assembly and the forms feed assembly is controlled by the latches. As shaît 98 rotates, the eccentrics associated with latches 83 and 84 separate the forms feed assembly from the print assembly.

The current necessary to fire the print wire -~ actuators is carried to the actuators via the cable - assemblies 73, Figs. 4 and 7, one for each group of ; 20 eight actuators. The cabling, such as cable 73a, Fig. 4, is set in the machine in a semicircular !~ ~, loop so that as carrier 31 reciprocates it allows the cable to roll about a radius and therefore not put excessive stress on the cable wires. This loop in the cable is formed and held in shape by a steel backing strap 74. In this case there is one cable assembly for each group of eight actuators or a maximum of eight cable backing strap groups on each ; machine.

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1 ~1 ~ibbon Drive Assembly The ribbon drive assembly 40 for the printer is ; shown in Figs. 3, 4 and 7 primarily. Spools 42 and 43 which contain the ribbon can be seen on either side of the machine near the front, Fig. 3. Th~se ` spools typically contain 150 yards of nylon ribbon that is one and a half inches wide. Gear flanges 118 and 119, Fig. 4. support ribbon spools 42 and 43, respectively. Drive for spool 43, as an example, is from motor 50, pinion gear 13Z to a matching gear ll9a formed on the underneath side of gear flange 119 then to spool 43. In one direction of feed, the ribbon path is from the left-hand spool 42 past posts 125 and 126, Figs. 3, 4 and 7 across the front of the ribbon drive assembly between the print heads 34 and forms 15, then past posts 127 and 128 back to the right-hand ribbon spool 43. The ribbon shield is generally located between posts 126 and 127 and is mounted on the two attachment spring members 130 and 131.
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Ribbon Shield Fig. 5 illustrates a ribbon shield 46 that is particularly useful in the printer of Figs. 3, 4 and 7. Fig. 6 is a cross-sectional view along the lines 6-6 in Fig. 7. Shield 46 has an elongated aperture 46a extending almost its entire length. The aper-ture enables the print wires 33 to press against the ' ribbon in the printer through the shield in order to print on forms 15. Shield 46 has slits 46b and 46c at opposite extremities to permit easy mounting in the printer on spring members 130 and 131 of the ': ' ~''''' ~
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' 15 ribbon drive assembly, Figs. 3, 4 and 7. Shield 46 and ribbon 41 are illustrated slightly on the bias in Figs. 5 and 7 which is their more normal relation-ship in the printer. The ribbon drive assembly 40 is also positioned on a slight bias relative to horizontal to accommodate the bias of shield 46 and ribbon 41. In this condition aperture 46a assumes a horizontal relationship with respect to the print wires 33 and forms 15.

Printing of Characters Characters that are printed are formed by ~ printing dots on the paper. These dots are printed - by wires that are mounted in groups of eight on - carrier 31 that moves back and forth adjacent to the print line. Printing is bidirectional with ' complete lines of print formed right-to-left and '~ ~ left-to-right.
''' A character is formed in a space that is eight dots high by nine dots wide. Two of the nine hori-zontal dot columns (1 and 9) are for spacing between characters. Any one wire can print a dot in four of the seven remaining horizontal dot positions (2 , ; through 8). The printer can print 10 characters per inch or 15 characters per inch.

Most or the characters printed use the top seven wires in the group to print a character in a format (or matrix) that is seven dots high and seven dots wide. The eighth (bottom) wire is used for certain lower case characters, special characters, and underlining.

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,: ' ' ., ~3~ 0 The number of print wire groups varies according to the printer model, and typically can be 2, 4, 6 or 8 groups. Printing speed increases with additional wire groups. There are, as an example, 16 character sets stored in the printer control unit. ~ny of these sets may be specified for use by the host system program.

Printer General Block Diagram Fig. 8 illustrates various printer blocks of interest. A power supply 245 supplies the unit with all the power to drive and to control. The on/off switch 240 controls power supply 245 being on and off. From the power supply the cover interlock switch 242 enables and disables the 48-volt drive which controls much of the printer logic 243.
Logic 243, once enabled, looks at operator panel 26 for information as to the operations to be performed.
Mode switch 65 tells the logic which type of opera-tion in testing procedures should be run. Print assembly 30 is controlled by the printer logic along with the forms assembly 20. Emitter devices 24 and 70 supply positional information to the printer logic. The printer logic also controls and talks with the interface panel 247 and passes information on the other parts of the printer. The ribbon motors 49 and 50 are controlled in an on/off fashion by printer logic 243 which accepts inputs from the ribbon assembly to determine when the end of r~bbon has occurred. Head servo 252 is a control block that insures that the print head is in the proper , position at the proper time for,the actuators to fire. Forms servo 253 is a control block that moves .

~ 3~3710 the forms to desired locations. Fans 254-258 are used to control temperature within the machine. As described in the Boynton et al patent application, printer logic 243 includes two microprocessor adapter blocks 200 and 210, The first one included is the communications adapter C~lA which accepts input and passes it to the second one which is the control adapter CTA that actually controls the printer.

Description of Ribbon Control and Drive System .

and Operating Modes Including Diagnostics The printer ribbon drive assembly consists of two stepper motors driving a ribbon between two reels.
While one motor drives the ribbon, the other motor is being pulled, and generates a voltage in one of its phases. This voltage is detected and buffered to give an emitter signal, the frequency of occurrence of emitter signals being dependent on relative ribbon diameters, and typically occurring every 4 to 6 motor steps. A motor step is defined as 2~ rotation of an individual motor. These emitter signals when present indicate that ribbon is present on the spool which is being pulled and that the ribbon is moving.
When the emitters stop, the motors are reversed with the one that was being pulled now driving and the other motor generating emitters. The criterion used to indicate turnaround is loss of emitter pulses for a given number of motor steps of the driving motor.

Fig. 9 illustrates the Ribbon Drive Control block 300. This drive block comprises a micro-computer 301 outputting drive signals to analog drivestages 302 and 303 for the left and right motors 49 and 50, respectively.

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The microcomputer includes its own ~ead Only Storage, Random Access Memory, Interface and timers and may be any of a number of commercially available products of this nature. An example is the Intel 8748 EPRO~I microcomputer that incorporates an eras-able programmable Read Only Memory. From drive stage 302 there is an emitter line 305 going bac~
to microcomputer 301 and from drive stage 303 there is an emitter line 306 going back to microcomputer 10 301. When line 312, Power on Reset, is activated, the block becomes operational, line 315, +Error, becomes active and line 316, +Busy, becomes inactive.
Before operation can begin, +Error line 315 is reset either by activating -Error Reset line 311 or by entering the diagnostic mode. Normal motion of the ribbon is activated by turning on the -Run line 310.
Block 304 is a Line Drive to improve the voltage going off the card.

When the ribbon drive is first activated, there is a possible problem. The microcomputer 301 which drives the ribbon assumes an initial direction for the motion. If this direction is such that there is no ribbon on one of the feed spools, no emitters will occur on either line 305 or line 306 and the ribbon will begin motion in the opposite direction.
If there is a little slack in the ribbon, no emitters will again be received. This will cause a ribbon error and a signal is provided on line 315.

Fig. 10 shows representative examples of wave-forms for one of the motors at different test points.
The Phase A and Phase B signals are included. At ;

~3fl7~0 each change of the motor phase, a current spike is developed on the +48-volt line. This current spike is caused by the changing of the phase and provides an initial 48-volt pulse to the motor to cause it to move. A saw tooth sustaining current is also shown.
The driver current in the different motors increases in a repeating pattern. These are shown as half a saw tooth waveform with a flat in the middle. This is because the driver current increases as it comes closer to the poles on the motor. The High Power ground current through the motor changes as shown with each motor phase change. The induced feedback wave-forms are not shown in Fig. lO, but they are a lower level signal, measured in tens of millivolts.

Ribbon ~iagnostic Motion and Slack Removal Provision is made in the Control and Drive cir-cuits of Fig. 11 to enable removal of slack in the ribbon with minimum stretching of the ribbon and to provide~ a diagnostic verification test as part of the slack removal process. This would occur after load-ing of a new ribbon or at Power on time, for example.

Summary ( Microcomputer 301 drives one motor for thirty-two steps, each motor step corresponding to 2 rotation of the motor. If no emitters are received, then the driving action is reversed and the other motor ls driven for thirty-two steps, while the . , 1~38~ 0 microcomputer tests for emitters. This driving and reversing rocess continues until an emitter is received or six reversals have taken place. If a failure occurs this indicates that too much slack was present.

Once an emitter is received, the motor which is driving continues driving 4x32=128 steps and then a reversal takes place. The other motor drives for 32 steps and if no emitter is received then an error is present. If an emitter is received in this direc-tion, then the test was passed.

; By this scheme, emitters have been received in both directions verifying correct operation of the ribbon and any slack has been removed without undue tension on the ribbon.

Detailed Description - Diagnostic Motion and Slack Removal ;, , The flowchart in Fig. 11 begins when the diag-nostic mode is entered and at this time drive is initiated for one of the motors, such as the right motor and a maximum of six turnarounds is permitted.
Then the routine goes to Junction "X" and proceeds to drive the right motor for thirty-two steps.
These are arbitrary numbers of turnarounds and steps.
Other values could be used if desired. As the right motor drives, microcomputer 301 looks for emitters from the other motor, that is the left motor, in this case. A test is made to determine whether an emitter change has been detected. If no emitter change has been detected after driving the right 3~P10 motor for thirty-two steps, a turnaround is enabled and drive of the other motor is initiated. Then a decision is made to test and find out how many turnarounds have occurred. If six have occurred, S this indicates that each motor has been driven three times with thirty-two steps and there is undue slack in the ribbon or no ribbon at all. The ribbon motion is stopped, and +Error line 315, Fig. 9, is activated.
If less than six turnarounds have occurred, the rou-tine returns to Junction "X" and begins driving theother motor, not previously selected. The routine continues in this loop until emitters have been received or six turnarounds have occurred. I~hen at least one emitter has been received, that indicates that ribbon is present and is on the spool currently being driven. The motor associated with that spool is driven 128 steps (4x32) which is again an arbitrary number chosen only to insure that there are enough emitters so that the difference in spool diameters has no effect on the final outcome. After 128 steps, a turnaround is initiated and the motor which was being pulled is now driving and emitters are tested in the opposite direction. The motor is now driven a full thirty-two steps to drive the ribbon in that direction and then a test is made to determine whether any emitters have been received for this ' direction. If no emitter has been received, that indicates that there has been an error and motion is stopped. If an emitter has been received, that indicates that at this state emitters have been received in both directions. There is ribbon on the spool and there are no problems in the ribbon system at this time.

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both spools and, therefore, there is less tension on the ribbon. Slack, if present, is removed. Also, diagnostic testing of the ribbon drive and emitters in both directions is provided.

Normal ~ibbon Motion - Summary Slack removal during normal ribbon drive opera-tions provides an inhibit period after turnaround in which emitter changes are not tested. During this time one motor is driving for a large number of steps in one direction. Referring to Fig. 12, the ribbon is first initialized and turnaround inhibit is en-abled, as an example, for 50 to 100 motor steps.
When a run signal is received, the first motor phases are advanced and turnaround enable is tested. If turnaround is not enabled yet, then a test for end of inhibit of turnaround is done and control is returned to junction "Y".
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When turnaround is enabled, emitter testing begins. If an emitter has been received within 32 motor steps, then control goes to junction "Z".
There microcomputer 301 tests the distance traveled by the ribbon since turnaround or starting. If a sufficient distance has been traveled, then the Turned Once indicator is set off, no record of a previous turnaround is known and control goes to Junction "Y".

Junction "W" is reached if no emitter change was received for 32 motor steps. There a test is made to see if there has already been a turnaround ~3l~7/lo (the Turned Once indicator is on). If so, then there have been two turnarounds within a short travel distance and a ribbon jam has occurred.

If no turnaround has recently been made, then this is the first and the Turned Once indicator is set, turnaround for a short turn sequence is inhibit-ed, and motion is enabled in the opposite direction.

Detailed Description - Normal Ribbon ~lotion , Fig. 12 is a description of the normal ribbon motion sequence. At the outset, certain initializa-tion procedures are performed and a turnaround inhibit indicator is set. Then control proceeds to Junction "Y". The program will remain at this point until the ; ribbon motion is activated by turning on the Run line 310, Fig. 9. At this time, one of the motors is advanced and a test is made to determine whether a possibility of making a turnaround is enabled. If the microprocomputer is in a period in which turnaround is disabled, it will modify a disable count, test for the ~ 20 end of this turnaround disable time period and then ;I return to Junction "Y". If turnaround is enabled, emitter testing will then proceed. Then a test is made to determine whether an emitter had been received within a time period of 32 motor steps. If an emitter has been received, then the program goes to Junction , "Z". At Junction "Z", the program tests and modifies a ribbon travel count indicating the total ribbon motion. If this travel count has reached a certain value, generally in the neighborhood of ten yards of ribbon motion, which is comparable to approximately 16,000 motor steps, then the Turned Once indicator is -- ~387~() reset and future ribbon turnarounds will not cause a ribbon jam. The program then goes to Junction "Y".
If a turnaround is detected while the Turned Once indicator is on, then a ribbon jam is indicated. If no emitter had been received for 32 motor steps, the routine proceeds to Junction "W". At this point, the Turned Once indicator is tested. If this indicator is on, that indicates that two turnarounds have been de-~` tected within a small amount of ribbon travel and a ribbon jam is present. If this indicator is off, that ; indicates that this is a first turnaround or that it is a turnaround after a long distance of ribbon travel.
' The Turned Once indicator is then set on and the ; Inhibit Turnaround indicator is also set on. The motion of the other motor is enabled and return ismade to Junction "Y".

While a preferred embodiment of the invention has been illustrated and described, it is to be understood that there is no intention to limit the invention to the precise constructions herein dis-closed a~nd the right is reserved to all changes and modifications coming within the scope of the ` invention as defined in the appended claims.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A ribbon control system for a printer unit having various assemblages for printing of characters on forms including a forms feed assembly and a print assembly, comprising:

a ribbon drive assembly in said printer unit including means to mount a pair of ribbon spools and motor drive means including a pair of ribbon drive motors individually associated with said ribbon spools, said ribbon spools and related motor drive means serving alternately as ribbon supply and ribbon takeup during printing operations;
means for mounting a ribbon on said spools for guiding past a print station in said printer unit; and a microcomputer interconnected directly with said ribbon drive motors and serving as a ribbon control element, said microcomputer issuing drive control signals as required in order to activate said ribbon drive motors on a selective and alternate basis during operations of said printer unit to thereby drive said ribbon between said ribbon spools and said microcomputer being further responsive to emitter signals provided by said motors to monitor for proper move-ment of said ribbon, and to perform other ribbon control functions such as diagnostics and ribbon slack removal.

2. The ribbon control system of claim 1, wherein:
said pair of ribbon drive motors comprise a pair of stepper motors, one stepper motor being individually connected to each of said ribbon spools and said stepper motors being alternately operable to drive their respective related ribbon spools responsive to said microcomputer drive control signals, each motor when not driving being pulled by the opposite motor by way of connection with said ribbon through the related ribbon spool of the motor not driving and providing emitter signals to said micro-computer to indicate ribbon movement.

3. The ribbon control system of claim 1 wherein:
said microcomputer provides drive signals to said ribbon drive motors on a selective basis during normal start-up and ribbon drive operations including ribbon turn-around operations and wherein said microcomputer is further operable to inhibit the sensing of emitter signals from the ribbon drive motor not activated for a predetermined number of motor steps upon such normal start-up of said ribbon driving or following any ribbon turnaround operation in order to remove slack in said ribbon.

4. The ribbon control system of claim 3, wherein:
said microcomputer is operable to provide signals to test for a minimum distance traveled by the ribbon since the last turnaround or since start-up during a normal ribbon feeding operation, the traveling of said minimum distance being indicative of satisfactory ribbon movement and wherein said microcomputer is further operable to provide signals to test for occurrence of at least two turnarounds within a minimum number of motor steps to detect a ribbon jam.

5. The ribbon control system of claim 1, wherein:
said microcomputer is operable upon each power-on or start-up occurrence of said printer unit to enable removal of slack with minimum stretching of the ribbon and to provide a diagnostic verification test by providing signals to drive and reverse said ribbon drive motors for at least a minimum number of ribbon reversals, the occurrence of such minimum number of reversals within a selected interval of time being indicative of the fact that too much slack is present in the ribbon.

6. The ribbon control system of claim 5, wherein:
said microcomputer is operable to check for the occurrence of at least one emitter signal during the diagnostic test, such occurrence being indicative that a ribbon is present on the ribbon spools in said printer unit.

7. The ribbon control system of claim 6 wherein said microcomputer is operable to test for the occurrence of emitter signals during movement of said ribbon in both directions.

8. The ribbon control system of claim 7 wherein the maximum number of ribbon reversals permitted is a value in the range of 6 indicating movement of each of said ribbon drive motors at least 3 times and wherein the maximum number of steps is in the range of 32 motor steps for each motor movement.

9. The ribbon control system of claim 8 wherein:
said microcomputer is operable to test for occurrence of at least one emitter signal indicative that ribbon is present on the ribbon spools, and wherein said microcomputer is further operable to provide drive signals to activate, said ribbon drive motors, on a selective basis for an arbitrary number of motor steps such as in the range of 128 in order to insure that no effect on the testing has occurred due to difference in diameter of ribbon on the individual ribbon spools.