referring now to the drawings and , more particularly , to fig1 there is shown one embodiment of an adjustment fixture 10 of the present invention interacting with a multicolor laser printer 12 . printer 12 includes a black toner cartridge 14 ( fig2 ), a magenta toner cartridge 16 , a cyan toner cartridge 18 and a yellow toner cartridge 20 with corresponding photoconductive drums 22 , 24 , 26 and 28 . each drum 22 , 24 , 26 and 28 engages a transfer belt 30 . each of printheads 32 , 34 , 36 and 38 scans a respective image onto a respective one of photoconductive drums 22 , 24 , 26 and 28 . a paper drive motor 39 moves paper into engagement with transfer belt 30 . adjustment fixture 10 includes a sensing unit 40 and a processing / feedback unit 42 in bi - directional communication with control electronics 44 of printer 12 . sensing unit 40 is placed inside laser printer 12 in the space that is normally occupied by the four printer cartridges 14 , 16 , 18 and 20 . sensing unit 40 registers itself to each corresponding datum axis of photoconductive drums 22 , 24 , 26 and 28 . sensing unit 40 then detects the skew between each datum axis of color photoconductive drums 24 , 26 and 28 relative to the reference datum axis of black photoconductive drum 22 and feeds this information to processing / feedback unit 42 . sensing unit 40 also detects the location in both the scan and the process direction of a particular energized spot at each end of the writing line ( 215 . 9 mm apart ) and feeds this information to processing / feedback unit 42 . processing / feedback unit 42 is located on the assembly operator &# 39 ; s bench external to printer 12 . processing / feedback unit 42 receives data from sensing unit 40 , processes the data , and provides the assembly operator with a visual indication of the desired skew target for the particular printhead being mechanically adjusted and an interactive visual indication of the orientation of the scan line relative to this target . processing / feedback unit 42 also communicates with printer controls 44 to trigger printer 12 to generate the necessary laser patterns to enable the skew adjustment by the operator and also the necessary patterns to achieve the electronic alignments needed . once printer 12 generates the desired patterns for the electronic alignment process , then sensing unit 40 gathers the data necessary to allow printer 12 to perform the desired electronic adjustments . processing / feedback unit 42 then receives one or more sets of data from sensing unit 40 in order to confirm that the electronic adjustments made by printer 12 have indeed achieved the desired alignment results . this confirmation is communicated to printer controls 44 for final storage of the results in printer memory 46 , which may be in the form of non - volatile random access memory . sensing unit 40 includes a very rigid plate 48 ( fig3 ) that has a drum fixture 76 rigidly attached thereto . this rigid plate 48 includes a fixed sensor 50 at each end of each nominal photoconductive drum 22 , 24 , 26 , 28 axis . these fixed sensors 50 detect in both the scan direction , indicated by double arrow 52 , and the process direction , indicated by double arrow 54 . sensors 50 are rigidly fixed such that a line between a pair of sensors 50 that correspond to a same one of photoconductive drums 22 , 24 , 26 , 28 is parallel to a v - block reference datum 56 ( fig4 ) of black photoconductive drum 22 . that is , there are a total of eight fixed sensors 50 , with each imaginary line that joins a corresponding one of the four pairs of fixed sensors 50 being parallel to black photoconductive drum datum 56 . sensors 50 are fixed relative to black photoconductive drum datum 56 , and each is located as accurately as possible to either a 0 mm location or a 215 . 9 mm location at an end of a corresponding one of photoconductive drums 22 , 24 , 26 , 28 . sensing unit 40 also includes six floating sensors 58 , each disposed at a corresponding end of a corresponding one of the non - black drum fixtures 78 , 80 and 82 . sensors 58 are rigidly mounted to each one of the non - black drum fixtures 78 , 80 , 82 which float such that they each align with a corresponding v - block 60 of photoconductive drums 24 , 26 and 28 when sensing unit 40 is lowered into place . floating sensors 58 are located on the outboard side of , and as close as possible to , fixed sensors 50 , which are fixed parallel to the black v - block reference datum 56 . each sensor 50 , 58 can be in the form of a charged coupled device ( ccd ) array or a dual axis position sensitive diode , for example . the floating sensors 58 may also be located on the inboard side of fixed sensors 50 . the laser scan for each color is used as the light source that is measured by sensors 50 , 58 on each end of the non - black drum fixtures 78 , 80 , 82 . the difference in the process direction 54 values obtained between the two readings on one end and the two readings on the other end characterizes the skew of the photoconductive drum v - blocks 60 for that color relative to the black reference photoconductive drum datum 56 . this information is then used to provide the operator with a target that is used to align the printhead for that non - black color . in the case of the black image printhead , only the fixed sensors 50 are used to align the black printhead to the black reference photoconductive drum axis . after each printhead skew has been set , fixed sensors 50 are used to measure the location of the first and last dots or picture elements ( pel &# 39 ; s ) that form the scan line . sensors 50 feed this location information to printer controller 44 . the locations of the first and last pel &# 39 ; s are determined by imaging a line of nominal length ( 215 . 9 mm ) across drum fixture 76 for example . at the start - of - scan end , if no beam is detected on sensor 50 , then the scan line must be started earlier in order to pull the first pel back onto sensor 50 . more particularly , the number of counts that occur after the horizontal synchronization signal and before the start - of - scan must be reduced . on the other hand , if the scan line extends completely across sensor 50 , then the scan line must be started later in order to push the first pel onto sensor 50 . more particularly , the number of counts that occur after the horizontal synchronization signal and before the start - of - scan must be increased . once the start of the print line is on sensor 50 , the location of this starting point is determined by the output of sensor 50 . a technique similar to the above technique of locating the start - of - scan pel is used to detect the location of the end - of - scan pel . if no scan line is detected on the sensor 50 that is disposed at the end - of - scan , then pel slices are added in order to lengthen the scan line . on the other hand , if the scan line extends completely across the sensor 50 that is disposed at the end - of - scan , then pel slices are removed in order to shorten the scan line . calibration is accomplished by mounting sensing unit 40 onto a very rigid calibration plate 62 which has the skew between photoconductive drum v - blocks 60 very accurately minimized . thus , sensing unit 40 now has the ideal “ no skew ” condition set up between the photoconductive drum v - blocks 60 . next , a glass standard 74 is placed between a collimated light source and the sensors . glass standard 74 has etched apertures 64 extending therethrough which are accurately aligned parallel to the black reference photoconductive drum v - blocks 60 . thus , the six sensors 58 that float with the v - blocks 60 for each of the non - black colors can be calibrated to the ideal “ no - skew ” values provided by the calibration fixture . likewise , the eight sensors 50 that are fixed relative to the black photoconductive drum datum 56 are calibrated for the “ no - skew ” location parallel to the black photoconductive drum datum 56 . in addition , the eight fixed sensors 50 use the etched apertures 64 on the glass reference standard 74 to establish the ideal scan direction 52 and process direction 54 locations for each color . the test sequence is controlled by the alignment fixture processing / feedback unit 42 . processing feedback unit 42 , when initialized by the operator mounting sensing unit 40 into printer 12 and connecting printer 12 to processing / feedback unit 42 , communicates to a raster image processor ( rip ) 66 in printer control electronics 44 to initiate the alignment sequence . raster image processor 66 generates a special page to turn on a constant laser scan line ( 100 % duty cycle ) which scans the entire tip - to - tip distance . raster image processor 66 then sends a request to print a diagnostic page to a print engine 68 . print engine 68 starts the page under the normal print sequence , but does not drive transfer belt 30 , cartridges 14 , 16 , 18 20 , the voltages on drums 22 , 24 , 26 , 28 , or paper drive motor 39 . dots 70 in fig4 indicate the locations of the spots on sensors 50 , 58 that were located through apertures 64 of glass reference standard 74 . the path of the scan line , also referred to as a “ scan path ”, produced by the laser beam is indicated by arrow 72 . the alignment fixture processing / feedback unit 42 determines , at each end of each non - black color scan , the process direction distance from the nominal calibration spot , indicated by dot 70 , to the centroid of the laser beam as it strikes fixed sensor 50 . this distance is indicated by y 1f at the start - of - scan end and by y 2f at the end - of - scan end . tester 10 also determines , at each end of each non - black color scan , the process direction distance from the nominal calibration spot 70 to the centroid of the beam as it strikes floating sensor 58 . this distance is indicated by y 1v at the start - of - scan end and by y 2v at the end - of - scan end . “ y ” values are considered to be positive if they are on the side of the nominal calibration point 70 that is away from the black photoconductive drum axis datum 56 . the calculated skew distance of the laser beam relative to fixed sensors 50 is : the calculated skew distance of the laser beam relative to the v - block sensors 58 is : as can be appreciated , if d v is nonzero , then scan path 72 is not linear in three dimensions . rather , scan path 72 is slightly arcuate due to the cylindrical outer surface of the photoconductive drum . the skew between a reference line through the v - blocks 60 of that particular color and the black reference photoconductive drum datum 56 is : in a similar manner the skew can be found for each of the non - black color v - blocks 60 relative to the black v - block reference datum 56 . these measurements can also be corrected for the distance x 1 between fixed sensor 50 and floating v - block sensor 58 . distance x 1 is known from alignment fixture 62 . an angle , ∀, of scan line 72 across fixed sensors 50 is : where d f is in mm to be consistent with the units of the 215 . 9 mm distance between fixed sensors 50 . thus , the predicted location of the laser beam on the v - block sensor 58 at fixed sensor 50 is : y ′ 1v = y 1v + x 1 tan ∀= y 1v + x 1 *( d f / 215 . 9 ) where d f is in units consistent with y and x 1 is in mm . clearly , this same technique works for distances between fixed sensors 50 that are different than the desired nominal print line distance of 215 . 9 mm . in a similar manner , all the y iv can be corrected to y ′ iv values projected onto the fixed sensor location . using the laser beam itself with the algorithm described above corrects for the skew of the laser beam itself and separates out the desired information about the skew in v - blocks 60 alone . black printhead 32 is adjusted mechanically to be in line with the black photoconductive drum datum 56 using corresponding fixed sensors 50 located at each end of the scan line . the non - black color printheads 34 , 36 , 38 are mechanically adjusted using the respective fixed sensors 50 located on each end of the scan line until the skew of the printhead relative to the black photoconductive drum datum 56 is : s b =( y 2f − y 1f )= 2 * s v . the alignment fixture processing / feedback unit 42 presents the operator with a display which shows where the desired laser scan beam should be for the required skew , s b = 2 * s v , and where the beam is currently located . this is a real time presentation to allow the operator to make the appropriate interactive skew adjustment and then tighten the screws to mount the printhead in the proper position . a fine adjustment may also be made after the printhead coarse adjustment and tie down has been completed . this skew adjustment is made for each color sequentially . raster image processor 66 generates a second special page to turn on a constant laser scan line ( 100 % duty cycle ) which starts at the first pel location at the start - of - scan and stops at the last pel location at the end - of - scan , corresponding to a nominal 215 . 9 mm location . these actual values can be changed if advantageous to the alignment process . fixed sensors 50 can be disposed along the lengths of the photoconductive drums at respective locations corresponding to desired side printing margins . the alignment fixture processing / feedback unit 42 determines from the start - of - scan fixed sensor 50 whether the laser scan beam is completely off sensor 50 , extends completely across sensor 50 , or begins on sensor 50 . based upon this information , the fixture processing / feedback unit 42 determines whether the first pel of the scan beam is beyond the desired target , short of the desired target , or on the desired target . processing / feedback unit 42 then commands raster image processor 66 to either decrease or increase the number of counts that occur after the horizontal synchronization signal and before the start - of - scan . the number of counts is decreased in order to pull the first pel back onto sensor 50 or , if the first pel is on sensor 50 but beyond the target , to move the first pel backward towards the target on sensor 50 . the number of counts is increased in order to push the first pel onto sensor 50 , or , if the first pel is on sensor 50 but short of the target , to move the first pel forward towards the target on sensor 50 . based upon the alignment fixture processing / feedback unit command , raster image processor 66 increases or decreases the number of counts dynamically down the page . once the first pel of a particular color laser scan line is in the correct position on the start - of - scan end sensor 50 , processing / feedback unit 42 informs raster image processor 66 . raster image processor 66 then stops increasing or decreasing the number of counts and stores that count value in printer nvram 46 as the correct starting margin value ns . this process is carried out simultaneously for all four colors . next , alignment fixture processing / feedback unit 42 determines , from the end - of - scan fixed sensor 50 , whether the laser scan beam is totally off sensor 50 , extends completely across sensor 50 , or , if the last pel of the scan beam is on sensor 50 , whether the last pel is short of the desired target or beyond the desired target . based upon these data , processing / feedback unit 42 then commands raster image processor 66 to either increase the number of counts that occur after the horizontal synchronization signal and before the start - of - scan in order to push the end - of - scan pel onto sensor 50 , decrease the number of counts in order to pull the end - of - scan pel back onto sensor 50 , or increase or decrease the number of counts based upon the location of the end - of - scan pel on sensor 50 relative to the desired target ( increase if short of the target , decrease if beyond the target ). based upon the alignment fixture processing / feedback unit command , raster image processor 66 decreases or increases the number of counts dynamically down the page . once the last pel of a particular color laser scan line is in the correct position on the end - of - scan end sensor 50 , processing / feedback unit 42 informs raster image processor 66 . raster image processor 66 then stops increasing or decreasing the number of counts . raster image processor 66 then uses this count value ne and the count value that was stored in nvram 46 as the starting margin count value ns to calculate how many pel slices are to be inserted or deleted from the respective scan line . the number inserted , ni , is calculated as : ni = ne − ns . because ne and ns are measured in clock pulses which are pel slices , ni is the number of pel slices to be added if it is positive , and deleted if it is negative . raster image processor 66 then stores that ni value in printer nvram 46 as the correct starting pel slice insertion value for that color . this process is carried out simultaneously for all four colors . in another embodiment , the black line length is left unchanged from its initial value , and all the non - black color line lengths are adjusted to be equal to the length of the black line , rather than to the nominal 215 . 9 mm line length . the exposure to adjustment fixture 10 is that the black line length might be such that the end - of - scan pel does not land on sensor 50 in the adjustment fixture . thus , a larger sensor 50 is required in order to insure that the location of the last pel is always sensed . next , the alignment fixture processing / feedback unit 42 uses the fixed sensors 50 at both ends of all four colors to determine the process direction displacement y 1f of the skew - adjusted laser beam from the nominal target at the start - of - scan end and the displacement y 2f in the process direction from the nominal target at the end - of - scan end for each color . likewise , the alignment fixture processing / feedback unit 42 determines the corrected process direction displacement y ′ 1v and y ′ 2v of the skew adjusted laser beam from the nominal target at each end of the v - block aligned sensors 58 for each color . assuming that each photoconductive drum 22 , 24 , 26 , 28 has the same diameter and is rotating at the same constant angular velocity , and that transfer belt 30 passes under black photoconductive drum 22 last , each color will image on transfer belt 30 relative to the nominal spacing between the black photoconductive drum datum 56 and that color &# 39 ; s nominal spacing as described by the following relationship : y vfi =( y 1f + y 2f ) i / 2 −( y ′ 1v + y ′ 2v ) i in the case of black photoconductive drum 22 , the procedure is to align the black laser beam parallel with the black photoconductive drum v - block axis , so the equation for the process direction location of the black laser beam relative to the reference black photoconductive drum axis datum 56 is : the alignment fixture processing / feedback unit 42 calculates the measured drum - to - drum spacing y di for the i th color using the nominal drum - to - drum spacing y ndi corrected by the measured distance y vfi from the sensor nominal target and the laser beam off - set y vfk of the black photoconductive drum 22 itself : this drum - to - drum spacing , relative to the black photoconductive drum image , is calculated for all three non - black colors . the alignment fixture processing / feedback unit 42 passes these y di values for each non - black color to the printer raster image processor 66 . based upon this y d number , raster image processor 66 determines the appropriate number of whole pel &# 39 ; s ( scans ) between photoconductive drum image points on transfer belt 30 , and the fractional pel needed to determine the horizontal synchronization phasing between black printhead 32 and that particular non - black color printhead . these values are then stored in nvram 46 in printer 12 . the printer registration for all four image planes is now complete except for the absolute top - of - page location of the entire four - color image plane . only this remaining top - of - page synchronization relative to the paper sensor location requires a test page to be run at the final assembly test station to set the proper vertical synchronization value into nvram 46 in printer 12 . while this invention has been described as having a preferred design , the present invention can be further modified within the spirit and scope of this disclosure . this application is therefore intended to cover any variations , uses , or adaptations of the invention using its general principles . further , this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims .