Sheet circulation in a duplex printer

A duplex printer (or copier) 10 has a copy sheet output path 92 through output rollers 84, curved chute 96, and output rollers 67 into output tray 86. Commonly utilizing a substantial portion thereof is a duplexing path 94 for returning copy sheets to be imaged on their opposite sides to make duplex copies. Rollers 67 alternatively, with reversal, feeds copy sheets into the duplex path 94, aided by rollers 84 at the path separation. The distance between output rollers 84 and the reversible ejecting rollers 67 along the chute 96 is substantial, preferably approximately one half the sheet dimension in the sheet feeding direction of the shortest sheet to be duplexed. The rollers 84 feed copy sheets through the copy sheet output path 92 to the reversible rollers 67 until about one half of the sheet extends out of their nip of these rollers 67. A reversed sheet can then be uninterruptedly reverse fed by, outside, of another sheet moving in the opposite direction in the same arcuate chute 96, for a substantial time period. Thus, a subsequent copy sheet may be fed downstream (upwardly) in the output path 92 simultaneously with, for a substantial time period, the reverse (downward) feeding of the preceding copy sheet into the duplex path 94, to enable slow, uncritical, reversals of rollers 67 even if the sheets are closely spaced in the output path. The duplex path 94 has an even greater path distance between the reversible rollers 67 and the first duplex path feed nip 90, which is separate from the rollers 84.

Cross-reference and incorporation by reference is made to a copending 
application by the same assignee, filed Apr. 20, 1989, as U.S. patent 
application No. 07/340,994, by Keith Gilliland, Christian G. Midgley, 
Francis W. Dana, and Mark C. Mutch, entitled "A MONITOR/WARRANTY SYSTEM 
FOR ELECTROSTATOGRAPHIC REPRODUCING MACHINES USING REPLACEABLE 
CARTRIDGES". If any claim may be made the benefit of the priority or 
filing date thereof it is hereby made. 
This invention relates generally to electrostatographic reproducing 
machines, and more particularly to a simplified duplexing system and path 
therefore whereby copy sheets may be printed on both sides with little 
additional machine cost or complexity and with easily accessible sheet 
transport paths for ease of sheet jam clearance. 
There is disclosed herein a simple, low cost copier or printer duplexing 
system with a simple but integrated copy sheet output and endless loop 
duplexing return path. 
There is disclosed herein a simple, low cost copier or printer duplexing 
system providing for closely spaced production of duplex copy sheets 
without sheet feeding interference in the sheet inverting operation for 
duplexing. 
Various types of duplexing systems are known in the art for copiers and/or 
printers. The following disclosures are particularly noted as examples, 
and cite other examples therein. U.S. Pat. No. 4,348,101 issued Sept. 7, 
1982 to A. Schonfeld, et al (Sperry Corporation) and U.S. Pat. No. 
4,825,245 issued Apr. 25, 1989 to K Fukae et al. (Kentek), shows a duplex 
printer with partially similar output and inverting paths. Another recent, 
but less compact, duplexing printer is disclosed in Hitachi U.S. Pat. No. 
4,806,979 issued Feb. 21, 1989 to Tokoro et al. Other patent examples of 
duplexing copiers of interest as showing duplexing paths including 
reversible sheet output rollers functioning as sheet inverters include 
Xerox Corporation U.S. Pat. No. 4,708,462 to (the same) D. J. Stemmle 
issued Nov. 24, 1987 and art cited therein, and Canon U.S. Pat. No. 
4,787,616, and Ricoh U.S. Pat. No. 4,692,020. Said U.S. Pat. No. 4,708,462 
to D. J. Stemmle is also of interest as disclosing an optional path choice 
of a trayless duplex loop path extending over and bypassing a duplex 
buffer tray. 
An example of a duplexing copier with a dual mode inverter/output path 
feeder system, with reversing rolls, of interest is in Xerox Corporation 
U.S. Pat. No. 4,487,506 issued Dec. 11, 1984 to Repp et al. 
An inverter per se of interest is disclosed in Xerox Corporation U.S. Pat. 
No. 4,346,880 issued Aug. 31, 1982 to George J. Roller, et al, wherein 
part of a preceding inverted sheet can be in the inverter simultanously 
with (overlapping) the feeding thereinto of the subsequent sheet. 
Other art of background interest includes U.S. Pat. No. 4,110,025 issued 
Aug. 29, 1978 to Tabata; U.S. Pat. No. 4,234,180 issued Nov. 18, 1980 to 
Looney; U.S. Pat. No. 4,272,181 issued June 9, 1981 to Treseder; U.S. Pat. 
No. 4,334,765 issued June 15, 1982 to Clark; U.S. Pat. No. 4,348,101 
issued Sept. 7, 1982 to Schonfeld et al.; U.S. Pat. No. 4,553,831 issued 
Nov. 19, 1985 to Dixon; U.S. Pat. No. 4,630,921 issued Dec. 23, 1986 to 
Watanabe; U.S. Pat. No. 4,699,503 issued Oct. 13, 1987 to Hyltoft; U.S. 
Pat. No. 4,758,862 issued July 19, 1988; U.S. Pat. No. 4,780,745 issued 
Oct. 25, 1988 to Kodama; U.S. Pat. No. 4,488,801 issued Dec. 18, 1984 to 
Gibson; Japanese Patent No. 58-118666(A); German Patent No. 31 13 
658.3-51. 
A specific feature of the specific embodiment disclosed herein is to 
provide a duplex printer or copier with a copy sheet output path and a 
connecting duplexing path for returning copy sheets to be imaged on their 
opposite sides to make duplex copies, said duplexing path including a copy 
sheet inverting system with reversible copy sheet output path rollers to 
alternatively eject copy sheets, or with reversal, to refeed copy sheets 
into said duplex path, the improvement comprising; output path roller nip 
means for feeding copy sheets therefrom downstream through said copy sheet 
output path to said reversible copy sheet output path rollers said output 
path roller nip means being positioned and driven to also function to 
direct copy sheets reversibly driven by said reversible copy sheet output 
path rollers into said duplexing path, copy sheet guide path means 
extending between, and providing an copy sheet guide path between, said 
output path roller nip means and said reversible copy sheet output path 
rollers, said copy sheet guide path comprising a portion of said copy 
sheet output path and an initial portion of said duplexing path, and 
having a copy sheet guide path length between said output path roller nip 
means and said reversible copy sheet output path rollers which is a 
substantial portion of the dimension of the copy sheet being fed but 
substantially less than said copy sheet dimension so that a substantial 
portion of the copy sheet is extendable through and downstream of said 
reversible copy sheet output path rollers before the copy sheet is 
released by said output path roller nip means, so that a subsequent copy 
sheet may be fed downstream in said arcuate copy sheet guide path by said 
output path roller nip means towards said reversible copy sheet output 
path rollers simultaneously with, for a substantial time period with, the 
reverse feeding of the preceding copy sheet by said reversible copy sheet 
output path rollers into said duplex path, and duplexing path roller nip 
means in said duplexing path for acquiring and feeding in said duplexing 
path copy sheets being reversibly driven by said reversible copy sheet 
output path rollers, said duplexing path roller nip means being mounted in 
said duplexing path to have a copy sheet guide path length between said 
duplexing path roller nip means and said reversible copy sheet output path 
rollers which is substantially greater than said copy sheet guide path 
length between said output path roller nip means and said reversible copy 
sheet output path rollers. 
Further specific features provided by the system disclosed herein 
individually or in combustion, include those wherein said copy sheet guide 
path length between said output path roller nip means and said reversible 
copy sheet output path rollers is approximately one half of the feeding 
dimension of the copy sheet; said output path roller nip means and said 
duplexing path roller nip means each comprise single nip pairs of sheet 
feed rollers which are substantially spaced apart, with said duplexing 
path roller nip means being separately positioned only in said duplexing 
path; and/or said output path roller nip means is in said arcuate copy 
sheet guide path and defining a part of said copy sheet output path but 
partially extends into said duplexing path to assist in deflecting sheets 
therein. 
All references cited in this specification, and their references, are 
incorporated by reference herein where appropriate for appropriate 
teachings of additional or alternative details, features, and/or technical 
background.

Describing now in further detail the exemplary embodiment with reference to 
the FIGURE, there is shown a duplex printer reproducing machine 10 by way 
of example of an automatic electrostatographic reproducing machine of a 
type adapted to implement the duplexing system of the present invention. 
In the example shown, reproducing machine 10 comprises a laser printer, 
respectively employing three different replaceable xerographic, developer, 
and toner cartridge units 12, 14, 16 designed to provide a preset number 
of images in the form of prints or copies. While the machine 10 is 
exemplified in the ensuing description and the drawing as a printer, other 
types of reproducing machines such as copiers, ink jet printers, etc., may 
be envisioned. Although the present system is particularly well adapted 
for use in automatic electrostatographic reproducing machines, it will be 
evident from the following description that it is equally well suited for 
use in a wide variety of copying systems including other 
electrostatographic systems and is not limited in application to the 
particular embodiments shown herein. 
Xerographic cartridge 12 includes a photoreceptor drum 20, the outer 
surface 22 of which is coated with a suitable photoconductive material, 
and a charge corotron 28 for charging the drum photoconductive surface 22 
in preparation for imaging. Drum 20 is suitably journaled for rotation 
within the cartridge body 25, rotating in the direction indicated by the 
arrow to bring the photoconductive surface 22 thereof past exposure, 
developer, and transfer stations 32, 34, 36 of machine 10 when cartridge 
12 is in the machine 10. To receive xerographic cartridge 12, a suitable 
cavity 38 is provided in machine frame 18, the cartridge body 25 and 
cavity 38 having complementary shapes and dimensions such that on 
insertion of cartridge 12 into cavity 38, drum 20 is in predetermined 
operating relation with exposure, developer, and transfer stations 32, 34, 
36 respectively. With insertion of cartridge 12, drum 20 is drivingly 
coupled to the conventional drum driving means (not shown) and the 
electrical connections to cartridge 12 are made. 
In the xerographic process practiced, the photoconductive surface 22 of 
drum 20 is initially uniformly charged by charge corotron 28, following 
which the charged photoconductive surface 22 is exposed by imaging beam 40 
at exposure station 32 to create an electrostatic latent image on the 
photoconductive surface 22 of drum 20. 
Imaging beam 40 is derived from a laser 42 modulated in accordance with 
image signals from a suitable source 44. Image signal source 44 may 
comprise any suitable source of image signals such as a memory, document 
scanner, communication link, tape drive, another computer, etc. The 
modulated imaging beam 40 output by laser 42 is impinged on the facets of 
a rotating multi-faceted polygon 46 which sweeps the beam across the 
photoconductive surface 22 of drum 28 at exposure station 32. I.e., a 
conventional laser printing system is provided. 
Following exposure, the electrostatic latent image on the photoconductive 
surface 22 of drum 20 is developed by a magnetic brush development system 
contained in developer cartridge 14. The magnetic brush development system 
includes a suitable magnetic brush roll 50 rotatably journaled in body 52 
of cartridge 14, developer being supplied to magnetic brush roll 500 by 
toner cartridge 16. To receive developer cartridge 14, a suitable cavity 
54 is provided in machine frame 18, cartridge body 52 and cavity 54 having 
complementary shapes and dimensions such that on insertion of cartridge 14 
into cavity 54, magnetic brush roll 50 is in predetermined developing 
relation with the photoconductive surface 22 of drum 20. With insertion of 
cartridge 14, magnetic brush roll 50 is drivingly coupled to the developer 
driving means (not shown) in machine 10 and the electrical connections to 
cartridge are 14 made. 
The toner cartridge 16 provides a sump 56 within which developer comprising 
a predetermined mixture of carrier and toner for the magnetic brush 
development system in developer cartridge 14 is provided. Alternatively, 
single component developer may be provided. A rotatable auger 58 mixes the 
developer in sump 56 and provides developer to magnetic brush roll 50. 
Magnetic brush roll 50 is suitably journaled for rotation in the body 52 
of cartridge 16. 
The developer cartridge 14 body 52 forms a cavity 62 for receipt of toner 
cartridge 16, cavity 62 of cartridge 14 and body 64 of cartridge 16 having 
complementary shapes and dimensions such that on insertion of cartridge 16 
into cavity 62, cartridge 16 is in predetermined operating relation with 
the magnetic brush roll 50 in developer cartridge 14. With insertion of 
toner cartridge 16, auger 62 is drivingly coupled to the developer driving 
means (not shown) and the electrical connections to cartridge 16 made. 
Any residual toner particles remaining on the photoconductive surface 22 of 
drum 20 after transfer are removed by a conventional cleaning mechanism 
(not shown) in xerographic cartridge 12. 
Prints of the images formed on the photoconductive surface of drum 20 are 
produced by machine 10 on a suitable support material, such as copy sheets 
68 or the like. Supplies of stacked copy sheets 68 may be provided in 
plural paper trays 70, 72, 74. The copy sheets may be of different sizes. 
The paper trays 70, 72, 74 here are removable and interchangeable cassette 
units, known per se. Conventionally mounted in the machine 10, to engage 
the top of the stack of sheets in each tray 70, 72, and 74 when the tray 
is inserted into the machine 10, are respective conventional sectored or 
segmented feed rolls 76 for feeding individual sheets seriatum from the 
stack of sheets in that tray. This sheet feeding is assisted by 
conventional stack corner snubbers 77 in the trays. Conventional 
intermittent drives for the feed rolls 76 are illustrated in phantom 
therewith. Sheets selectively fed on demand from a tray 70, 72, or 74 are 
all fed to a common registration pinch roll pair 78 in the machine 10 
paper path. Following this conventional sheet registration at stalled 
pinch roll pair 78, the sheet is forwarded on by those rolls to transfer 
station 36 in proper timed relation with the developed image on drum 20. 
There, the developed image is transferred to one side (the upper surface) 
of the copy sheet 68. Following transfer, the copy sheet 68 bearing this 
toner image is separated from the photoconductive surface 22 of drum 20 
and advanced to fixing station 80 where a roll fuser 82 fixes this 
transferred powder image thereto. After fusing the toner image to the copy 
sheet 68, the copy sheet 68 is advanced downstream to print discharge 
rolls 84, which it turn feed the copy sheet downstream towards print 
output tray 86. A suitable sheet sensor 85 senses each copy sheet as it 
passes from fixing station 80 to output tray 86. The final discharge of 
the copy sheet or print to output tray 86 is by elastomer copy sheet 
output path rollers nipped with mating spring loaded baffle plate 67a. 
The duplex printer 10 has a copy sheet output path 92, shown in a dot-dash 
line with arrows from fuser 80 through output path roller nip 84 rollers 
on up through curved baffles or chute 96 through copy sheet output path 
rollers 67 to eject sheets out into output tray 86. Connecting with and 
utilizing a substantial portion of this output path 92 is a duplexing path 
94, shown here in dashed lines and arrows, for returning copy sheets to be 
imaged on their opposite side to make duplex copies. This duplexing path 
94 includes a copy sheet inverting system provided by reversal of copy 
sheet output path or ejecting rollers 67. Rollers 67 alternatively eject 
copy sheets, or with reversal, transport copy sheets into the duplex path 
94. 
Preferably the distance between output rollers 84 and the reversible 
ejecting rollers 67 is approximately one half the sheet dimension, in the 
sheet feeding direction, of the shortest sheet to be duplexed. Thus, for a 
conventional 11" long letter size sheet 68 fed short edge first this 
preferable distance between nips 84 and 67 is approximately 7". Thus, the 
rollers 84 feed copy sheets therefrom downstream through the copy sheet 
output path 92 to the reversible rollers 67 until about one half of the 
sheet extends downstream out of the nip of these output rollers 67, 
without losing control of the sheet. That is, the chute 96 provides a copy 
sheet guide path length between said output path roller nip 84 and the 
reversible copy sheet output path rollers 67 which is a substantial 
portion of the dimension of the copy sheet being fed but substantially 
less than that copy sheet dimension, so that a substantial portion of the 
copy sheet is extendable through and downstream of the output rollers 67 
before the copy sheet is released thereby. 
The plane of the nip of the reversible rollers 67 with their engaging 
surface 67a, and the curve of the baffles or chute 96, and the position of 
the rollers 84, are such that a copy sheet reversibly driven by the 
reversal of rollers 67 is automatically driven into the duplexing path 94. 
The chute 96 provides an arcuate copy sheet guide path, against the 
outside of which a reversed sheet fed back by reversed rollers 67 can 
uninteruptedly pass by the next sheet, which is moving downstream in the 
same chute 96 towards rollers 67. Thus, a subsequent copy sheet may be fed 
downstream (upwardly) in the arcuate copy sheet guide path 96 
simultaneously with, for for a substantial time period with, the reverse 
(downward) feeding of the preceding copy sheet backwards into the duplex 
path 94, even if the inter-copy gap or pitch space is only about 5 cm. 
Sheets 68 reverse fed back into the duplexing path 94 are fed fromm rollers 
67 down through arcuate chute 96 into the nip of duplexing path rollers 90 
in the duplexing path. These duplexing path rollers 90 are positioned 
substantially further in sheet path distance from reversible rollers 67 
than are output path rollers 84, and are substantially separated from 
rollers 84, and rollers 84 have only one opposing pair of rollers, unlike 
a conventional three or four roller inverter. With this separate and 
further downstream path location of duplexing path rollers 90, only that 
one additional set of rollers 90 is needed for providing duplex path 
feeding in this system. However, rollers 90 are spaced from rollers 67 by 
a sheet path distance slightly less than (within) the feeding dimension of 
the shortest sheet being duplexed, so as to not to release these sheets 
and to provide positive nip feeding in at least one nip at all times. 
As shown by its rotational arrow in the FIGURE, the outer rollers 84 rotate 
towards, but are spaced from, the outer wall or baffle of chute 96, 
thereby helping urge a reverse-fed sheet 68 (from reversed rollers 67) 
into the duplexing path 94. The (now) led edge of a reverse driven sheet 
which might hit this roller 84 is urged to flip over into the duplex path. 
The duplexing path 94 at that point diverges from the output path 92 and 
passes by the outside of the rollers 84. This urging of any reverse moving 
sheet into the duplexing path 94 is also assisted by the curvature of 
chute 96 and the beam strength of the sheet, which also urges the sheet 
towards the outside wall of chute 96. However, the chute 96 need not 
necessarily be arcuate. The outer wall of chute 96 is diverging away from 
output path 92 and rollers 84 to form the duplex path 94 at that point. 
Note that no separate inverter chute is required as in most inverter 
designs. Here there is only one single inverter chute 96 and it is an 
integral part of the output path, and also of the duplexing path. The 
sheet reversing for inverting function is integral the normal exit 
transport in a single paper path. When output of the sheet is desired, 
rollers 67 simply continue to rotate in the same forward or downstream 
feeding direction until the sheet is fully ejected, instead of reversing 
after only about one half of the sheet is extending therefrom. 
The long path distance between the nips of rollers 84 and the nips of 
reversible rollers 67 allows ample time for the reverse feeding of the 
proceeding sheet out of the nip of rollers 67 into the duplex path 94 
before the lead edge of the next copy sheet in the ouput path 92 reaches 
the rollers 67 (at which point the rollers 67 must be reversed again to 
drive that sheet out into tray 86). Thus an expensive high speed or 
critical reversal system is not required for the rollers 67. Yet the 
overall path lengths are such that 2, or even 3, sheets can be 
continuously circulated in the combined output and duplex path loop 
without pitch skips or copying rate reductions. For duplexing, clean 
sheets may be alternatingly intermittently fed from any of trays 70-74 to 
be copied on their first sides alternately and intermixed with the return 
of those sheets through the duplex path for their second side imaging and 
outputting into output tray 86. 
The forward or ejecting sheet drive velocity of reversible rollers 67 may 
be about the same as the reverse or duplexing sheet velocty. However, by 
increasing or decreasing the reverse drive speed and the rollers 90 speed, 
the duplex path 94 velocity may be changed relative to the simplex or 
output path speed 92. That allow for a different pitch in the duplex path, 
e.g., to give a choice of efficient duplex loops for either two or three 
sheets. (Two sheets requires less page buffer memory.) A faster duplex 
path can return sheets faster to the transfer station for a second side 
image. 
The duplex return rollers 90 feed the sheet being duplexed down onto the 
top of, and over an upper cover surface 100 of, the uppermost cassette 
tray 70. The rollers 90 feed the sheet along that tray cover surface 100 
to the cassette feeder 76, feeding the sheet under a baffle plate 102 in 
the machine which is spaced above and parallel to the tray cover surface 
70. Thus the feeding baffle or chute for the sheet being duplexed is 
defined by a fixed upper baffle 102 in the machine 10 and a mating 
opposing lower baffle 100 which is a part of the removable paper tray 
cassette 70, and removable therewith. 
The duplex return feed rollers 90 are positioned, in the duplex printer (or 
copier) 10 itself, to be upstream of feed rollers 76 and just above cover 
100 when the cassette 70 is inserted into its mating insertion aperture in 
the printer 10, for feeding copy sheets in the duplex path between the 
fixed baffle arrangement 102 and the top cover member 100 of cassette copy 
sheet tray to the other end of the cassette 70 without requiring any 
transporting or driving means in the cassette 70 itself. Not only is that 
desirable in itself, but also, when the tray 70 is removed, there is no 
obstruction to removal or retention of a sheet, which is free to drop by 
gravity and be both readily visible and removable from that entire 
substantial portion of the duplexing path through the regular cassette 
loading aperture. This is true here even is the trail edge of the sheet 
being removed is still in the nip of rollers 90. That is in contrast to 
normal sheet jam recovery which normally requires operator opening of 
machine doors and opening of sheet roller nips. 
Note that the paper tray cassette 70 is not being used as a duplex tray 
here. Here, the cassette tray 70 is only a conventional source of clean or 
blank copy paper for the first side copying operation, and is not a source 
of sheets during the duplexing or second side copying operation. Here, the 
sheets being duplexed (the sheets in the duplex path 94), do not stack or 
go into the tray 70, they slide over the top of the tray 70 and the stack 
of clean sheets therein. 
The cassette feeder 76 for tray 70 is normally disengaged, as shown, with 
its open or cut-away roller segments overlying and spaced from the stack 
of sheets in the tray. Thus, the sheets being duplexed can freely pass 
under the feeder 76 feed rollers and on to the illustrated sheet feeding 
rollers carrying the sheets to the registration rollers 78. Then the sheet 
68 being duplexed can be imaged on its oposite side at transfer station 
36, with the appropriate electronically reordered iamge, in the same way 
it was imaged on its first side, and fed to the output tray 86 via output 
path 92 like a simplex copy sheet, this time without reversing the rollers 
67. The sheet being duplexed is turned over, only once, in the natural 
inversion in the paper path provided between tray 70 and transfer station 
36. 
If desired, the cassette feeder 76 can be operated or utilized to assist in 
the duplex path feeding by rotating the feed wheels thereof after the 
sheet being duplexed has been fed under feeder 76 from rollers 90. The 
feeder 76 will thus treat the sheet being duplexed as if it were forward 
feeding an already separated top sheet of the stack of sheets in the tray, 
sliding that sheet over the top of the stack. 
Optionally, fingers (not shown) may be provided over the open or exposed 
front portion of the top of cassette tray 70, extending between the feed 
wheels of the feeder 76 (as an extension of the cover 100 in the area 
where the cover 100 does not extend), to preclude any possibility of the 
sheet being duplexed from catching on the corner snubbers 77. However, 
that has not been a problem. 
As noted, the use of the upper cover surface 100 of a cassette tray as the 
lower baffle or sheet guide surface for a major portion of the sheet 
second pass or duplex path provides a signifiant advantage, not only in 
cost and simplicity, but also in jam clearance. Many duplex paths are 
difficult to clear of paper in the event of a feeding jam. But here, 
simply by removing the cassette tray 70, as the operator is accustomed to 
doing anyway for paper loading that part of the duplex path is fully 
exposed through the cassette loading entrance, and a jammed sheet therein 
is removed with the tray. Only one tray 70 is actually needed, but here 
trays 72 or 74 may be desirably substituted in the top cassette tray 
location and also provide a duplex path in the same manner, simply by 
using a standardized cassette upper surface 100 for all cassettes. 
To control operation of machine 10, a suitable control panel 87 with 
various control and print job programming element is provided. Panel 87 
may additionally include a suitable message display window 88 for 
displaying various operating information to the machine operator. 
Conventional or readily programmable software microprocessor controls may 
be used for all machine and paper path operational controls, as is well 
known in the art. 
A simplex-only version of the disclosed printer embodiment has been 
successfully operating as commercial "Compact Laser Printer" Models 10, 
20, 30, and 40, products of Fuji Xerox Corporation, since about December 
1987. The present invention adds full duplex capability thereto with only 
a few dollars in incremental parts costs, since the only parts which need 
be added for this added function with this system include one more 
conventional feed roller nip 90 and associated baffling for the duplex 
return path, plus the clutch or reversing motor for the rollers 67 and, 
optionally, an additional conventional sheet path sensor. Furthermore, 
this duplex version provides duplex copies at the same copying rate and 
with the same small inter-sheet pitch spacing as simplex copies. 
Conventional paper trays may be utilized, as in these products or the Fuji 
Xerox "2970" copier cassettes or many other copier cassettes. No special 
or dedicated duplex buffer tray or associated extra sheet feeders or 
separators therefore are required with the present duplexing system. 
While the embodiment disclosed herein is preferred, it will be appreciated 
from this teaching that various alternatives, modifications, variations or 
improvements therein may be made by those skilled in the art, which are 
intended to be encompassed by the following claims: