the present invention will hereinafter be described in detail with respect to some embodiments thereof . fig1 is a perspective view of a sheet conveying device to which an embodiment of the present invention is applied , fig2 is a side view showing the state thereof during manual sheet feeding , and fig3 is a side view showing the state thereof during automatic sheet feeding . in fig1 reference numeral 1 designates a platen rotatably supported on the left and right side plates 2 of the device f by a platen shaft 1a . a gear 3 is fixed to one end of the platen shaft 1a outside the side plate 2 of the device f . a manually operated knob 30 which will later be described is secured to the platen shaft 1a further outside the base plate 2 . the platen 1 is a roller made of rubber and conveys a sheet s in a predetermined direction ( the direction of arrow b ) by being rotated ( in the direction of arrow a ). a group of gears 4a , 6 , 6a and 16 for transmitting the drive force from a motor 4 are disposed near the gear 3 . the motor 4 is fixed to the frame ( not shown ) of the device with its output shaft 4b being parallel to the platen shaft 1a . the gear 4a is fixed to the output shaft 4b of the motor 4 and is in mesh engagement with the gear 6 supported on a shaft 5 extending from the side plate 2 and parallel to the platen shaft 1a . a frusto - conical space 6c ( as shown in fig2 and 3 ), gradually reduced in diameter toward the gear 6 is formed inside the pinion gear 6a , and a serration 6b is formed on the inner peripheral surface of the space 6c . also , a gear 7 is fitted for rotation and sliding movement ( in the directions of bilateral arrow c ) on the shaft 5 inside the gear 6 and the side plate 2 . the gear 7 is biased toward the gear 6 by a spring 8 wound around the shaft 5 between the gear 7 and the side plate 2 . a frusto - conical serration portion 7a is formed integrally with the side 7b of the gear 7 which is adjacent to the gear 6 . this serration portion 7a is in mesh engagement with the serration 6b formed on the inner peripheral surface of the pinion gear 6a . on the other hand , the gear 7 is in mesh engagement with a gear 10 rotatably supported on the side of the side plate 2 through a shaft 9 . a detent gear 11 is formed integrally with the gear 10 , and above this detent gear 11 , a detent lever 12 is supported for rotation relative to the side plate 2 by a shaft 13 . one end of the detent lever 12 provides a ratchet 12a which can mesh with the teeth of the detent gear 11 . a spring 14 is extended between the other end 12b of the detent lever 12 and the upper portion of the side plate 2 , and the detent lever 12 is biased in counter - clockwise direction ( the direction of arrow d ) as viewed in fig1 by the spring 14 . on the other hand , the pinion gear 6a integral with the gear 6 is in mesh engagement with a gear 16 rotatably supported on the side plate 2 by a shaft 15 , and the gear 16 in turn is in mesh engagement with the gear 3 fixed to one end of the platen shaft 1a . a solenoid 17 is fixed below the above - described group of gears , and the lower end of a pivotable lever 18 is pivotably connected to the end of the rod 17a of the solenoid 17 . the intermediate portion of the pivotable lever 18 is pivotably supported on the fixed portion of the device through a shaft 19 , and a bulged portion 18a is formed on the upper end portion of the pivotable lever 18 , as shown in fig2 and 3 . this bulged portion 18a is in contact with the side 7b of the gear 7 which is adjacent to the gear 6 . operation of the present embodiment construction as described above will now be described . first , where sheet feeding is to be effected by manually rotating the platen 1 , the supply of electric power to the solenoid 17 is cut off as shown in fig2 . in this state , the rod 17a is in its extended position , the pivotable lever 18 is in its substantially vertical position , the gear 7 is moved toward the gear 6 by the resilient force of the spring 8 , and the serration portion 6b and the serration portion 7a are in mesh engagement with each other . in this state , the gears shown in fig1 are all in mesh engagement with one another . that is , a drive transmitting system ( motor 4 → gear 4a → gear 6 → pinion gear 6a → gear 16 → gear 3 ) for transmitting the drive force of the motor 4 to the platen 1 to rotate the platen 1 by the drive force of the motor 4 is connected to a restraining force transmitting system ( detent lever 12 → gear 10 → gear 7 ) for transmitting a rotation restraining force produced by the detent lever 12 . the ratchet 12a of the detent lever 12 is lowered by the force of the spring 14 and meshes with the teeth of the detent gear 11 . in this state , of course , the supply of electric power to the motor 4 is cut off . in such a state of manual operation , the rotation restraining force of the ratchet 12a of the detent lever 12 relative to the detent gear 11 is transmitted to the gear 3 integral with the platen shaft 1a , through the gears 10 , 7 , 6a and 16 which are in mesh engagement with one another . the platen 1 cannot be rotated unless the force of the spring 14 is overcome and the platen shaft 1a is rotated through the intermediary of the knob 30 . that is , the platen 1 is positioned in a predetermined position . so , where the platen 1 is to be rotated to effect feeding of the sheet s by manual operation , if the knob 30 is grasped by fingers and such a degree of rotational force that the ratchet 12a rides over the detent gear 11 against the force of the spring 14 is imparted to the knob 30 , the platen 1 will be click - rotated in response to the manual operation and the recording sheet s can be conveyed in the predetermined direction at a predetermined pitch . in this manner , in the case of manual feeding , feeding of the sheet s can be accomplished by rotating the platen 1 at a predetermined pitch determined by the pitch of the teeth of the detent gear 11 . on the other hand , where automatic feeding of the sheet s is to be effected by the utilization of the motor 4 , the motor 4 is operated and the solenoid 17 is operated . the operation of the motor 4 and the solenoid 17 can be accomplished by a signal produced by a return key or a line feed key being depressed , or a signal produced when a line feed code is detected during the processing . when the solenoid 17 is operated , the rod 17a is retracted as shown in fig3 and the pivotable lever 18 is rotated in a counter - clockwise direction ( in the direction of arrow e ). as a result , the upper end portion of the pivotable lever 18 pushes the outer side 7b of the gear 7 by the bulged portion 18a and therefore , the spring 8 flexes and the gear 7 is moved toward the side plate 2 . as a result , the serration portions 6b and 7a separate from each other and the gear 6 becomes disengaged from the gear 7 . in this state , the rotation restraining force of the ratchet 12a of the detent lever 12 through the detent gear 11 is not transmitted to the gear 3 on the platen shaft 1a . the revolution of the motor 4 is transmitted to the gear 3 through the pinion gear 4a , the gear 6 and the gear 16 , and the platen 1 is rotated in conformity with the revolution of the motor 4 , whereby desired feeding of the sheet s can be accomplished . where such automatic sheet feeding utilizing the motor is effected , the rotation restraining force of the ratchet 12a of the detent lever 12 is absent and therefore , the load of the group of gears as a whole is very small . so , even if the output of the motor 4 is not so great , the platen 1 can be rotated and a very compact , inexpensive motor of a small output can be chosen . the present invention is not restricted to the above - described embodiment . for example , in the absence of electric power being supplied to the solenoid 17 , the pivotable lever 18 can be manually pivoted to thereby separate the gear 7 and the gear 6 from each other . also in the case of manual sheet feeding , the rotation restraint by the detent gear 11 and the ratchet 12a can be eliminated , whereby the platen 1 can be rotated to accomplish sheet feeding by a slight force . the device of the present invention can also be used in such a special state of use , but when the pivotable lever 18 is returned to its original position and the serration portions 7a and 6b are brought into mesh engagement with each other , the mesh engagement therebetween deviates by an amount corresponding to the pitch of the serration because the serration portions mesh with each other not in the same state as that when they were separated from each other . however , the portion of the drive transmitting system between the gears formed with the serrations and the platen 1 is sufficiently decelerated , i . e ., the diameter of pinion gear 6a is sufficiently smaller than that of gear 3 , such that any deviation imparted to the position of the platen 1 by slight deviation in the engagement of serrations 7a and 6a is very small . the positional deviation of the mesh engagement between the serration portions occurs also when automatic sheet feeding is changed over to manual sheet feeding , but the influence of such deviation of the mesh engagement can be almost neglected as described above . the force with which the solenoid 17 is operated and the gear 7 is moved to the platen side through the pivotable lever 18 to thereby separate the gear 7 from the gear 6 may be enough to flex the spring 8 and therefore , the solenoid utilized may be of a small output , which leads to the possibility of greatly saving the electric power . the embodiment which will hereinafter be described is such that both of drive force transmitting means and restraining force transmitting means can be selectively connected and disconnected . fig4 is a perspective view of a sheet conveying device to which said another embodiment of the present invention is applied , fig5 is a side view showing the state thereof during manual sheet feeding , and fig6 is a side view showing the state thereof during automatic sheet feeding . in the present embodiment , members functionally similar to those in the previous embodiment are given the same reference numerals . also , in fig4 the platen 1 , the side plates 2 , the manually operated knob 30 and the sheet s are not shown because they are identical to those shown in fig1 . referring to fig4 - 6 , a gear 4a is fixed to the output shaft 4b of a motor 4 and meshes with a gear 21 supported on a shaft 23 extending from the side plate ( not shown ) and parallel to a platen shaft 1a . a frusto - conical space 21c ( as shown in fig5 and 6 ), gradually reduced in diameter toward the gear 21 is formed inside the pinion gear 21a , and a serration 21b is formed on the inner peripheral surface of the space 21c . also , inside the gear 21 and the side plate , a gear 22 is fitted on the shaft 23 so as to be rotatable and slidable ( in the directions of bilateral arrow c ). frusto - conical serration portions 22a and 22b are integrally formed on the opposite sides of the gear 21 . the serration portion 22b can mesh with a serration portion 21b formed on the inner peripheral surface of the pinion gear 21a . the other serration portion 22a can mesh with a serration portion 20b formed on the inner peripheral surface of a pinion gear 20a integral with a detent gear 20 . the ratchet 12a of the detent lever 12 is in mesh engagement with the detent gear 20 . the gear 22 is in mesh engagement with a gear 16 supported on the side plates ( not shown ) through a shaft 15 . the gear 16 is in mesh engagement with the gear 3 . on the other hand , a solenoid 31 is fixed below the above - described group of gears , and the lower end of a pivotable lever 32 is pivotably connected to the end of the rod 31a of the solenoid 31 . the intermediate portion of the pivotable lever 32 is pivotably supported on the fixed portion of the device through a shaft 33 , and a u - shaped portion 34 is formed at the upper end of the pivotable lever 32 , as shown in fig5 and 6 . this u - shaped portion 34 can bear against either one of the opposite sides 22c and 22d of the gear 22 . a tension spring 24 is secured to the lower portion of the lever 32 and biases the lever 32 in a counter - clockwise direction . operation of the present embodiment constructed as described above will now be described . where sheet feeding is to be effected by manually rotating the platen , the supply of electric power to the solenoid 31 is cut off as shown in fig5 . in this case , the lever 32 is biased the counter - clockwise direction by the biasing force of the spring 24 , and the protrusion 34a of the u - shaped portion 34 moves the gear 22 toward the gear 20 . so , the serration portions 20b and 22a come into engagement with each other . accordingly , the restraining force of the detent lever 12 is transmitted to the gears 20 , 20a , 22 , 16 and 3 , and the platen ( not shown ) is rotated under the influence of the detent force by a knob ( not shown ) being manually turned . in this case , the gear 22 and the gear 21 are disconnected from each other and the rotation of the gear 3 is not transmitted to the motor 4 . on the other hand , where automatic feeding of the sheet s is to be effected by the utilization of the motor 4 , the solenoid 31 is operated . the actuated of the motor 4 and the solenoid 31 is accomplished , for example , by a signal produced by depressing a return key or a line feed key , or a signal produced when a line feed code is detected in the processing . when the solenoid 31 is actuated , the rod 31a is retracted as shown in fig6 and the pivotable lever 32 is pivoted in the clockwise direction ( in the direction of arrow f ). as a result , the upper end of the pivotable lever 32 pushes the side 22d of the gear 22 by means of the protrusion 34b thereof and therefore , the gear 22 is moved toward the gear 21 and the serration portions 22b and 21b come into engagement with each other . so , the drive force of the motor 4 is transmitted to the gears 4a , 21 , 21a , 22 , 16 and 3 to thereby rotate the platen . as the result of the serration portions 22b and 21b coming into engagement with each other , the serration portions 20b and 22a are separated from each other and the gear 20 and the gear 22 are separated from each other . in this state , the rotation restraining force of the ratchet 12a of the detent lever 12 through the detent gear 20 is not transmitted to the gear 3 on the platen shaft 1a . accordingly , the motor 4 is not subjected to the influence of the restraining force of the detent when it rotates the platen . thus , in the present embodiment , during automatic feeding , the motor 4 may put out a force necessary to drive the platen without being subjected to the influence of the restraining force . also , during manual feeding , the platen may be manually operated against the load of the platen and the load by the restraining force without being subjected to the influence of the drive source , for example , the influence of the stationary residual torque in a case where the motor is a pulse motor , and the platen can be driven by a small force . the platen is not limited to the roller shown in the embodiments , but may be any rotational member capable of conveying the sheet , such as a belt passed over pulleys . the means for displacing the gears is not limited to the solenoid shown in the embodiments , but may also be a plunger or the like . as described above , the present invention provides a sheet conveying device which is capable of automatic sheet feeding and manual sheet feeding and in which , during automatic feeding , the detent force does not provide a load .