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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a window covering, and more particularly to a roman shade, which is operated by a cord with safety device. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1  shows a conventional roman shade  1  having two cords  2  and  3 , and a shade member  4 . The cords  2  and  3  have ends fastened to the bottom of the shade member  4 , and then have another ends passing through rings  5  on the shade member  4  to form two operating sections  2   a  and  3   a  hung at top of the shade member  4  that user may operate the operating sections  2   a  and  3   a  to fold or unfold a cloth  4   a  of the shade member  4 . The cords  2 ,  3  of such roman shade  1  have their ends fastened and the operating sections  2   a  and  3   a  hung outside that children may be strangulated by the cords  2 ,  3  when they play with the roman shade  1 . To avoid such accident, some shades with safety device are provided in the present market. For example, a safety device is provided between the cords and the shade member that the cords will be detached from the shade when the cords are pulled with a great force. 
         [0005]    A common safety device has an elastic buckle and a buckle base. However, such safety device will fail when the elastic buckle is fatigue after buckled and unbuckled for too many times. Another safety device is shown in  FIG. 2  which has a magnet  7  at an end of the cord  6  and a magnet  9  on a top rail  8  of the shade. In a normal condition, the magnets  7  and  9  are coupled together by a magnetic force, and they will be separated when the cord  6  is pulled by an external force F which is greater than the magnetic force between the magnets  7  and  9 . 
         [0006]    The magnet safety device does not have the same drawbacks as the buckle safety device, however, the magnet  7  at the end of the cord  6  might wind around the neck or other body parts of the child pulling the cord or the children nearby. Besides, it is inconvenient that people have to stand high to reconnect the magnet  7  of the cord  6  to the magnet  9  near the top rail  8 . In conclusion, the conventional safety devices for roman shade have to be improved to avoid danger and inconvenience. 
       SUMMARY OF THE INVENTION 
       [0007]    The primary objective of the present invention is to provide a roman shade, which has good safety mechanism and convenient assembly and installation. 
         [0008]    To achieve the objective of the present invention, a roman shade includes a shade member and a cord assembly is introduced as follows. The shade member has a top rail, a bottom rail, and a valance with opposite ends connected to the top rail and the bottom rail. The cord assembly has a positioning unit, a first device, a second device, and at least a cord. The positioning unit is provided on the shade member adjacent to the top rail. In one embodiment, the first device is provided on the shade member adjacent to the bottom rail. The second device is fastened to the cord to be magnetically attracted by the first device. In another embodiment, the second device is provided on the shade member adjacent to the bottom rail. The first device is fastened to the cord to attract the second device. The cord passes through the positioning unit for manipulation to fold and unfold the valance. 
         [0009]    In an embodiment, the second device is surrounded by a flexible device to avoid the shade being damaged by the second device or the first device attached to the cord when they are separated from each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of the conventional roman shade; 
           [0011]      FIG. 2  is a perspective view of the conventional safety device for the roman shade; 
           [0012]      FIG. 3  is a perspective view of a preferred embodiment of the present invention in the unfolded condition; 
           [0013]      FIG. 4  is a perspective view of the preferred embodiment of the present invention in the folded condition; 
           [0014]      FIG. 5  is a partial sectional view of the preferred embodiment of the present invention in the folded condition; 
           [0015]      FIG. 6  is a partial enlarged view of  FIG. 3 ; 
           [0016]      FIG. 7  is a perspective view of the ring of the preferred embodiment of the present invention; 
           [0017]      FIG. 8  is a sketch diagram of the magnet devices; 
           [0018]      FIG. 9  is a; sketch diagram of the weight; 
           [0019]      FIG. 10  is a sketch diagram of another preferred embodiment of the present invention in the folded condition; and 
           [0020]      FIG. 11  is a partial enlarged view of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The present invention provides a roman shade  100 .  FIG. 3  shows that the roman shade  100  of the present invention is unfolded, and  FIG. 4  and  FIG. 5  show that the roman shade  100  is folded. The roman shade  100  of the present embodiment of the present invention includes a shade member  10  and a cord assembly  20 . 
         [0022]    The shade member  10  includes a top rail  11 , a bottom rail  12  and a valance  13 , which is a cloth with opposite sides connected to the top rail  11  and the bottom rail  12 . The valance  13  is fastened with several parallel bars  14 , whose weight may stretch the valance  13 , as shown in  FIG. 3  and  FIG. 6 .  FIG. 4  shows the positions of the bars when the valance  13  is folded. The valance  13  is fastened with several pairs of couplers  15  and a pair of rings  16  to connect the bars  14 . As shown in  FIG. 7 , the coupler  15  is a C-ring having a gap  151  and a curved-inward guiding portion  152 . The rings  16  are secured at the bar closest to the bottom rail  12 . 
         [0023]    The cord assembly  20  has a positioning unit  21 , a left first device  22 , a right first device  23 , a left second device  24 , a right second device  25 , two flexible members  26 , a left cord  27 , a right cord  28 , and a connector  29 . 
         [0024]    In the present invention, the positioning unit  21  includes a reel  211  and a stopper  212 . The stopper  212  has a ratchet wheel  212   a  and a paw  212   b . The reel  211  is a pulley in the present invention, and it may simply be a round frame as well. 
         [0025]    The left first device  22  and the right first device  23  are two round magnets.  FIG. 8  shows the left first device  22  having a through hole  221  and a recess  222  around an end of the through hole  221 . The ring  16  passes through the through hole  221  of the left first device  22  to fasten the left first device  22  on the valance  13  adjacent to the bottom rail  12 . In the same way, the right first device  23  is fastened on the valance  13  by the other ring  16 . It still has some other ways to fasten the first device  22 , such as passing a rope through the through hole  221  of the left first device  22 , and then stitching the rope to the valance  13 . 
         [0026]    The left second device  24  and the right second device  25  are two round irons. As shown in  FIG. 8 , the left second device  24  has a through hole  241  and a recess  242  around an end of the through hole  241 . The left second device  24  and the right second device  25  can be connected with a flexible device  26 , which is a rubber ring fitted to the second devices  24  and  25 . 
         [0027]    As shown in  FIG. 3 ,  FIG. 5 , and  FIG. 8 , the left cord  27  has an end passing through the through hole  241  of the left second device  27  and tied a knot  271 , and the other end thereof passing through the couplers  15  on left side of the valance  13  and around the ratchet wheel  212   a , and then passing through a space between the ratchet wheel  212   a  and the paw  212   b  to form a hung operating section  272 . The right cord  28  has an end passing through the through hole of the right second device  25  and tied a knot, and the other end passing through the couplers  15  on right side of the valance  13  and around the reel  211 , and then passing through the space between the ratchet wheel  212   a  and the paw  212   b  to form a hung operating section  281 . As shown in  FIG. 3 , the attraction of the first devices and the second devices connect the left and right cords  27  and  28  to the valance  13 , and the knots of the cords are received in the recesses of the second devices, and the second devices have the sides with the recesses contacting the first devices. 
         [0028]    In the present invention, the connector  29  is connected to the distal ends of the left and right cords  27  and  28 . A rope  30  has an end connected to the connector  29  and a weight  32  is connected to the other end of the rope  30 . User may operate the weight  32  to control the left and right cords  27  and  28  at the same time for folding and unfolding the valance  13 . The rope  30  and the weight  32  may avoid the left and right cords  27  and  28  tangling also. As shown in  FIG. 9 , the weight  32  is embedded with a metal  33  therein that the weight  32  is attracted by the left first device  22  (or the left second device  24 ) when the valance  13  is folded, as shown in  FIG. 4 , to fasten the suspended cords  27  and  28 , the rope  30  and the weight  32 . The metal  33  within the weight  32  may be a magnet also. The connector  29  may be omitted and its function can be replaced by simply connecting the two separate weights  32  to the left and right cords  27  and  28 . 
         [0029]    It has to be mentioned that the first device  22 ( 23 ) and the second device  24 ( 25 ) may either be both magnets, or one of them is a magnet and another one is a device attracted by magnet, such as iron. 
         [0030]    While a child is wound around by the cord  27  or  28  and struggling, the first device  22  or  23  will be separated from the second device  24  or  25 . Also the cord  27  or  28  while moving along the couplers  15 , the guiding portions  152  may leave the couplers  15  via the gaps  151  to provide a further protection to the child. 
         [0031]    Also, the first device  22  or  23  and the second device  24  or  25  are located at a position adjacent to the bottom rail  12  that the separated cord  27  or  28  will be stopped by the positioning unit  21  on the top rail  11  so the cord  27  or  28  still has an end connected to the roman shade  100  to avoid the dropped cord winding around the neck or other body parts of the child pulling the cord or other children nearby. 
         [0032]    After that, user may easily pull down the second device  24  or  25  from the positioning unit  21  and reconnect the second device  24  or  25  to the first device  22  or  23  so that the roman shade  100  of the present invention may be normally operated again. 
         [0033]    The flexible devices  26  on the second device  24  and  25  may protect the roman shade  100  from damaging surrounding furniture or window glasses when the second device is separated from the first device  22  and/or  23 . 
         [0034]    As shown in  FIG. 10  and  FIG. 11 , a roman shade  200  of another preferred embodiment of the present invention is provided, which is similar to the roman shade  100  of above, except that: 
         [0035]    The roman shade  200  has a cord assembly  40 , which includes a positioning unit  42  having two reels  421 ,  422  on a top rail  52  of a shade member  50  and a driving device  423  between the reels  421 ,  422 . The driving device  423  includes a spring  423   a  and a transmission unit  423   b . The transmission unit  423   b  includes several bevel gears to connect the spring  423   a  and the reels  421 ,  422  that the spring  423   a  may drive the reels  421 ,  422  turning. 
         [0036]    The cord assembly  40  has a left cord  44  and a right cord  46  having ends passing through the top rail  52  and fastened to the reels  421 ,  422  respectively and the other ends passing through couplers  54 , and then fastened to second devices  48 . The second devices  48  are attracted by first devices  49  on a lower portion of a valance  56 . The operation and function of the roman shade  200  are the same as the roman shade  100  above. 
         [0037]    The major difference between the roman shades  100  and  200  is that the roman shade  200  provides the driving device  423  driving the reels  421  so that there is no free-standing operating sections of the left cord or right cord. 
         [0038]    The description above is a few preferred embodiments of the present invention and the equivalence of the present invention is still in the scope of claim construction of the present invention.

Summary:
A roman shade of the present invention is operated by a cord to fold and unfold it. The cord is connected with a second device to be attracted by a first device provided on a valance adjacent to a bottom rail. The second device and the first device will separate when the cord is pulled with strength greater than a magnetic force between second device and the first device to avoid the cord dropping down and winding around neck or other body parts of the kids nearby.