Abstract:
An electronic chassis or modular rack has elements and methods for reducing emission of electromagnetic waves or interference (EMI) and at the same time ensuring that the components within the chassis or rack are properly cooled. A face plate which attaches to a printed circuit board or blank board for insertion and removal of the board from the chassis and limiting EMI emission while allowing connector and indicator accessibility. The face plate has an elongated frame having at least two planar body portions. The planar body portions are integrally formed to each other with the planes of the two planar body portions at an angle of between 25 and 55 degrees of each other. The elongated frame has a pair of free edges adjacent to the planar surfaces and the pair of side edges. The face plate has a pair of mounting devices. Each mounting device is carried by the elongated frame in proximity to one of the free edges. The pair of mounting devices are offset from each other about the planar body portions. The air impedance module or board inserts into the chassis for controlling air flow in the chassis. The top and bottom edges are adapted to be received in the chassis. An air impedance protrusion is formed integrally with the planar portion adapted for limiting the flow of air. The limiting of the flow of air in one portion results in the air being redirected to printed circuit boards that have active components that need cooling.

Description:
BACKGROUND OF THE INVENTION 
     Electrical components generate an electromagnetic field when the electrical components are in an activated state. This electromagnetic field can be generated by an act as simple as a current passing through a wire. However, the concern related to the electromagnetic field creating interference is typically related to complex circuitry. This electromagnetic field generated by one component can interfere with the workings of other components in proximity to it. With this concern, various groups and organizations such as the Federal Communications Commission establish requirements regarding the emission of electromagnetic waves or fields from components or units, such as chassises. This emission of electromagnetic waves is typically referred to as electromagnetic interference or EMI. 
     It is known to create a metal cage surrounding the components to block or “knock down” the radiating electromagnetic waves. As the frequency within the circuitry increases the wavelength decreases, therefore the size of the opening through which the electromagnetic wave can exit the chassis is decreased. For example, as the frequency approaches 622 MHz, the size of the opening through which a wave may pass is in the range of 2 inches. While the height of the wave depends on the frequency, the width or thickness of the wave is infinitesimal. Therefore, while the opening needs to be approximately 2 inches in height for the frequency of approximately 622 MHZ to pass through, the opening could be the thickness of this paper and the wave could pass through. 
     While there is a desire to reduce the size of openings in the chassis containing the electronic components, there is still a need to access the components and a need to replace components or interchange components as desired. It is known to have a series of panels or plates, such as face plates, which are removable to grant access to a component such as a printed circuit board located within the unit. In addition, the face plate may include openings to allow connectors to be accessible and indicators, such as lights, to be visible from the outside of the unit. 
     In addition to the desire to limit the emission of electromagnetic waves from the component, there is a desire or need to expel heat from the unit. It is therefore a balance of apparently opposing goals to allow sufficient airflow through the unit to allow proper cooling while at the same time preventing the emission of electromagnetic waves. 
     SUMMARY OF THE INVENTION 
     The invention relates to an electronic chassis or modular rack having elements and methods for reducing emission of electromagnetic waves or interference (EMI) and at the same time ensuring that the components within the chassis or rack are properly cooled and accessible. 
     The invention relates to a face plate which attaches to a printed circuit board or blank board for insertion and removal of the board from the chassis and limiting EMI emission while allowing connector and indicator accessibility. 
     The invention relates to an air impedance module or board for limiting air flow therein allowing redirection of the flow. 
     The face plate according to the invention has an elongated frame having at least two planar body portions. Each planar portion has a pair of planar surfaces and a pair of side edges. The planar body portions are integrally formed to each other with the planes of the two planar body portions at an angle of between 25 and 55 degrees of each other. The elongated frame has a pair of free edges adjacent to the planar surfaces and the pair of side edges. The face plate has a pair of mounting devices. Each mounting device is carried by the elongated frame in proximity to one of the free edges. The pair of mounting devices are offset from each other in parallel planes parallel to one of the planar body portions. 
     In a preferred embodiment, the face plate has a plurality of mounting bosses or printed circuit mounts carried on the inner surface of the planar body portions for attachment to a front edge of a printed circuit board. The mounting devices include at least one ejector pivotably mounted to the face plate adapted for seating and securing the face plate to a chassis. The planar portion of the elongated frame has an integral protrusion on the outer surface for pivotably receiving the ejector. 
     In a preferred embodiment, the face plate has an opening on one of the planar body portions adapted to receive an optical interface. In a preferred embodiment, the optical interface is an optical transceiver. 
     In a preferred embodiment, the elongated frame of the face plate is die cast. The planar portion of the elongated frame has an integral die cast protrusion on the outer surface for pivotably mounting an ejector adapted for seating and securing the face plate to a chassis. One of the side edges of the elongated frame has a flange with a groove. The groove receives a metallic seal adapted to seal with an adjacent surface. 
     The invention relates to a board for insertion in a chassis for controlling air flow in the chassis. The board has a planar portion having a pair of sides and a plurality of edges. The top and bottom edges are adapted to be received in the chassis. An air impedance protrusion is formed integrally with the planar portion and is adapted for limiting the flow of air. The limiting of the flow of air in one portion results in the air being redirected to the printed circuit boards that have active components that need to be cooled. 
     In a preferred embodiment, the air impedance board has a plurality of air impedance protrusions formed integrally with the planar portion of the chassis and protruding from the planar portion on the same side for limiting the flow of air. The board has a stiffener protrusion projecting generally non-parallel to the air impedance protrusion. 
     In a preferred embodiment, the air impedance board is formed of a flame retardant thermoplastic such as Kydex-T. In a preferred embodiment, the electronic chassis has a fan tray with a plurality of slots underlying the fan blades to allow air to flow through and is solid under the hub for fire protection purposes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a left front perspective view of a modular rack, switching box, with portions broken away; 
     FIG. 2 is a sectional view of the modular rack taken along line  2 — 2  of FIG. 1; 
     FIG. 3A is a side view of a printed circuit board with a face plate exploded away; 
     FIG. 3B is a side view of the printed circuit board with the face plate attached; 
     FIG. 4A is a front perspective view of the face plate with the ejectors exploded away; 
     FIG. 4B is a rear perspective view of the face plate; 
     FIG. 5 is a right front perspective view of the ejector handle in an open position relative to the rail; 
     FIG. 6 is a right front perspective view similar to FIG. 5 with the ejector handle in a closed position relative to the rail; 
     FIG. 7 is a side sectional view of the switching box with the circuit board installed and the cable routed; 
     FIG. 8 is an exploded perspective view of a fan cage; 
     FIG. 9 is a perspective view of an impedance board; 
     FIG. 10 is a front view of the impedance board of FIG. 9; 
     FIG. 11 is a side view of the impedance board of FIG. 9; and 
     FIG. 12 is a top view of the impedance board of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures, like numbers are used to indicate like elements. FIG. 1 shows a modular rack with a face plate  20  and an air impedance module  22  of the invention. 
     Referring to FIG. 1, the modular rack  26  in a preferred embodiment is for use in optical switching. It is recognized that the invention as described can be used for other systems such as conventional copper wire and integrated circuit switching systems, transceiver units, processing units, or other units or systems. 
     The modular or mounting rack  26  has a housing  28  with a front  30 , a pair of sides  32 , a top  34 , a base, and a back  38 , as seen in FIG.  2 . The front  30  has an opening  40  adapted to receive printed circuit boards (PCB)  42 , as seen in FIG. 2, into the housing  28  of the module rack  26 . The PCB  42  has an interface  44 , such as a High Density Metric (HDM) female module sold by Molex, which is adapted to be received by an interface  46 , such as a HDM male backplane connector sold by Molex, located on a backplate  48  carried within the modular rack  26 . 
     The modular rack  26  has upper and lower card guides  50  and  52  each formed of metal in a preferred embodiment. Each card guide  50  and  52  has numerous openings  54  through which air can pass as described in additional detail below. In addition, the upper card guide  50  and the lower card guide  52  each have formed within an upper rail  56   u,  as seen in FIG. 5, and a lower rail  56   l  respectively. The rails  56  each define a channel  58 , a guide channel, to receive the upper and lower edge of the PCB as installed. 
     The modular rack  26  has a pair of anchor surfaces  62 , also referred to as stop plates. The anchor surfaces  62  are abutted by the face plates  20  when the face plates  20  are installed. The anchor surfaces  62  each have a plurality of threaded openings  64 , as best seen in FIGS. 5 and 6, used in securing the face plates  20  as described below. In a preferred embodiment, the anchor surfaces  62  are formed as part of the card guides  50  and  52 . 
     The modular rack  26  has a pair of ejector rails  68 , a upper ejector rail and a lower ejector rail. The ejector rails  68  are located along the upper edge and the lower edge of the opening  40  on the front  30  of the housing  28 . While referred to as ejector rails  68 , the rails are used for both injection and ejection of the face plate  20  as explained in further detail below. The ejector rails  68  are adjacent to the anchor surfaces  62 . The plane of each of the ejector rails  68  is perpendicular to the plane of the respective anchor surfaces  62 . The ejector rails  68  interact with the front panel  20  as described below. 
     In a preferred embodiment, the housing  28  is formed from sheet metal having a thickness of 0.062 inches. In order to stiffen the housing, especially around the opening  54 , in addition to the card guides  50  and  52  and the ejection rails  68 , the modular rack  26  has a pair of extruded aluminum side rails  69 , which extend from the upper card guide  50  to the base of this housing. 
     The modular rack  26  has a cable or fiber management chamber  70 , also referred to as cable channel or trough, and seen in FIGS. 2 and 7, which is accessible to a pair of openings  72 , one seen in FIG. 1, located on the sides  32  of the housing  28 . The front  30  of the housing  28  has a hinged panel  74  which opens to allow access to the cable channel  70 . In addition, a plurality of slots  76 , one seen in FIG. 7, located on the front panel  30  of the housing  28  of the modular rack  26  below the opening  40  allow access of cables into the cable channel  70 . 
     The upper portion of the housing  28  receives a fan casing or tray  80  containing a plurality of fans  82 , as best seen in FIG. 8, for drawing air through the housing  28 . The fan casing  80  is slid into an opening  84  in the front  30  of the housing  28  above the opening  40 . The front of the fan casing  80  has a pair of optical LEDS for indicating the status of the fan. A fan faceplate or decorative bezel  86  is located in front of the fan casing  80 . 
     The housing  28  of the modular rack  26  has a plurality of holes  90  located on the hinged panel  74  to the cable channel  70  and a plurality of holes  92  located on the sides  32  and the back  38  of the housing  28  of the modular rack  26  to allow airflow. The openings, which are holes  90  and  92  in the housing  28 , are positioned such that all activated circuitry which would generate an electromagnetic field are not along a direct line of sight with such opening to the outside and any electromagnetic fields being generated needs to be redirected by at least one metal surface, therefore reducing the electromagnetic field prior to exiting the modular rack  26 . 
     Still referring to FIG. 1, the face plate  20  has a frame  95  with a plurality of planar portions  96  and in a preferred embodiment has three planar portions  98 ,  100 , and  102 . In a preferred embodiment, the second planar portion  100  is integral with the other planar portions  98  and  102  and is angled between the two other planar portions  98  and  102  which are parallel but offset. The face plate  20  extends from one of the anchor surfaces  62  to the other anchor surface  62 . The anchor surfaces  62  are parallel to each other, but offset from each other, as best seen in FIG.  7 . Located at the ends of the face plate  20 , the free ends of the planar portions  98  and  102  are the ejectors  106  which engage the ejector rails  68  as described in further detail below. 
     In certain situations the modular rack  26  does not require that each slot which is capable of receiving a printed circuit boards (PCB)  42  receive a PCB  42  with components. In these instances, instead of installing a blank PCB  42  with the face plate  20 , an air impedance module  22  is installed with the face plate  20 . Therefore, the modular rack  26  may have one or more of the air impedance modules  22 . The air impedance modules  22  each have a plurality of protrusions  108  for limiting the air flow as further explained below. 
     FIG. 2 is a cross sectional view of the modular rack  26  showing sections of the modular rack  26  at various elevations in the rack  26  looking downward. The sections shown, looking left to right in FIG. 2, are at elevations through a mid-portion of the face plate  20 , one of the two ejector rails  68 , the fan tray or casing  80 , and the cable channel or trough  70 . 
     The first quarter of the figure shows three face plates  20  and a portion of a fourth face plate  20 . A groove  112  in the housing  28  of the modular rack  26  receives a metallic seal  114  for engaging the first face plate  20 . The first three face plates  20  each are shown with a groove  112  on the right side of the plate  20  for accepting the metallic seal  114  and engaging the adjacent face plate  20 . 
     As indicated above, in a preferred embodiment a pair of extruded aluminum side rails  69  extend along the side of opening in the housing  28 . The groove  112  in the housing  28  is in the side rail  69  as seen in FIG.  2 . 
     A printed circuit board  42  is seen extending from the first and second face plates  20 . Each of the circuit boards  42  has a plurality of components  116 . The circuit boards  42  shown each have an optical transceiver  118  in proximity to the face plate  20  of their respective PCB  42 . An opening  120  in the face plate  20  receives an adapter  122 , such as a LC—LC duplex adapter, which allows fiber optic cabling to extend, in two segments, from the optical transceiver  118  through the interface  122  and extend outward and enter the cable channel or trough  70 . The interface  44 , such as a HDM module, of each of the PCBs  42  is accepted by a compatible interface  46 , such as a HDM connector, on the backplate  48 . 
     The third and fourth face plate  20  each are connected to a blank board which is one of the air impedance boards  22 . The air impedance boards  22  each have an air impedance protrusion  108  which projects from a planar portion  126  to minimize the air gap to the adjacent printed circuit board  42  or blank air impedance board  22 . The function of the air impedance board  22  with the air impedance protrusion  108  is explained in further detail below. 
     The second quarter of FIG. 2 shows a portion of a pair of teeth  128  of the ejectors  106  extending through holes  130  in the ejector rail  68 . The teeth  128  and a back engaging surface  210  of the ejector  106  interact in moving the face plate  20  with the PCB  42  or the blank air impedance board  22  in and out of the housing  28 . The ejector  106  has a handle portion  132 , as best seen in FIGS. 3A-6, for assisting in the installation by rotating the ejector  106  as explained in further detail below. The upper card guide  50  with the plurality of openings  54  is seen from above with the channel  58 . A few components  116  on a PCB  42  and an air impedance protrusion  108  on an air impedance board  22  can be seen through openings  54  in the upper card guide  50 . 
     Still referring to FIG. 2, the third portion or quarter of the figure shows a section through the fan casing or tray  80 . The fan casing or tray  80  has a plurality of fans  82 , one seen in FIG.  2 . In a preferred embodiment, the fan casing  80  has three fans  82  such as seen in FIG.  8 . Each fan  82  has a plurality of blades  136  and is driven by a motor to draw air up through the housing  28  between the printed circuit boards  42  and the blank air impedance boards  22 . The fan casing or tray  80  has a control unit  140  with a pair of LEDs (light emitting diodes)  142  for indicating the status of the fan casing or tray  80 . The fan faceplate  86  covers the front of the fan casing or tray  80  and has a pair of light channels  144  for transmitting the light from the LEDs  142  to the front  30  of the modular rack  26 . 
     The fourth portion of FIG. 2 is a section through the cable channel or trough  70 . The cable channel  70  has openings  72  through the housing  28  at each side  32 , one seen in FIG.  1  and the other in FIG. 2, for routing cables, also referred to as cabling, out of the cable channel  70 . The cable channel  70  is above a duct  148 ; the duct  148  allows air to flow, as seen best seen in FIG. 7, to a plenum  150 , seen in FIG. 2, located behind the cable channel  70 . 
     Referring to FIG. 3A, the face plate  20  and a printed circuit board  42  are shown exploded away from each other. The face plate  20  has three planar portions  96  wherein the first and third planar portions  98  and  102  are parallel and offset from each other. The interposed second planar portion  100  projects at an angle from the first planar portion  98  to the third planar portion  102  to create such offset. The planar portions  96  have an inner surface  152  which faces the printed circuit board  42  and an outer surface  154 . The edge portion  156  of the first planar portion  98  and the third planar portion  102 , the portion not adjacent to the second planar portion  102 , define a free end  158 . 
     Located at the free ends  158  of each of the first and third planar portions  98  and  102  is a protrusion  160  on the outer surface  154  as best seen in FIGS. 3B and 4. Each protrusion  160  receives one of the ejectors  106  as explained in further detail below with respect to FIG.  4 . The face plate  20  has a plurality of mounting features or bosses  162  formed integral on the inner surface  152  of the planar portions  96 . The printed circuit board  42  has holes  164  aligned with the holes  166  in the bosses  162  such that a fastener  168 , such as a screw, may secure the printed circuit board  42  and the bosses  162  of the front plate  20  to each other. With the use of five connection points, the force transferred between the face plate  20  and the printed circuit board  42  is more evenly distributed. The HDM female module interface  44  is shown on the printed circuit board  42 . In addition, a plurality of components  116  are shown on the printed circuit board  42  including an optical transceiver  118  and a plurality of light emitting diodes (LEDs)  170 . 
     Referring to FIG. 3B, the face plate  20  is shown attached to the PCB  42 . The fasteners  168  extend through the PCB  42  into the holes  166  in the bosses  162 . The second planar portion  100  is at angle α of between 25° to 55° and in a preferred embodiment the angle α is 35°. 
     Referring to FIG. 4A, a face plate  20  for use with a printed circuit board  42 , not shown in this figure, having different components than those shown in FIG. 3, is shown in perspective with the ejectors  106  exploded away. The face plate  20  has a plurality of openings  172  on the first planar portion  98  for light channels, such as the light channels  144  shown in FIG. 2, which project the light from a LED mounted on the printed circuit board  42  to the outside of the face plate  20 . The panel  98  in addition has a slot  174  through which a PCMCIA card reader extends outward. The second planar portion  100  has a pair of slots  176  through which pin connectors such as a standard series nine pin connector jack may extend from the printed circuit board  42 . 
     The protrusions  160  located at the top of the first planar portion  98  and the bottom of the third planar portion  102  in proximity to the free end  158  each have a hole  178 . A pivot pin  180  extends through the hole  178  allow pivotal movement of the ejector  106  relative to the face plate  20 . Located on one side of each of the protrusions  160  is a slot  184  in the planar portion  98 . The slots  184  receive a planar portion  186  of an engaging portion  188  of the ejector  106  to allow movement of the ejector  106  (The ejector  106  is generally defined as having the engaging portion  188  and the handle portion  132 ). Located on the other side of the protrusion  160  near the side edge is a notch  190  in the planar portion  98  which receives the other planar portion  186  of the ejector  106  as best seen in FIG.  4 B. Each of the ejectors  106  takes approximately one half of the width of the planar portion  98  of the frame  95  of the face plate  20 . 
     The frame  95  of the face plate  20  has a captive fastener  194  located adjacent to the ejector  106  on the other half of the planar portion width. The captive fastener  194  is sometimes referred to as a floating screw. 
     In a preferred embodiment, selected face plates  20  can have an alignment pin  196  such as seen in the bottom of FIG. 3B, projecting from the inner surface  152  of the frame  95  for alignment with an alignment hole  198 , such as seen in FIG. 5, on the anchor surface  62 . The alignment pin  196 , sometimes referred to as identifying pin, and the respective alignment hole  198  located on the anchor surface  62 , can be positioned in various locations, such as only on the upper end, only on bottom, both, shifted to the left, right, etc. so that the improper printed circuit board  42  and associated face plate  20  are not installed into the wrong slot. In this preferred embodiment, it is recognized that only one or a few face plates  20  would have the alignment pin  196 . It is recognized that the interface  44  to interface  46  of PCB  42  to the backplate  48  can incorporate alignment devices also to insure the proper circuit board  42  and face plate  20  are installed. 
     The ejector  106 , as seen in FIG. 4A, has a handle portion  132  and an engaging portion  188 , as indicated above. The bottom ejector  106 , as seen in FIG. 4A, is rotated to better show the two planar portions  186  spaced by an opening  200  of the engaging portion  188 . Each planar portion  186  of the engaging portion  188  is carried by the handle portion  132  and has a hole  202 . The holes  202  are aligned with the hole  178  on the protrusion  160  of the frame  95  when the ejector  106  is installed. The pin  180 , which pivotally connects the ejector  106  to the frame  95 , is in a preferred embodiment a steel rail. The pin  180  is shown exploded away from the ejector  106  and the protrusion  160  on the outer surface  154  of the frame  95  of the face plate  20 . The pin  180  extends between the two holes  202  in the ejector  106  and through the hole  178  in the protrusion  160  of the frame  95 . The hole  178  of the protrusion  160  is a clearance hole to allow the pin  180  to rotate within this hole  178  of the protrusion  160 . The pin  180  is press fit within the holes  202  of the planar portions  186  of the engaging portion  188  of the ejector  106  of the face plate  20 . 
     As indicated above, one of the planar portions  186  of the engaging portion  188  of the ejector  106  passes through the slot  184  and the other planar portion  186  extends through the cut away or notch  190  in the frame  95  of the face plate  20  and an adjacent face plate  20 . Each of the planar portions  186  of the engaging portion  188  has a notch  206  formed with a front engaging surface  208 , the back engaging surface  210 , and a bottom stop surface  212 . The top of each of the notches  206  is open. The back engaging surface  210  extends between the two planar portions  186 . In addition, each of the planar portions  186  has a front surface  214 . The front engaging surface  208  and the front surface  214  define the tooth  128 . The tooth  128  interacts with the ejector rail  68 , as seen in FIG. 6, and further described below. 
     The frame  95  of the face plate  20  has a flange  246  on each of the side edges  248  that extends along a majority of the side edge  238 , as best seen in FIG.  4 B. Referring back to FIG. 4A, the frame  95  of the face plate  20  has the groove  112  formed along the one side for receiving the compressible metallic strip  114 . The groove  112  is formed in the flange  236 . The compressible metallic flexible strip  114 , which is shown exploded away from the face plate, fills the gap between adjacent face plates to form a continuous electromagnetic shield. The compressible strip  114  is shown in three segments which abut each other and in a preferred embodiment are glued into the groove  112 . The metallic strip  114  in a preferred embodiment is a wire mesh or wire covered nylon yard over a foam core, such as soft-shield® 2000 sold by Chomerics. The flexible strip  114  compresses when mated with the adjacent face plate  20 , or housing. 
     The board, the printed circuit board (PCB)  42  or the air impedance module board  22 , is slid into the modular rack  26  such that the upper and lower edges of the board are received by the rails  56  formed on the upper and lower cages  50  and  52 , which are located above and below the printed circuit board area of the rack  26 . When the board is nearly completely in, the ejector  106  is rotated such that the notch  206  is positioned so that the notch  206  opens towards the rail, the ejector rail  68 , and the ejector  106  is extending generally outward as seen in FIG.  5 . 
     With the ejector  106  in this position, the board and face plate  20  can be slid into a position where the bottom stop surface  212  of the notch  206  engages the outer surface  218  of the rail  68 , and the teeth  128  on the ejector  106  are located underneath the rectangular openings  130  in the ejector rail  68 . With both ejectors  106  in this position and a printed circuit board  42  nearly installed, the user rotates the handle portion  132  of the ejector  106  towards the frame  95  of the face plate  20  to a position such as seen in FIG. 6 therein forcing the board and face plate  20  into the closed position by the force of the tooth  128  against the surface of the opening  130  on the ejector rail  68 . This rotation of the ejectors  106  also results in the seating of the interface  44 , such as a HDM female module on the printer circuit board  42  into the interface  46 , such as a HDM male backplane connector on the backplate  48 , such as seen in FIG.  7 . The anchor surface  62  in addition to positioning the face plate  20  structural, is a part of the chassis ground path. 
     To remove the board and the face plate  20  from the modular rack  26 , chassis, the captive fasteners  194  are unscrewed and the handle portion  132  of the ejectors  106  are rotated away from the frame. The rotation of the ejectors  106  against the ejector rail  68  causes the board to unseat from the back plate  48 . 
     Referring to FIG. 7, a sectional view of the modular rack  26  is shown. The printed circuit board  42  shown has an optical transceiver  118  adjacent to the face plate  20 . The optical transceiver  118  has an exit port  220  which projects through an opening  120  in the second planar portion  100 . With the tight tolerances that are possible with the die cast frame  95  of the face plate  20 , the optical transceiver  118  can interface directly through the opening  120  without an intermediate jack or interface  120 , such as shown in FIG. 1, which seats on the frame  95  of the face plate  20  and which would require connectors and interfaces. It is recognized in certain embodiments it may desirable to locate the optical transceiver  118  away from the opening, so that EMI which may radiate from the exit port  220  of the optical transceiver  110  is distant from the opening  120 . 
     The printed circuit boards  42  each have an interface  44 , such as a HDM female module, which is seated in an interface  46 , such as an HDM male backplane connector on the backplate  48  to provide power to the components, such as the optical transceiver  118 , and electronically connect those components to other components. 
     A fiber optics cable  222  extending from the optical transceiver  118  is routed through a slot  76  located in the housing  28  into the cable channel or trough  70 . The cable  222  is routed through the openings  72  on the side of the housings. 
     Still referring to FIG. 7, underlying the cable channel or trough  70  is the duct  148  from the holes  90  on the front  30  and side  32  of the housing  28  to the plenum  150  behind the cable channel  70 . Air which is drawn from outside the housing  28  passes through the duct  148  and plenum  150  and is then drawn through the lower cage  52 , passes by and cools the printed circuit board  42  and passes up through the upper cage  50 . The housing  28  receives a filter  228 , which seats at the upper edge of the duct  148  and extends into the plenum  150  to filter particles out of the air prior to passing over the printed circuit board  42 . The filter  228  also acts as a diffuser to create a more uniform air flow. 
     The fan casing or tray  80  is located above the upper cage  50 . The fan casing  80  has a plurality of fans  82  which are each driven by a motor  138 . The air drawn through the housing  28  and the fans  82  is forced out the holes  92  on the side  32  and the back  38  of the housing  28  near the top of the housing  28 , which is sometimes referred to as “evacuation style cooling.” The fan faceplate or bezel  86  is located in front of the fan casing  80  to give a finished look. 
     In a preferred embodiment, the frame  95  of the face plate  20  is formed of a metal, such as Zinc-Alloy (ZA-12) in a die-cast process. In a preferred embodiment, the ejector  106  is also formed of Zinc-Alloy (ZA-12). The use of die-cast face plates  20  allows a high tolerance of the frame  95  of the face plates  20  therein minimizing any gaps between adjacent planer portions  98  of adjacent frames  95  of adjacent face plates  20 . In addition, in view of the increased precision for example of the second planar portion  100  relative to the remaining portions of the face plate  20  including the other planar portions  98  and  102  of the frame  95 , those openings created in the frame  95  of the face plate  20 , which are formed by secondary machining operation, to allow components such as pin connections to extend through the face plate  20  are more accurately positioned. These accurately positioned openings therefore do not need to be oversized, therein minimizing the possibility of electromagnetic interference waves passing through openings. 
     In a preferred embodiment, the mounting bosses  162  are formed in the die-cast process with a slight taper to facilitate removal of the frame  95  of the face plate  20  from the mold. The mounting bosses  162  are machined to form a surface to abut the PCB  42  of the air impedance module  22 . Likewise, the groove  112  in the flange  236  for the metallic seal  114  is machined and holes  178  in the protrusions  160  are drilled after the part is die-cast. 
     The modular rack  26  with the tight tolerance face plate  20  and the metallic compressible strip  114  compressed between face plates  20  and between the end face plates  20  and side of the opening  54  prevents electromagnetic interference waves from passing through the opening  54 . The interface of the top and bottom of the frame  95  with the stop plates  52  and the ejectors  106  with the ejector rails  68  likewise prevent openings through which EMI may pass. 
     The arrangement of other components as addressed above likewise limits EMI emission. In addition, the holes or openings in the housing  28  and in the card guides  50  and  52  are sized in addition to prevent insertion of adult size fingers through the openings. 
     Referring to FIG. 8, the fan casing or tray  80  is shown in exploded view. The fan casing  80  has a metal housing  230  that slides into the fan opening  84  in the housing  28 , as seen in FIG.  7 . The metal housing  230  has a bottom frame  232  with a plurality of curvilinear slots  234  that position in three sets under the fan blades  136  of the three fans  82 . The portion of the bottom frame  232  is solid underlying the hub of the fan for fire protection purposes. Each of the fans are driven by a DC motor located within the hub of the fan  82  in a preferred embodiment. 
     An upper frame  236  of the metal housing  230  has a plurality of openings  238 . The fan casing or tray  80  has an interface  240  and a control card  242 . A pair of rails  244  for sliding the tray  80  into the housing  28  are shown exploded away. 
     Metallic seals, such as the compressible metallic seals  114 , can be carried on the housing  230  of the fan tray  80  to engage the housing  28  to minimize EMI emission. However, the position of the fan tray  80  relative to the components, the interposed caged guide  50  and the geometry of the fan tray  80  results in reduced emission of EMI. 
     Referring back to FIG. 1, with the components  116  on the printed circuit boards  42  in the modular rack  26  operating and the face plates  20  covering the opening  40 , which allows access to the printed circuit boards  42 , to reduce any electromagnetic interference being generated by the components  116  within the modular rack  26  from escaping from the rack  26 , these same components  116  are generating heat, which builds up in the rack  26  if not addressed. The fans  82  located in the fan casing or tray  80  are drawing air from the bottom portion of the modular rack  26  and sending it out the exit ports/holes  92  located on the upper portion of the modular rack  26 . It is recognized that such airflow would take the path of the least resistance. The modular rack  26  in a preferred embodiment may have spaces to receive a plurality of cards, such as fifteen printed circuit board  42  in the embodiment shown. It is recognized that in many configurations, not all those fifteen slots will be filled with printed circuit boards  42  having components  116  in that this number of cards or components are not necessary for the use of the rack  26  at that time. If the printed circuit board  42  is not located within a rack  26 , the open space will create a path of least resistance for the airflow to pass from below the lower cage  52  to above the upper cage  50 . In such circumstances, the narrow space between two printed circuit boards  42  which has components additional filling the space receive minimal airflow. Even a blank card, which is lacking components, creates less air resistance than a filled circuit board  42  and the air will flow past the blank cards. 
     Referring to FIG. 9, an isometric view of the air impedance module  22  is shown. The air impedance module  22  has similar mounting connections as that of a printed circuit board  42  for connecting to the face plate  20 . The air impedance module  22  has three protrusions/ribs  108  which project off the plane  226  to block airflow. 
     In addition to the three protrusions  108 , the air impedance module  22  has a vertical stiffener  228  which extends between the top and bottom ribs. The vertical stiffener  228  is to provide mechanical stiffness to minimize vibration and has a minimum effect on restricting flow, since the air flow is in the same direction. 
     Referring to FIGS. 10-12, the air impedance module  22  has the plane  226  with a plurality of protrusions  108 . In a preferred embodiment, the air impedance module  22  has three protrusions or ribs  108 . The protrusions  108  do not extend all the way to the front edge, as best seen in FIGS. 10 and 11, so that the air impedance module  22  does not interfere with the flange  246 , EMI gasketing flange, on the face plate  20 . The vertical stiffener  228  of the air impedance module  22  stiffens the plane. 
     The holes  165  in the plane  226  are positioned at the same location as those holes  164  on the PCB  42  in a preferred embodiment. The face plate  20  can secure to the air impedance module  22  using the mounting blocks  162 , such as seen in FIGS. 3 and 7. 
     The air impedance module  22  is formed in a preferred embodiment of thermoform plastic, preferable a flame retardant (UL94V-O rated material) thermoform plastic such as Kydex-T. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.