Abstract:
A circuit pack blank occupies an unequipped position in an electronic equipment shelf that is cooled by forced-air convection. The blank incorporates two diffuser blades that protrude into a cooling space in the shelf adjacent to the blank. When air is forced through the cooling space at a constant velocity, a backpressure is generated that is equivalent to backpressures generated at cooling spaces adjacent to equipped positions. In addition, the blank has a faceplate that incorporates one or more cable trays used to retain cables at the unequipped position for future use.

Description:
FIELD OF THE INVENTION 
     This invention relates to the packaging of electronic apparatus in an equipment shelf. More particularly, the invention relates to the use of a blank in an unequipped shelf position to equalize airflow back pressure in the shelf, and to retain optical cables provided for future use at the unequipped position. 
     BACKGROUND OF THE INVENTION 
     Complex electronic apparatus may require the interconnection of thousands of individual electronic devices. To manage the large number of required interconnections, such apparatus is often configured by affixing devices to a circuit boards (circuit packs), and by interconnecting these circuit packs via one or more printed wire backplanes. Collections of interconnected circuit packs are often contained in housings referred to as equipment shelves. 
     As circuit and power densities increase, heat dissipation presents a significant challenge. Increasingly, forced air convective cooling is used in equipment shelves to provide the required cooling capacity. In a typical shelf configuration, circuit packs are positioned vertically with sufficient spacing to allow for airflow between adjacent packs. Airflow is generated by fans located above or below the shelf, and directed past the packs by enclosing the sides, rear and front of the shelf to be relatively airtight. Air flowing past the boards carries heat away from the electronic devices on the boards, and is then exhausted from the shelf. Ambient air is drawn into the shelf by the fans to refresh and continue the airflow. 
     The effectiveness of this approach can be impacted by imbalances in the equipment shelf. For example, to allow for future increases in system capacity, fewer than all circuit pack positions may be equipped. Fan assemblies for shelves that are partially equipped may generate uneven airflow backpressures across the equipped and unequipped positions. 
     In an equipped position, electronic devices are present in the airflow path, and these devices create resistance and an associated pressure drop over the airflow path. In an unequipped position, no electronic devices sit in the airflow path, and pressure differentials over the path are substantially less. Because air moving through the shelf seeks a path of least resistance, the presence of unequipped positions leads to an increased airflow through the unequipped positions and a decreased airflow through the equipped positions. While fans with increased capacity can substantially overcome this problem, they may also generate increased noise, require added space, consume additional power, generate additional cost, and experience reduced life. In addition, fan failures may occur, increasing the risks of overheating and reducing the life of the electronic devices. 
     SUMMARY OF THE INVENTION 
     Uniform airflow backpressure is substantially maintained across airflow paths in partially-equipped equipment shelves through use of a novel circuit pack blank for unequipped positions. Each blank includes a base for retaining the blank in an unequipped position, and one or more diffuser blades attached to the base to create resistance in an adjacent air plenum. The diffuser blade is designed to generate a backpressure in the adjacent air plenum that is substantially equivalent to backpressures generated in air plenums adjacent to the equipped positions. 
     In a preferred embodiment of the blank, two diffuser blades are attached to the base to provide resistance both at the entrance and exit of the adjacent airflow path. In addition, the blank incorporates a faceplate that supports insertion and removal, and that contains a novel tray for retaining associated cables in position for future use. In a preferred embodiment of the shelf, the blank is inserted into an unequipped position in the shelf via an apparatus guide, and positioned so that an air plenum is formed between the blank and an adjacent equipped position. The diffuser blades attached to the blank effectively reduce the cross-sectional area of this air plenum, so that a backpressure generated by air flow in the air plenum is substantially equivalent to a backpressure generated by air flow in an air plenum adjacent to the equipped position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the invention may be obtained by reading the following description of specific illustrative embodiments of the invention in conjunction with the appended drawing in which: 
     FIG. 1 illustrates a preferred embodiment of the circuit pack blank, which includes a base, two diffuser blades, a faceplate and a cable tray; 
     FIGS. 2A and 2B illustrate the positioning of the blank of FIG. 1 within an apparatus shelf, and diagram an airflow through this position; 
     FIG. 3A shows the structure of the cable tray; 
     FIG. 3B illustrates the positioning of the cable tray with the faceplate; 
     FIGS. 4A and 4B depict a spring mechanism as illustrated in the base of FIG. 1 for reducing vibration in the base; and 
     FIGS. 5A and 5B depict a spring mechanism as illustrated in the base of FIG. 1 for retaining the blank in position. 
    
    
     For consistency and ease of understanding, those elements of each figure that are similar or equivalent share identification numbers that are identical in the two least significant digit positions (for example, diffuser blade  104  of FIG. 1 is equivalent to diffuser blade  204  of FIG.  2 ). 
     DETAILED DESCRIPTION 
     An illustrative embodiment of the circuit pack apparatus blank  100  is shown in FIG.  1 . The blank  100  includes a diffuser blade  104  that is perpendicularly attached to a base  102 , in such manner that it faces a lower edge  111 . In addition, the blank includes a diffuser blade  105  perpendicularly attached to base  102 , in such manner that it faces an upper edge  109 . When inserted into an unequipped position in an equipment shelf, diffuser blades  104  and  105  serve to reduce the effective cross-sectional area of an airflow path over the blank  100 , and thereby generate an increased airflow backpressure. Diffuser blades  104  and  105  may be integrally molded within base  102 , or attached in a variety of ways well known in the art. In the embodiment of FIG. 1, braces  106  and  113  are added to improve the structural integrity of the diffuser blades  104  and  105 , respectively. 
     As will be apparent to one skilled in the art, the base  102  may be effectively constructed in a variety of ways, provided that it can be inserted in the equipment shelf via edges  109  and  111  in a manner equivalent to the insertion of circuit boards in equipped positions. For example, the base  102  may simply be formed as a uniform member with rectangular dimensions equivalent to those of the circuit boards. As depicted in FIG. 1, the base includes ribs  101  to conserve material and add rigidity. In an alternate embodiment of the invention, material is selectively removed from some of the cells  113  within the ribs  101  to conserve additional material. Guide springs  112  are also formed within the base  102 , and provide a mechanism for reducing vibration of the blank when inserted into an unequipped position. 
     Faceplate  108  is perpendicularly attached to the base  102 , at a front edge  107  of base  102 . As will be apparent to one skilled in the art, the faceplate  108  may be attached to the base  102  in a variety of ways. In the embodiment illustrated in FIG. 1, the faceplate  108  is integrally molded with the base  102 , and incorporates braces  114  for added rigidity and strength. 
     In the embodiment of FIG. 1, faceplate  108  also includes spring latches  103 , which provide means for securing blank  100  to the unoccupied position in the shelf. The latches  103  may be designed in a number of conventional ways. Faceplate  108  also includes one or more cable trays  110  that are used, for example, to retain optical cables ready for future interconnection with a circuit board that will replace blank  100  in the unequipped position. 
     As will be readily apparent to one skilled in the art, the components of blank  100  can be constructed from a variety of materials. For example, an injection molding process may be used to form the blank  100  from a variety of plastic resins. Acrylonitrile-Butadine-Styrene (ABS) is one such resin providing reasonable strength at a low cost. Alternatively, materials such as Polycarbonate (PC) may provide improved impact strength at higher cost. Various blends of ABS and PC may be used to obtain a good balance between strength, cost and ease of manufacture. An example of such a blended material is General Electric&#39;s C6200 engineering plastic. The entire blank  100  may be molded integrally from such a material. 
     FIG. 2A illustrates how blank  100  of FIG. 1 functions in an unequipped position in an apparatus shelf  200  to provide airflow backpressure through the position. In FIG. 2A, an upper edge  223  and a lower edge  221  of a base  202  locate a blank  202  in apparatus position guides  222 ,  220 , respectively. Apparatus position guides  222 ,  220  are affixed to an apparatus shelf frame  232  by one of a variety of conventional methods. For example, apparatus position guides  222 ,  220  may be produced as an integral stamping in the associated frame surface  234 . 
     A fan  224  promotes airflow in a fan plenum  225  interconnected to an apparatus position plenum  226 . The apparatus position plenum  226  is effectively enclosed by the blank  202  and a next apparatus position surface  230  along a distance defined by the lower edge  221  and the upper edge  223 . At or near the lower edge  221 , a diffuser blade  204  meets the oncoming airflow driven by a fan  224 . 
     As shown in FIG. 2B, the diffuser blade  204  reduces the nominal plenum height  229  to an effective plenum height  228 . Although this height is enlarged once the airflow passes diffuser blade  204 , a second diffuser blade  205  located at or near the upper edge presents an additional barrier that again reduces the effective height at the exit from the plenum. 
     In a preferred embodiment of the invention, diffuser blades  204 ,  205  are designed to be about 0.950 inches high in an plenum that is nominally 1.575 inches high. As a result, the effective plenum height  228  is about 0.625 inches. With this configuration, at an air velocity of 400 feet per minute (fpm), a pressure drop of 0.40 inches of water (in H2O ) from the lower edge  221  to the upper edge  223  has been observed. At equipped positions experiencing similar air velocities, pressure drops of 0.2 in H2O  to 0.5 in H2O  have been observed. 
     FIG. 3 further details the structure of an illustrative embodiment of the cable tray  110  of FIG.  1 . Cable tray  330  includes a retention surface  332  that is designed to be of sufficient area, for example, to hold a connector at the end of an optical cable. A retention slot  336  perpendicularly pierces the surface  332 , and is sufficiently wide to permit free passage of, for example, an optical cable having a cordage in the range of 1.6 to 3.0 millimeters. An entrance slot  334  of substantially similar width also perpendicularly pierces the surface  332  and the retention slot  336 . When the cable tray  330  is affixed to the faceplate  108  of FIG. 1 so that the longitudinal axis of the retention slot  336  is parallel to the faceplate  108  and perpendicular to the base  102 , the entrance slot  336  permits entry of the cable into the retention slot  336  from the front of the faceplate  108  via the entrance slot  334 . 
     Walls  338 ,  340 ,  342  and  344  are perpendicularly provided at the edges of retention surface  332  so that retention surface  332  will continue to retain a cable connector when it is positioned non-horizontally. Because the entrance slot  334  is of sufficient width to pass a cable and not of sufficient width to pass a connector, the connector end of a cable to be retained must be lifted over the wall  338  both for entry into and removal from the cable tray  330 . 
     FIG. 3B illustrates a cross-section of the cable tray  330  as attached to faceplate  308 . In this embodiment, the retention surface  332  is positioned at an inclination angle Θ  346  with respect to a horizontal plane perpendicularly piercing faceplate  308 . A cable connector  350  is retained by retention surface  332 , with further support, for example, from wall  342 . The positioning of retention surface  332  at inclination angle Θ  346  allows a cable  348  connected to cable connector  350  to be easily placed in retention slot  336  through faceplate  308 , for example, without substantially bending cable  348 . 
     FIGS. 4A and 4B illustrate the operation of springs  112  of FIG.  1 . In FIG. 4A, base  402  is inserted into apparatus position guide  420  in order to be placed in an unequipped position. By sliding base  402  through guide  420 , spring members  460  are compressed by guide walls  462 , and thereby exert a frictional force against the walls  462  that assists in restraining base  402  within guide  420 . As a result, vibrations in base  402  that would otherwise arise from the forced-air cooling and other forces are substantially reduced. 
     As shown in FIG. 4B, when base  402  is withdrawn from guide  420 , spring members  460  return to an uncompressed position. The distance  466  across the spring extremities in an uncompressed state shown in FIG. 4B is greater than the distance  464  across these extremities as shown in FIG. 4A when the base  402  is inserted in the guide  420 . 
     FIGS. 5A and 5B illustrate the operation of spring latches  103  of FIG. 1, used to retain the blank  100  in the unequipped position. In FIG. 5A, a spring latch  503  includes a spring member  514  attached to a fixed member  502  and an operating lever  504 . The fixed member  502  positively attaches the spring latch  503  to the base  102  of FIG.  1 . The spring member  504  can be compressed by moving the operating lever  504  towards the fixed member  502 , thereby lowering a variable surface  506  so that retention tab  508  may pass horizontally over the variable surface  506 . The retention tab  508  is positively attached to the equipment shelf near one of the apparatus position guides  222 ,  220  of FIG. 2A as an anchor for retaining the blank  100  of FIG. 1 in its unequipped position. 
     As an alternative to moving the operating lever  504 , the retention tab  508  of FIG. 5A can be brought into direct contact with the variable surface  506 . The height of the variable surface  506  increases gradually as it nears the latch stop  510 . When the retention tab  508  makes contact with the variable surface  506  and moves horizontally toward the latch stop  510 , the retention tab  508  exerts a force on the variable surface  506 . This force causes the operating lever  504  to move toward the fixed member  502 , so that the spring member  514  is compressed. As a result, the variable surface  506  becomes horizontally oriented, enabling the retention tab  508  to continue its horizontal movement until it reaches the latch stop  510 . 
     FIG. 5B shows the spring latch  503  and retention tab  508  in position to enable the blank  100  of FIG. 1 to be retained. As the retention tab  508  passes over the variable surface  506 , it aligns with a holding surface  512 . Since the holding surface  512  is below the adjacent portion of the variable surface  506 , the spring member  504  decompresses to bring the holding surface  512  into contact with the retention tab  508 . Once in contact with the holding surface  512 , the latch stop  510  and an intermediate surface  516  restrict the retention tab  508  from any further horizontal movement. 
     The exemplary embodiment of this method described above is but one of a number of alternative embodiments of the invention that will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Various other alternatives can be devised by a worker skilled in the art without departing from the teachings of this invention. For example, to provide the pressure-increasing function described above, the blank  100  of FIG. 1 could contain a single diffuser blade  104 . Alternatively the blank  100  could incorporate a raised surface extending across a portion or all of the base  102  with a cross-sectional shape and area substantially similar to the shape and area of diffuser blade  104 .