Abstract:
A computer module/module chassis system is provided with a keying system having a binomial design having 2 X  keying arrangements where X is the number of bits or keying elements in the keying system. The keying system is especially useful in preventing misinstallation of a module in a slot having an incompatible electrical connection system. The usefulness of this keying system is enhanced by the fact that it can be installed late in the manufacturing process or at an installation site.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to keying devices for module systems. More particularly, it relates to keying devices for computer peripheral modules having the same, or confusingly similar, physical sizes. Such keying devices are used to prevent module installation mistakes.  
           [0003]    2. Description of Related Art  
           [0004]    Many computer systems include a cabinet or chassis which houses a number of modules having similar, if not identical, physical dimensions. Thus, the cabinet will have a number of similar or identical module-receiving slots. The modules might include mass storage devices, central processing units, controllers for input-output devices and power supplies. These modules sometimes need to be repaired, updated or augmented with additional modules. Repair usually involves removing a defective module and replacing it with a correctly functioning module of the same type. Some computer systems even go so far as to incorporate redundant modules so that a given module can be removed and repaired while the remainder of the system remains in operation.  
           [0005]    Obviously, when a repaired module is replaced (or when an additional module is placed in an open slot in a module chassis), it is essential that the subject module be of the correct type. If a module of one type is inserted into a site designed for a module of a different type, serious damage may occur to the module and/or to the entire computer system. Valuable data can be lost as well.  
           [0006]    Obvious differences in the shape of many modules precludes their insertion into an open slot in a chassis that is not designed to receive that particular module. For example, a hard disk drive unit and a power supply unit normally are of very different sizes and shapes. Moreover, they have electrical connectors that are mechanically incompatible. Therefore, it is virtually impossible to insert either of these two module types into a slot designed for the other—and then electrically connect such a misinstalled unit. Other types of modules are, however, purposely designed in a standard physical form so that they can be inserted into physically identical slots in a common housing. Consequently, owing to the fact that electrical connections to modules of different types usually differ, electrical component damage may result from the fact that an appropriately sized module is mistakenly installed in a slot whose electrical connections are not designed to accept that module. And even if there is no damage to the hardware, loss of its functionality may result from misinstallation of a given module.  
           [0007]    The prior art has recognized that misinstallations of this kind can be precluded by placement of a first mechanical keying device at the face of a given module-receiving slot in a housing, and placing a second mechanically cooperating keying device at the rear end of a module that may otherwise physically fit into the module-receiving slot. For example, U.S. Pat. No. 5,733,149 (“the &#39;149 patent”) teaches a computer peripheral module having keying elements that are molded into a front corner of the module. The module can be inserted into a receiving slot in the chassis only if keying holes molded into the front of the module mechanically cooperate with keying pins in the intended receiving slot in the chassis. Thus these cooperating keying elements ensure that the module can only be inserted into the correct receiving slot, thereby protecting the mechanical and electrical functioning of that module.  
           [0008]    Such molding of the key elements into the module housing requires complicated and costly molding procedures, including mechanical core pulls of an injection mold system. Moreover, such molding operations represent a commitment to a given key system at a very early stage of the manufacturing process i.e., when the module housing unit is first molded into its intended shape. Such “molded in” keying systems work well enough in those parts of a computer system where a given slot is intended to accept one and only one type of module. They do not, however, work very well in module systems where the slots are designed to accommodate any one of several different types of modules having the same physical dimensions. For example, in some computer systems each input/output controller is comprised of two identically sized modules. The first module normally contains most of the logic board circuitry of the controller. The second module usually contains specialized interface circuitry that is electrically connected to other components of the computer device, or to a communication line. If the first or second module is plugged into the wrong slot, damage may result from electrical arcing or power supply voltage drops.  
           [0009]    Consequently, methods for preventing power delivery to an incompatible module/module-receiver slot also have been proposed. For example, U.S. Pat. No. 4,800,462 (“the &#39;462 patent”) discloses an electromechanical interconnect system for interconnecting a set of pin-compatible module pairs in a manner such that power delivery to electrically incompatible modules is precluded. More specifically, the module includes a control switch having two control terminals. Each terminal is connected to a keying connector. Keying connectors on various modules are coupled so that the control terminals are only conductively connected when electrically compatible modules are interconnected. It should be noted, however, that electrical keying systems of this kind perform their electrical power damage prevention function only after the module has been fully installed in the receiver slot.  
           [0010]    From a manufacturing standpoint, it is extremely advantageous in module-oriented products, such as disk arrays or controller systems, to use as many components for as many different applications as possible. This allows better use of design and manufacturing resources to create as many products as possible using as many commonly used parts as possible. For example, a module chassis may be provided with a number of slots to receive similarly shaped disk arrays, controller systems, power supplies, fans, etc. The extended use of such common hardware components requires a substantial amount of physical commonality between the chassis enclosures and the modules they house. As a general rule, a manufacturer usually wants to preserve a given item&#39;s individuality as long in the manufacturing process as possible. Stated another way, a manufacturer wishes to distinguish a given item as late in the manufacturing process as possible. On the other hand, in order to create different products, one or more modules of a computer system will be made different from their predecessor modules in order to provide some different functionality. Thus, an inherent tension exists between the economic desire to use as many common parts as possible (for as long in the manufacturing process as possible) and the desire to improve module-oriented products.  
           [0011]    The prior art keying systems noted above have not fully solved the above noted manufacturing economy versus end product versatility dilemma. For example, keying based upon molding a keying system into the module (a la the teachings of the &#39;149 patent) must be carried out very early in the manufacturing process. Consequently, permanent commitment to a given key design is made at this early stage. This commitment will to some degree limit or even prevent future change in the module system. It also makes keying changes impossible in the field. Keying done at the electrical connection level (a la the teachings of the &#39;462 patent) also is done relatively early in the manufacturing process. It also requires substantial or complete installation of the module before its “incorrectness” is determined. At the very least, valuable time is lost in misinstalling an electrically incompatible module.  
           [0012]    Thus, there is a continuing need to prevent similarly sized, but electrically incompatible, modules from being inserted into “wrong” module-receiving slots. Preferably, such a misinstallation prevention device will be entirely mechanical in nature and function at the very first attempt to misinstall a module. Such a device also should be capable of being easily installed late in the manufacturing process (e.g., long after the module molding and chassis construction processes have taken place). Indeed, in its most preferred embodiments, such a device would be an accessory item capable of being installed in the field, i.e., at a computer installation site. This ability to install such a device at an installation site greatly widens the possibilities for future product modifications. Keying expandability, low cost and portability also are desirable attributes in such devices. The keying devices of this patent disclosure provide all of these desirable attributes.  
         SUMMARY OF THE INVENTION  
         [0013]    Generally speaking, use of applicant&#39;s invention will produce a computer module system comprising: (1) a chassis having two or more substantially identically-configured slots and wherein at least one of said two or more slots is provided with a first binomial keying element; and (2) at least two modules each having a substantially identical configuration and capable of being inserted into the two or more slots in the chassis and wherein at least one of said two modules is provided with a second binomial keying element that mechanically cooperates with the first binomial keying element to prevent misinstallation of a module. The mechanically cooperating first and second binomial keying elements thus, together, constitute applicant&#39;s binomial keying system.  
           [0014]    The binomial keying systems of this patent disclosure mechanically cooperate (or fail to cooperate) near the rear end of the module and near the front end of the slots. Preferably these binomial keying systems employ “snap on” or otherwise easily installed, and easily removed, first and second binomial keying elements. That is to say that these first and second binomial keying elements are preferably in the nature of accessory items, wherein the term “accessory” can be taken to mean that the first and second binomial keying elements (and hence the keying systems) of this patent disclosure can be installed at an end point installation site using easily transportable tools (e.g., screwdrivers, pliers, wrenches, drills and the like). Obviously, if these keying devices can be installed in the field, they can be installed during the module/chassis manufacturing processes as well.  
           [0015]    The individual first and second binomial keying elements of the binomial keying systems of this patent disclosure can be positioned on the top, bottom or side of the module they are to protect. Preferably, applicant&#39;s binomial keying systems will have a three bit or four bit design. In some of the more preferred embodiments of this invention, the binomial keying system will be comprised of 3 bits having lug elements and lug passage keyway elements that are rectangular (or square) in configuration. The lug passage keyway elements allow an appropriate lug to pass through an appropriate lug passage keyway element. Such keys can be installed very late in the manufacturing process. Indeed, they even can be changed in the field should replacement modules be required by reason of upgrading and/or so-called rolling manufacturing changes. Again, the binomial keying systems of this patent disclosure can be thought of as “accessory items”. Thus, their use anywhere in the manufacturing process (or installation process) does not foreclose future changes in a modular product. Again, there are great practical and economic advantages associated with keying module/chassis systems “late” in the manufacturing process or at the installation site. Moreover, the binomial keying devices of this patent disclosure readily permit keying expandability owing to their use of certain readily expandable binomial keying concepts hereinafter more fully described. In these descriptions, computer module/chassis systems will be used to illustrate this invention. It should be appreciated however that the binomial keying systems of this patent disclosure also could be used in other chassis-housed module systems (including spare parts drawers) where there is a need to keep a module in a “correct” slot. Preferably, the keying elements can be installed and/or set up using simply hand tools or hand operated key element set up mechanisms.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of a representative prior art module array in a module chassis.  
         [0017]    [0017]FIG. 2 is a front view (or rear view) of a module/module chassis system provided with a keying system designed according to the general teachings of this patent disclosure.  
         [0018]    [0018]FIG. 3 is a perspective view of a module provided with a keying system based upon the teachings of the present patent disclosure, about to be inserted into a keyed slot in a module chassis.  
         [0019]    [0019]FIG. 4 is a perspective view of a module, provided with a keying system on its right side, about to be inserted into a cooperating keyed slot in a module chassis.  
         [0020]    [0020]FIG. 5 is a detailed side view of a module provided with a donor key component and a chassis receiver slot that is shown provided with a donor key passing component.  
         [0021]    [0021]FIG. 6 is a side view of the module/slot system shown in FIG. 5 wherein the module is partially inserted into the slot.  
         [0022]    [0022]FIG. 7 is a side view of the module/slot system shown in FIG. 5 wherein the module is fully inserted into the slot.  
         [0023]    [0023]FIG. 8 is a side view of a module provided with a donor key passing component and a chassis receiving slot that is provided with a donor key component.  
         [0024]    [0024]FIG. 9 is a side view of the module/slot system shown in FIG. 8 wherein the module is partially inserted into the slot.  
         [0025]    [0025]FIG. 10A depicts a keying system having a 3 bit design and a binary value of zero.  
         [0026]    [0026]FIG. 10B depicts a keying system having a 3 bit design and a binary value of 1.  
         [0027]    [0027]FIG. 10C depicts a keying system having a 3 bit design and a binary value of  2 .  
         [0028]    [0028]FIG. 10D depicts a keying system having a 3 bit design and a binary value of  7 .  
         [0029]    [0029]FIG. 11 depicts an instance where the keying system has assured that the module and the chassis slot are properly associated.  
         [0030]    [0030]FIG. 12 depicts an instance where the keying system has prevented an improper module/chassis slot association.  
         [0031]    [0031]FIG. 13 shows a lug passage element attachably mounted on top of a module chassis.  
         [0032]    [0032]FIG. 14 shows a binomial keying device of this patent disclosure having four rectangular keying elements.  
         [0033]    [0033]FIG. 15 shows a binomial keying device of this patent disclosure having five round keying elements.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    [0034]FIG. 1 shows a prior art module/chassis system  10 . Such a system is generally comprised of an array of modules  12 ,  14 ,  16  and  18  in a chassis or housing  20 . These modules may be installed from the front end of the chassis  20 , or from its rear end. Thus, FIG. 1 could just as well be a rear view of the module/chassis system  10 . For the sake of simplicity, the chassis  20  of FIG. 1 is shown provided with four slots ( 20 A,  20 B,  20 C and  20 D) that respectively receive modules  12 ,  14 ,  16  and  18 . Those skilled in this art will however appreciate that such a chassis could have many more such slots. Moreover, several such chassis could be placed side by side to produce systems having very large numbers of module-receiver slots of several different sizes.  
         [0035]    [0035]FIG. 1 also illustrates the circumstances that create the need for the present invention. Module  12  (residing in slot  20 A) has the same cross sectional configuration and size as module  14  (residing in slot  20 B). Hence, module  12  could be mistakenly placed in slot  20 B. Module  14  could likewise be mistakenly installed in slot  20 A. The same possibility for misinstallation exists with respect to modules  16  and  18 . That is to say that module  16  is larger than modules  12  and  14  and therefore will not fit into slots  20 A or  20 B, but module  16  could be misinstalled in slot  20 C since it is of the same size as module  18 . Hence, both sets of modules ({fraction (12/14)} and {fraction (16/18)}) could benefit from a keying system that prevents misinstallations. Again, those skilled in this art also will appreciate that many computer module chassis units have many more module-receiver slots than the four ( 20 A,  20 B,  20 C and  20 D) shown in FIG. 1. Hence, as the number of such slots increases, so does the potential for misinstallation of any given module unit having a physical size that is substantially the same as other modules in the subject module system.  
         [0036]    [0036]FIG. 2 shows a generalized front view (or rear view) of a module  22  inserted in a slot  24  of a chassis  26  (such as the chassis  26  depicted in FIG. 3). FIG. 2 also shows a generalized binomial keying system  28  located in the upper left corner of the slot  24 . The generalized binomial keying system  28  is intended to depict a binary system having three bits or keying elements  30 ,  32  and  34 . Because this system is binary in nature, each bit or keying element can be thought of as being in one of two states (yes/no, go/no-go, positive/negative, male/female, etc.). Thus, the number of possible keyed configurations is 2 x  where X is the number of bits or keying elements. This number could be quite large; but as a practical matter keying systems wherein X is from 2 to 5 are highly preferred for the practice of this invention. In the system shown in FIG. 2 for example, the number of keying possibilities is 2 3  or 8. If the number of bits or keying elements were 4 (as depicted in FIG. 14), the number of keying possibilities would be 2 4  or 16, and so on.  
         [0037]    [0037]FIG. 3 is a perspective view of a chassis  26  designed to house four modules ( 22 ,  36 ,  38  and  40 ). Module  22  can be regarded as being identical to the module  22  depicted in FIG. 2. These four modules are shown provided with respective binomial keying systems  28 ,  28 (A) . . .  28 (C) that are each comparable to the binomial keying system  28  shown in FIG. 2. In FIG. 3, module  22  is shown about to be inserted into slot  24  of chassis  26 . Such a module can have any cross sectional (e.g., rectangular, square, round, parallelepiped, etc.), but rectangular modules are by far the most common and therefore will be used as an example throughout this patent disclosure. In any case, the first binomial keying element  28 ( 1 ) associated with a module, such as module  22 , is preferably mounted near the rear end  42  of a given module such as module  22 . In all cases, the keying element  28 ( 1 ) should be positioned such that it (in conjunction with a second binomial keying element) prohibits engagement of an electrical connector element of a module with an inappropriate electrical connector element of a chassis. The second binomial keying element  28 ( 2 ) associated with a slot such as slot  24  is likewise preferably mounted at (or near, e.g., within  3  inches of) the front face  44  of the chassis  26 . Thus, if the key bit elements  30 (B),  32 (B) and  34 (B) mounted on the front  44  of the chassis  26  are not all properly cooperating with their counterpart key bit elements  30 (A),  32 (A) and  34 (A) on the rear  42  of the module  22 , the module  22  will be “blocked”, rejected, etc. as soon as one tries to install (i.e., misinstall) said module  22  in slot  24 . As can be better seen in FIG. 5, the module  22  also can be provided with a lock element (not shown), e.g., in its lower left rear corner region (or its lower right rear corner region). Such a lock element could cooperate, in known ways, with a second lock element (not shown) on the lower, left, front corner (or lower right front corner) of slot  24  to securely hold the module  22  in the slot  24  once said module  22  is fully installed (see also FIG. 5).  
         [0038]    [0038]FIG. 4 is another perspective view of a chassis  26  designed to house four modules ( 22 ,  36 ,  38  and  40 ). In this system however, the keying systems  28 ,  28 (A),  28 (B),  28 (C) are positioned near the center of the right side of each of the four respective module-receiving slots. Thus, module  22  is shown having the three key bit elements  30 (A),  32 (A),  34 (A) of its first binomial keying element  28 ( 1 ) located near the center of the front end  42  of its right side  46 . The location of these keying elements is not, however, in any sense crucial. They can be on the top, bottom or sides of the module  22 .  
         [0039]    [0039]FIG. 5 is a side view of a portion of the chassis/module system shown in FIG. 3. It depicts a module  22  about to be installed in a receiving slot  24  of a chassis  26 . To aid in such operations the slot  24  is shown provided with rollers  27 (A),  27 (B) and  27 (C) that are inserted in a channel  29  on the side of module  22 . Such roller/channel arrangements are well known in the chassis/drawer making arts. The keying system  28  depicted in FIG. 5 is shown positioned to prevent insertion of the module  22  into slot  24  if its lug element  30 (A) does not pass through the lug passage keyway  30 (B) in the appropriate key bit element in the face  44  of the slot  24 . Thus, FIG. 5 shows an embodiment of the present invention wherein key bit element  30 (B) of the binomial keying system  28  is an open passage keyway  58  that receives and passes a lug  50  that constitutes key bit element  30 (A).  
         [0040]    [0040]FIG. 5 also shows the module  22  provided with a pin system  52  for making an appropriate electrical connection between the module  22  and an electrical connector located in the rear end of the slot  24 . The individual pins in the array of pins in pin system  52  are intended to be received into a cooperating pin holes  53  having the proper pin hole sizes and locations relative to the individual pins in the pin system  52 . In other words, the array of pins in pin system  52  are intended to fit into a mechanically cooperating array of pin-receiving holes  53  in a pin receiving block  54  positioned in the rear of slot  24 . Assuming proper mechanical cooperation between the pins and pin holes, a proper electrical connection can be made between the module  22  and the pin-receiver block  54 .  
         [0041]    Those skilled in the computer peripheral manufacturing arts will of course appreciate that many Personal Computer Memory Card International Association (“PCMCIA”) form factors have become de facto standards for certain computer peripherals such as memory, disk drives and modems. Even components that are designed to be permanently built into a host device typically abide by these form factors. For example, one often employed PCMCIA standard defines a 68-pin interface between a pin array and a socket or slot into which it is inserted. Included in that specification is a requirement for connector end keying, which prevents mismatched, connecting elements from being mated. Consequently, this invention is also concerned with assuring that modules of the same physical size also conform to a predetermined PCMCIA standard that calls for use of a keying system. Be these PCMCIA standards as they may, FIG. 5 suggests that the mating between the individual pins in the pin system  52  and the individual pin-receiving holes  53  in the pin receiver block  54  of FIG. 5 will only take place if the lug  50  that comprises bit element  30 (A) passes completely through the lug passage keyway  48  that comprises key bit element  30 (B). This passage is suggested by arrow  56 . This passage assumes that the other two bit element components ( 32 (B)/ 32 (A) and  34 (B)/ 34 (A)) also have mechanically cooperated in the manner prescribed by a previous key setup operation.  
         [0042]    [0042]FIG. 6 shows the results of all three key bit elements ( 30 (B),  32 (B),  34 (B)) on the chassis  26  successfully cooperating with all three of their counterpart key bit elements  30 (A),  32 (A) and  34 (A) on the module  22 . Thus, FIG. 6 also suggests that mating between the individual pins in the pin system  52 , and the individual pin receiver holes  53  in the pin receiver block  54 , will only take place if all three of the keying elements have mechanically cooperated in a previously prescribed (“keyed”) manner to allow full insertion of the module  22  into the slot  24 .  
         [0043]    [0043]FIG. 7 shows the final results of the keying system  28  having successfully mechanically cooperated to allow the module  22  to be fully inserted into the module receiver slot  24  in chassis  26 . The pin system  52  shown in FIGS. 5 and 6 now resides in the appropriate pin holes  53  of pin receiver block  54 . Thus, the front end  62  of the receiver block  54  is shown abutting against the front end  63  of the module  22 . Any one of several locking devices  23 (A)/ 23 (B) well known to this art can be used to lock the module  22  in the slot  24 .  
         [0044]    [0044]FIG. 8 is a cross sectional view showing key bit element  32 (B) of chassis  26  in the form of a lug  64 . This lug  64  is intended to pass through a lug keyway passage  66  in key bit element  32 (A) of a first binomial keying element  28 ( 1 ) mounted on module  22 . This passage is also suggested by arrow  68 . Thus, FIG. 8 illustrates the concept that a lug e.g., lug  64  can be a keying element for any (or all) of the key bit elements  30 (B),  32 (B) or  34 (B) of a second binomial keying element  28 ( 2 ) placed in the slot  24 . Likewise, any or all of the key bit elements  30 (A),  32 (A) or  34 (A) could be a passage (e.g., passage  66 ) for a lug such as lug  64 . Thus, module installation “blockage” will be produced in situations where both key bit elements of a given bit (e.g.,  32 (A) and  32 (B)) are both lugs.  
         [0045]    [0045]FIG. 9 is a cross sectional view showing successful passage of module  22  into the module-receiving slot  24  of chassis  26 . Here again, this will only take place if the lug  64  that constitutes bit element  32 (B) passes through the lug passage keyway  66  that constitutes key bit element  32 (A). This outcome also assumes that the other two key bit elements shown in FIG. 3 (i.e.,  30 (B)/ 30 (A) and  34 (B)/ 34 (A)) also have mechanically cooperated in a manner that allows full insertion of the module  22  into slot  24 . Had key bit element  32 (A) also been a lug, passage of the module  22  into the slot  24  would have been prevented.  
         [0046]    [0046]FIGS. 10A to  10 D are intended to illustrate various generalized, representative, rule applications for the binary keys of a representative keying system  28  of this patent disclosure. In effect, it is the application of such rules that enable applicant&#39;s keying systems to perform their intended function (prevention of misinstallation of a module). For example, in the keying system  28  shown in FIGS. 10A to  10 D, each bit location (Bit  0 , Bit  1 , Bit  2 ) must have either a bit element located on the chassis  26  or a bit element located on the module  22 . If a bit location is not occupied on either the module or chassis, misinstallation of an alternate module may be possible—or that bit must be dropped from the calculation of the maximum number of keying possibilities. Obviously, the two sets of key bit elements  30 (A),  32 (A), and  34 (A) and  30 (B),  32 (B),  34 (B) must be physically aligned in the system so that no bit element affects an adjacent bit element.  
         [0047]    [0047]FIG. 11 illustrates a circumstance wherein a keying system  28  allows the correct module  22  to be installed in an appropriate keyed slot  24  of a chassis  26 . By way of example only, FIG. 11 suggests that the slot  24  is intended to accept a module  22  with a key bit element for bit  0  installed. The module will therefore be keyed with bit  0  installed. Thus there will be no interference at this bit location. Assuming the other two bits of the three bit system cooperate in the manner in which they are keyed, the module  22  can be installed in its intended slot  24 .  
         [0048]    [0048]FIG. 12 illustrates a circumstance wherein a keying system will prevent an incorrect module  22  from being installed in a given keyed slot  24 . For example, if Bit  0  of the module key is a lug, that lug is intended to pass through a passage in the slot key at the Bit  0  location. If the slot key at Bit  0  is also a lug there is an interference on bit  0  that will prevent the module from being installed. In other words, these two conflicting key bit elements (e.g., lugs) will collide and prevent misinstallation of the module  22 .  
         [0049]    [0049]FIG. 13 shows a representative keying system  28  of this patent disclosure. A second binomial keying system element  28 ( 2 ) includes a key bit element  34 (B) mounted to the chassis  26 . A cooperating key bit element  34 (A) of a first binomial keying system element  28 ( 1 ) is mounted to a module  22 . Key bit element  34 (B) is shown having a passage keyway element  70  through which a lug  72 , mounted on module  22 , can pass. The second binomial keying element  28 ( 2 ) is shown mounted by bolts to the underside  26 (U) of a chassis component. The second binomial keying element  28 ( 2 ) containing key bit element  34 (B) could be mounted through use of glue, magnetism or various lock or fastener means. In much the same way, the top surface of module  22  is shown provided with a first binomial keying element having a key bit element  34 (A) that is secured to the top surface of the module  22  by means of two bolts.  
         [0050]    [0050]FIG. 14 shows a binomial keying system  28 ′ having four bits (bits  0 ,  1 ,  2  and  3 ). Such a system is therefore capable of providing 2 4 = 16 keying arrangements. It should also be noted that an initial three bit keying system having BIT  0 ,. BIT  1  and BIT  2  could be expanded by installing a fourth bit, Bit  3 , by attaching it to the body of a binomial keying element containing the other three bits. Thus, the keying possibilities of the original three bit system can be expanded by adding a fourth bit (BIT  3 ), fifth bit, etc. to the binomial keying system.  
         [0051]    [0051]FIG. 15 depicts a keying system  28 ″ wherein there are 5 bits ( 0 ,  1 ,  2 ,  3  and  4 ) having round, keywayed configurations. Since there are 5 such bits, this keying system  28 ″ is capable of providing 2 5  or 3 2  different keying possibilities. Thus, FIG. 17 suggests that the rectangular lug/lug passage examples used previously can be replaced by keying elements having other geometries (e.g., round, triangular, etc.).  
         [0052]    This invention has been described with reference to certain preferred embodiments. Substitutions and modifications will be apparent to those skilled in the art. In particular, the number of bits is not material to this invention. Similarly, lugs and lug passing passages have been used to illustrate the concepts of this invention, but any other types of mechanically cooperating elements could be utilized as well. Accordingly, it is intended that the invention not be limited except as provided by the appended claims.