Patent Application
20050243535
This invention relates generally to communications systems and, more specifically, to circuit packs and backplanes used in such systems.
Complex electronic apparatus may require the interconnection of thousands of individual electronic devices. To manage the large number of required interconnections in a communication system, for example, such apparatus is often configured by affixing devices to circuit boards (also referred interchangeably hereinafter as circuit packs or circuit cards), and by interconnecting these circuit boards using a printed wire backplane. Collections of interconnected circuit boards and backplanes are often contained in housings referred to as equipment shelves. The backplane of an equipment shelf includes connectors for electrically mating with the circuit boards to facilitate the transport of electronic signals. In one known arrangement, the backplane connectors consist of a plastic housing and metal-plated (e.g., gold-plated) pins, often referred to as contacts. The circuit boards include a connector tab on one edge with metallic conductive elements (e.g., gold-plated “fingers”) that mate electrically with the corresponding metal contacts of the backplane connector when inserted.
Equipment shelves are not always fully populated with circuit boards. For example, full capacity may not be used at all times or at least initially upon system installation. As such, some circuit board positions may remain unequipped, i.e., open, until more capacity is demanded and a new board is inserted. However, unequipped positions in an equipment shelf can be problematic. For example, circuit boards or portions of circuitry on circuit boards that are in positions adjacent to the unequipped position are vulnerable to environmental effects (e.g., temperature variations, dust particles, etc.) as well as inadvertent tampering or intrusion. Unequipped positions can also disrupt airflow in a convection-cooled shelf by diverting cooled air from operational circuit boards.
Unequipped positions in an equipment shelf can also degrade operational performance. For example, some communication systems include protection features to maintain communications in the event of equipment failures, e.g., when a faulty circuit board must be removed. More specifically, one circuit board slot in an equipment shelf may be designated as a “protection” slot for carrying the signal transmission in the event of the removal of a failed circuit board from one of the “service” slots. To facilitate a switchover in signal transmission from service to protection, a protection bus is included on the backplane for connecting the signals. For example, the protection bus carries a signal transmission to and from any one of the service slots to the protection slot. Normally, when a circuit board is removed from a service slot, the signal transmission carried by that failed circuit board is routed via the protection bus to the protection slot. Such routing is accomplished via wiring from shorting contacts in the backplane connector corresponding to the circuit board that is removed.
For example,
Continuing with the above protection switching example, the shorting contacts in a backplane connector provide the electrical connection to the protection bus when the shorting contacts are closed, i.e., when a circuit board has been removed. Stated otherwise, when a circuit board is inserted and the shorting contacts are open, e.g., during normal operation when signals are processed through the working circuit board, the service slot is isolated from the protection bus. When the failed circuit board is removed, the shorting contacts close and provide the electrical connection from the circuit board slot to the protection bus so that transmission can be routed to the protection slot.
While protection switching is a desirable feature for handling equipment failures, a problem arises when a circuit board slot is intentionally left unequipped, such as when full capacity of the equipment shelf is not used, and a switchover to the protection bus is both unnecessary and undesirable. Because the shorting contacts in the unequipped position remain closed, the wiring/cabling corresponding to the unequipped slot essentially becomes a stub that is connected to the wiring path of the protection bus that is carrying actual customer traffic. As with any unterminated transmission line, the signal will propagate down the stub in this unequipped slot, be reflected, and add to and/or subtract from the desired signal, thus interfering with the ability of the receiver to properly discern the signal.
Several circuit board “blanks” are known which have been used to fill unequipped positions in an equipment shelf. For example, circuit blanks have been developed for use in equipment shelves to equalize airflow backpressure. However, these circuit blanks are not suitably designed to engage the backplane connectors and therefore do not address the aforementioned problem associated with the operation of the shorting contacts in the backplane connectors. Another type of circuit blank shown in
However, these similarities also result in some significant disadvantages. Because of the density of the contact field (e.g., number of contacts) in the dual density edge connectors, thickness of the printed circuit boards, and the metal-to-metal mating of contacts, a considerable amount of insertion force is required for inserting blank 200 into a backplane. The amount of required insertion force, as well as the potential misalignments between the metallic contacts on the blank and those in the backplane connection during insertion, results in increased stress and failure of these circuit blanks and/or backplane connectors over the course of repeated insertions. Additionally, blank 200 is relatively expensive and traditionally has long lead times to manufacture. Furthermore, because the applied insertion force must follow the lever assembly as it rotates, usually one circuit blank is inserted at a time.
In an equipment shelf that utilizes backplane connectors having metallic contacts, a circuit card blank that separates the contacts, requires a minimal amount of insertion force, and substantially reduces wear and tear on the backplane connectors is realized according to the principles of the invention using an integrally molded design that includes a non-metallic connector tab adapted for insertion into a backplane connector such that the connector tab serves as an insulating layer thereby preventing electrical connection between the contacts. Because the circuit card blank is integrally molded without metallic contacts on the connector tab, fabrication cost, complexity and lead time are significantly less than with prior circuit blanks. Moreover, multiple circuit card blanks can be inserted into the equipment shelf simultaneously because of the reduced amount of required insertion force.
According to one illustrative embodiment of the invention, a circuit card blank includes a base for positioning the blank in an unequipped position of an equipment shelf and a non-metallic connector tab integrally molded with and located at an edge of the base. The connector tab is adapted for insertion into a backplane connector having metallic contacts, including shorting contacts, such that the connector tab serves as an insulating layer thereby preventing electrical connection between the shorting contacts. A snap-fit clip can also be integrally molded with the base for retaining the circuit card blank in the equipment shelf and preventing self-extraction.
A more complete understanding of the principles of the invention may be obtained from consideration of the following detailed description in conjunction with the drawing, with like elements referenced with like references, in which:
It should be noted that the terms “circuit board”, “circuit card”, and “circuit pack” may be used interchangeably throughout the description and are not being used in any limiting manner with regard to the description of the various embodiments. In particular, the present invention would be applicable to various implementations, whether on a board, pack or card type of apparatus, which are used in conjunction with some type of equipment housing such as an equipment shelf, bay, compartment, and so on.
It should also be noted that while the embodiments described herein are particularly well-suited for equipment shelves that utilize backplane connectors of the type having shorting contacts for protection switching functionality, it will be apparent to those skilled in the art that the circuit blank according to the principles of the invention can also be used with an equipment shelf or housing that has contacts that may operate similarly (e.g., open and closed positions), but which provide different functionality other than protection switching. Moreover, a circuit blank according to the principles of the invention can also be used in equipment shelves having different types of backplane connectors including, for example, dual density edge connectors, single density edge connectors, connectors with or without shorting contacts, and so on. As such, the types of equipment shelves and backplane connectors that are described herein for use with the various embodiments of the present invention are meant to be illustrative only and not limiting in any manner.
Blank 400 also includes handle mechanism 406, as shown in
As shown in
As will be apparent to one skilled in the art, blank 400 may be effectively constructed in a variety of ways, provided that blank 400 can be inserted in an equipment shelf in a manner equivalent to the insertion of operational circuit boards in equipped positions. For example, base 401 may simply be formed as a uniform member with rectangular dimensions equivalent to those of the circuit boards. As depicted in
It will also be apparent to those skilled in the art that circuit blank 400 can be constructed from a variety of materials. For example, an injection molding process may be used to form circuit blank 400 from a variety of commercially available plastic resins. Acrylonitrile-Butadine-Styrene (ABS) is one example of an amorphous polymer resin that provides 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 also be used to obtain a desired balance between strength, cost and ease of manufacture. Polyetherimide is yet another material that is suitable for circuit blank 400. Other currently known and commercially available materials, as well as materials yet-to-be developed, which are suitable for use in a circuit blank according to the principles of the invention, will be readily apparent to those skilled in the art in view of the teachings herein as well as routine design considerations. As such, the teachings herein are only meant to be illustrative and not limiting in any manner.
Other design considerations will also be apparent to those skilled in the art when selecting an appropriate material for constructing the molded circuit blank according to the principles of the invention. For example, existing or to-be-developed safety and performance standards may dictate certain compliance requirements, such as flame retardation. Presently, some applicable standards might include Underwriters Laboratories Standard 94 V-O and Telcordia GR-63-CORE by way of example.
In contrast to prior arrangements that use a printed circuit board (PCB) construction, such as a rough-machined fiberglass (FR-4) type of surface with gold-plated fingers, the molded plastic construction of circuit blank 400 results in less wear and tear on backplane connectors because of the elimination of the metallic contacts on the circuit blank. In particular, there is no metal-to-metal mating of contacts between the blank and the backplane connector as with the prior art circuit blanks. Another common problem that contributes to wear and tear in the prior arrangements is damage to the metallic contacts (both on the prior art circuit blanks and to the backplane connector) caused by misalignments during insertion of the circuit blank. That is, the metallic gold-plated fingers of the prior art circuit blanks are supposed to mate with corresponding metal contacts within the backplane connector when the board is inserted. However, misalignment can sometimes occurs when the prior art circuit blank is inserted into a backplane connector. These misalignments can remove some of the metallic coating and therefore damage both backplane connectors and circuit cards, especially with repeated insertions and removals of circuit cards. By contrast, the molded plastic construction of the circuit blank according to the principles of the invention eliminates this problem altogether. Additionally, the electrical connection resulting from the mating of metallic contacts on prior art circuit blanks with the metallic contacts of a backplane connector can result in other problems including, for example, arcing between the ground contact and power contact on a circuit blank and/or backplane connector. The molded plastic construction of the circuit blank according to the principles of the invention eliminates this problem as well.
According to another aspect of the invention, circuit blank 400 requires less insertion force than conventional circuit blanks and also substantially reduces wear and tear on both the circuit blank itself as well as the backplane connectors in an equipment shelf. In the illustrative embodiment shown in
The variable thickness of circuit blank 400, as shown in
Because of the reduced amount of insertion force needed to insert circuit blank 400 into an equipment shelf as compared with prior arrangements, the mechanism for inserting circuit blank 400 into an equipment shelf is also greatly simplified over the prior arrangements and in a manner that reduces the stress on the circuit blank itself.
As previously described, the prior arrangements that use lever assemblies, such as that shown in
By contrast, circuit pack blank 400 can be inserted into an equipment shelf (such as shelf 500 in
Additionally, because of the reduced insertion force as well as the construction of handle mechanism 406, a technician can simultaneously install multiple circuit blanks in an equipment shelf much more easily and rapidly in contrast to the prior arrangements that use lever assemblies in which usually a single board is inserted at a time because of the nature of the lever operation and the amount of required insertion force.
According to another aspect of the invention, circuit pack blank 400 engages and is secured in an equipment shelf using snap-fit clip 404 with locking tab 405, which can also be integrally molded with faceplate 403 and/or base 401 as shown in
The integrally molded snap-fit clip 404 is also less costly and easier to manufacture than prior latch/lever assemblies and, because there are less moving parts and no need for a substantial rotational moment couple, mechanical stresses on the circuit pack blank and equipment shelf are substantially reduced as compared to the aforementioned arrangements that use conventional latch/lever assemblies. In another illustrative embodiment, snap-fit clip 404 can also be a separate part that is fastened or otherwise attached to base 401 and/or faceplate 403, e.g., by rivets, screws, heat posted, and so on. Other means for fabricating and attaching snap-fit clip 404 to circuit pack blank 400 will be apparent to those skilled in the art.
According to another aspect of the invention, circuit pack blank 400 also prevents inadvertent access or intrusion into the otherwise unequipped slot and distributes airflow more uniformly within the equipment shelf. Moreover, the plastic construction results in improved electro-static discharge (ESD) performance as compared to prior circuit blanks.
The foregoing is merely illustrative of a few exemplary embodiments according to the principles of the invention. Those skilled in the art will be able to devise numerous arrangements, which, although not explicitly shown or described herein, nevertheless embody those principles that are within the spirit and scope of the invention. Such modifications are therefore contemplated by the teachings herein. Accordingly, the scope of the invention is only limited by the claims appended hereto.
