Abstract:
A system and method for constructing a resistance integrated coupler adapted to be coupled between a data bus and a terminal device. The system and method includes the steps of calculating a desired resistance of a set of data-bus windings, wherein the desired resistance is substantially (1.5×Zo), wherein Zo is a selected data bus cable nominal characteristic impedance; winding the set of data-bus windings to form a first part of the resistance integrated coupler using a specified amount of high resistance wire, the specified amount of high resistance wire having a total resistance substantially equal to the calculated desired resistance; and winding a set of terminal device windings to form a second part of the resistance integrated coupler. The resistance integrated coupler is constructed substantially in accordance with the guidelines of MIL-STD-1553B.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of data bus fault isolation couplers and methods therefor, and more particularly, is a coupler and method for a data bus fault isolation coupler featuring a coupling transformer with integrated resistance in a set of windings. 
     2. Description of the Related Art 
     Modern aircraft, ground and ocean platforms employ many on-board computer systems. These computer systems are concerned with command functions, platform control operations, and many other types of electronic system interfacing within or on the platform. In the military context, the installation and interconnection of the electronic systems with various computer systems is controlled by Military Specification requirements. 
     The Military Specifications for aircraft, spacecraft, ground vehicles and ocean vessels specify the linking of the various computer systems and electronic systems onboard with a Local Area Network type network built to military specifications. Typically, these interconnect systems onboard are called data bus networks. Within the United States military aircraft data bus networks are built to MIL-STD-1553B requirements, currently in revision B. Additionally, European military aircraft data bus networks are built to MIL-STD-1553B and STANAG 3838 requirements. The contents of MIL-STD-1553B and STANAG 3838 are incorporated herein by reference. 
     Both the United States and the European specifications specify that these data bus networks have electrical fault protection placed between the main data bus and each separate component, computer system, or terminal connected thereto. Specifically, each connection to the main data bus requires that a data bus coupler, constructed to the standards of MIL-STD-1553B and/or STANAG 3838 requirements be utilized between the main data bus and the component, computer system, or terminal connected to the main data bus. The MIL-STD-1553B and STANAG 3838 specifications require that directly coupled data bus couplers be built with internal fault isolation resistors to be placed in series with a step-up transformer to provide for protection to the main data bus in the event of a fault or short in a connected component or computer system. This interrelation of main data bus, data bus coupler, and component or computer system according to the prior art is shown in FIG.  1 . 
     Presently, all data bus couplers are constructed using two wire non-inductive wound resistors connected in series with a coupling transformer. This configuration of components is usually packaged on a printed wring board (PWB), or potted into a Mil Spec MIL-T-21038 transformer module. 
     However, many drawbacks to this system and method of constructing data bus couplers exist. The addition of the two separate wire wound resisters increases the cost of the data bus coupler because not only the transformer must be purchased, but also the two wire wound resistors must be purchased, and all these components must meet the requirements of the applicable Military Specifications. Additionally, all the components must be electrically coupled together internal to the data bus coupler. This internal coupling of components effects the Mean Time Between Failures (MTBF). MTBF is an indicator of expected system reliability calculated on a statistical basis from the known failure rates of various components of the system, is usually expressed in hours, and may address both systems and individual population items. MTBF in a system is also directly proportional to the number of components and the number of electrical couplings, or joints, between each component. As the addition of the resistors adds two components and two joints, the MTBF will therefore be affected. Finally, the space required for the combination of the transformer and the two wire wound resistors is much greater than merely for a transformer alone. 
     Therefore a need existed for a system and method of removing the two separate wire wound resistors from a data bus coupler in order to reduce the cost, reduce the size of the coupler, and increase the MTBF of the coupler. Additionally, a need existed for a system and method of incorporating the required fault protection resistance into the transformer windings in order to meet the protection requirements of MIL-STD1553B. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a system and method of constructing data bus couplers without the two separate wire wound resistors in the data bus coupler in order to reduce the cost, reduce the size of the coupler, and increase the MTBF of the coupler. 
     Another object of the present invention is to provide a system and method of constructing data bus couplers incorporating the required fault protection resistance into the transformer windings in order to meet the protection requirements of MIL-STD-1553B. 
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with one embodiment of the present invention, a resistance integrated coupler adapted to be coupled between a data bus and a terminal device is disclosed. The resistance integrated coupler comprises; a set of terminal device windings, and a set of data-bus windings comprised of high resistance wire. 
     In accordance with another embodiment of the present invention, a method for constructing a resistance integrated coupler for coupling between a data bus and a terminal device is disclosed comprising the steps of: calculating a desired resistance of a set of data-bus windings, wherein the desired resistance is substantially (1.5×Zo), wherein Zo is a selected data bus cable nominal characteristic impedance; winding the set of data-bus windings to form a first part of the resistance integrated coupler using a specified amount of high resistance wire, in which the specified amount of high resistance wire has a total resistance substantially equal to the calculated desired resistance; and winding a set of terminal device windings to form a second part of the resistance integrated coupler; and wherein the resistance integrated coupler is constructed substantially in accordance with the guidelines of MIL-STD-1553B. 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG.  1 . is a simplified schematic of the interrelation of prior art; data bus, data bus coupler, and component or computer system. 
     FIG.  2 . is a simplified schematic of the interrelation of; data bus, data bus coupler, and component or computer system showing the integrated resistive winding of the data bus coupler of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction 
     MIL-STD-1553is a military standard that defines the electrical, mechanical and timing specifications for a dual-redundant communication network, that interconnects cooperating digital units in a system. This communication network, also referred to as a data bus, is typically used in avionics systems, but is also used in submarines, tanks and missiles. MIL-STD-1553B is a 1 Mbit/sec bus. It is a highly reliable bus, both because of its extremely low error rate (one word fault per 10 million words), and because of its dual-redundant architecture. 
     MIL-STD-1553B is a data bus that interconnects up to 31 terminals (referred to as Remote Terminals, or RTs), controlled by a single controller unit (referred to as the Bus Controller, or BC). All of the RTs, and the BC, are connected using two separate busses. These are named the Primary Bus (or bus ‘A’) and the Secondary Bus (bus ‘B’). Messages are normally transferred only on the primary bus. If a message fails to be transferred over one bus, transfer may be retried on the other bus. An RT is usually a single box, that has some information to send out, and needs other information to be sent in. (A sensory unit, for example, would only need to send data out, while a fuel control unit would need to transfer data in and out.) An RT sends or receives data only when instructed to do so by the BC. The role of the bus controller is typically assigned to one of the more powerful units on the bus (e.g., the mission computer). 
     Referring to FIG. 1, a simplified schematic of the interrelation of prior art; data bus, data bus coupler, and component, computer system, or terminal is shown (“the prior art system  10 ” hereinafter). The main data bus  32  is terminated at each end with the cable characteristic impedance in order to minimize reflections caused by transmission line mismatch. With no connections or couplings to the main data bus  32 , the main data bus  32  looks like an infinite length transmission line and therefore there are no disturbing reflections. When the data bus couplers  20  are added for connection of the RTs  40 , the main data bus  32  is loaded locally and a mismatch occurs with resulting reflections. The degree of mismatch and signal distortions caused by reflections are a function of the impedance (Z) presented by the data bus coupler  20  and RT  40  input impedance. In order to minimize signal distortion, it is desirable that the data bus coupler  20  maintain a high impedance. This impedance is reflected back to the main data bus  32 . At the same time the impedance needs to be kept low so that adequate signal power will be delivered to the RT  40  input. Therefore, a trade-off and compromise between these conflicting requirements is necessary to achieve the specified signal-to-noise ratio and system error rate performance. MIL-STD-1553B specifies transformer coupling with stubs, and requires a data bus coupler  20  box, separate from the RT  40 , located near the junction of the main data bus  32  and stub  42 . The coupling transformer  24  used within the data bus coupler  20  is specified by MIL-STD-1553B §4.5.1.5.1.1, which states that a coupling transformer  24  shall be required, and shall have a turns ratio of 1:1.41 ±3.0 percent, with the higher turns on the main data bus  32  side of the stub  42 . The coupling transformer  24  characteristics defined in MIL-STD-1553B provide a compromise between the signal level and distortion characteristics delivered to the RTs  40 . The coupling transformer  24  turns ratio (1:1.41) provides impedance transformation for both RT  40  reception and transmission. The improvement of stub  42  load impedance is a result of impedance transformation, which is proportional to the square of the turns ratio, assuming an ideal coupling transformer  24 . 
     In addition to the coupling transformer  24 , MIL-STD-1553B §4.5.1.5.1.2 specifies that an isolation resistor  22   a  &amp;  b  shall be placed in series with each connection to the main data bus  32  cable to provide protection for the main data bus  32  in case of a short circuit in the stub  42  or RT  40 . Each resistor  22   a  &amp;  b  shall have a value of (0.75×Zo) plus or minus 2.0 percent, where Zo is the selected cable nominal characteristic impedance. The impedance placed across the main data bus  32  cable shall be no less than (1.5×Zo) ohms for any failure of the coupling transformer  24 , cable stub  42 , or RT  40  transmitter/receiver. 
     An additional requirement within MIL-STD-1553B §4.5.1.5.1.3, is that all coupling transformers  24  and isolation resistors  22 , as specified in §4.5.1.5.1.1 and §4.5.1.4.1.2, shall have continuous shielding  28  which will provide a minimum of 75 percent coverage of the data bus coupler  20  and that the shielding  28  shall couple to the bus shield  30 . The isolation resistors  22  and coupling transformer  24  shall be placed at the minimum possible distance from the junction of the stub  42  to the main data bus  32 . 
     Integrated Fault Isolation Data Bus Coupler 
     Referring to FIG. 2, the Integrated Fault Isolation (“IFI” hereinafter) data bus coupler, data bus, and component or computer system relationship of the present invention is shown, (the “IFI system  100 ” hereinafter.) The IFI data bus coupler  120  replaces the two resistors  24   a  &amp;  b  (see FIG. 1) that are series wired with the primary winding  24  (see FIG. 1) of the coupling transformer  24  (see FIG. 1) with an integrated fault isolation package comprised of a single primary winding  124   a.    
     The IFI package replaces the primary winding  24   a  (see FIG. 1) with a special gage of high resistance wire that has been drawn to meet specific length and resistance properties. This length of high resistance wire is equal to the sum of the resistance of the two isolation resistors  22   a  &amp;  b  (see FIG. 1) added to the resistance of the primary winding  24   a  (see FIG.  1 ). The length of high resistance wire is wound directly onto the inductor core  124   c  of the coupling transformer  124  and thus constitutes the new primary winding  124   a . The inductive properties of the inductor core  124   c  have been balanced to provide the required transformation ratio specified in MIL-STD-1553B. Thus, the required fault isolation resistance is integrated directly within the inductor core  124   c  of the coupling transformer  124 . This integration of the resistance, and therefore the removal of the two resistors  22   a  &amp;  b  (see FIG. 1) also results in a perfect balance between the two sides of the data bus coupler  120 , and also reduces reflections due to the absence of the two additional joints required by the resistors  22   a  &amp;  b  (see FIG.  1 ). 
     The functional advantage of the IFI data bus coupler  120  is that one component now performs the functions of three components. Less components mean fewer interconnects and thus this offers a more reliable main data bus  132  network. The IFI data bus coupler  120  design eliminates the two resistors  22   a  &amp;  b  (see FIG. 1) and the two additional solder joints, which contributed to a 650%, increase in the MTBF of each IFI data bus coupler  120 . Additionally, elimination of the resistors  24  (see FIG. 1) offers a smaller overall module volume—this translates to a smaller and lighter package. The IFI data bus coupler  120  module whose schematic is shown in FIG. 2 is 2 grams lighter than a T-21038 potted data bus module design, having the resistors  22   a  &amp;  b  of FIG. 1, currently marketed by Phoenix Logistics of Phoenix, Arizona. Lower manufacturing costs are realized by eliminating the two Mil Spec MIL-R-39007 wire-wound resistors  22   a  &amp;  b  (see FIG.  1 .). The new high resistance wire winding simply replaces the original inductor primary winding  24   a  (see FIG. 1) wire, without significantly affecting manufacturing cost of the coupling transformer  124 . 
     All other details regarding the coupling of the stubs  142 , the coupling of the shielding  128  to the bus shield  130 , and the coupling of the primary winding  24   a  wire to the main data bus  132  are essentially the same as the prior art system  10 , or as is according to methods and techniques well known in the art. 
     Although the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and detail may be made therein without departing from the spirit and scope of the invention.