The interconnected backbone of a MIL-STD-1553B vehicle depends on the electrical and mechanical reliability of the components, the design, and the installation. With the vast array of MIL-STD-1553B specified sub-assemblies, there are factors that can go overlooked for passive components such as couplers, cables, and connectors. Often built with kilometers of wiring, aircraft in particular can be difficult to troubleshoot and must be inspected frequently for potential failures such as intermittent connections, shorts, and corrosion. Still, these passive components that make up the network can be individually assessed to operate with a high level of integrity.

Bus Couplers Considerations

MIL-STD-1553B specifies that data bus couplers must be placed between the main data bus and the vehicle subsystems, computer system, or terminal in order to protect the integrity of the entire network. The data bus coupler is often called a ‘stub coupler’ where a ‘stub’ is simply a pair of wires connecting avionics components to the main bus. MIL-STD-1553B and STANAG 3838 further specifies that each stub coupler come equipped with fault isolation resistors and a step-up transformer (1:1.41) to avoid shorts, improve common mode rejection, and provide lightning immunity for the terminals connected to the bus.

In-line couplers have the benefit of high reliability and space savings with the tradeoff of flexibility during installations and overhauls.

While bus couplers are necessary to protect the internal wiring and circuitry, they inevitably add a degree of mismatch. The main bus (often 78Ω twinax) has a consistent impedance along the transmission line until a stub where the discontinuity causes an abrupt change in impedance resulting in reflections and loss. MIL-STD-1553B specifies that the longest stub length is 20 feet for transformer coupled stubs in order to minimize the impedance load on the main bus. Still, this number can be exceeded as there is a delicate balance in introducing loads on the bus in order to achieve the specified signal-to-noise ratio and systems error rate performance as specified in MIL-HDBK-155A.

The effect the stub has on the bus waveform depends on the rise/fall time as compared to the time it takes for a wave to propagate from the bus to the end of the stub and back. The reflection can occur before the waveform has changed, causing waveform distortions. In essence, a high impedance of the coupling stub can minimize signal distortion but since this impedance is reflected back to the main bus, the impedance has to be kept below a certain threshold in order to deliver an adequate amount of power at the receiving end. The total load and total characteristic impedance can potentially have an adverse effect on the performance of an installation. Oftentimes, it is desirable to have reserve couplers in order to access extra remote devices whenever deemed necessary. Still, the hazard that the extra load can cause makes it so that reserve couplers are not used in a bus line system unless absolutely required[1].

In-Line Stub Coupler

In-line couplers are spliced directly into the main bus cable, allowing for small form factors and weight savings when compared to the box style stub coupler. These couplers can be conveniently rolled up in wiring bundles without the need to preplan the arrangement of couplers. Highly integrated wiring systems in small aircraft may require in-line couplers due to their limited weight requirements to maintain the desired power-to-weight ratio. This comes with the cost of modularity as the in-line stubs are often difficult to repair/replace in the case of a failure due to a lack of transformer integrity where the windings could be open or shorted. During installation, a complete cable harness cannot be dismantled and reorganized due to unexpected circumstances (damage during fitting) by the operators in charge of fitting the equipment – potentially costing more in time and pushing back deadlines for aircraft construction. This can inhibit productivity on the industrial scale as the exact arrangement of the aircraft components and interconnections must be thoroughly gauged in order for the whole process to run smoothly.

Box Stub Coupler

Couplers housed in boxes allow for a highly modular configuration in aircraft but the increase in connections can decrease their level of reliability as compared to in-line stubs.

Boxed couplers can be an asset in that they are relatively simple to install and replace, thereby mitigating any hassle that can occur upon installation. Still, designers must plan for mounting points for couplers and their respective wiring as the bulky housing occupies much more space and weight. The splicing necessary to install a boxed coupler is more unreliable, as there are far more connections between the bulkhead connectors on the box and the clamp/solder or crimp joints of the wiring. These connections can quickly fatigue under the duress of an aircraft’s high vibration environment, increasing the mean times between failures (MTBF). MTBF is a direct measure of a system’s reliability based on known failure rates of subassemblies in a 1553B network. This indicator is also directly proportional to the number of components and joints between each component. In the occurrence of a fault, technicians connect bus analyzers to the twisted shielded pair bus and then twist and wring the wires in order to locate the source of the short. This can further damage the bus wires, posing another potential risk for failure to a network installation[2].

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