Every MIL-STD-1553B vehicle has a backbone of interconnected electrical components. When designing the vast array of sub-assemblies, factors can be overlooked regarding passive components such as couplers, cables and connectors. In particular, aircraft, with all their wiring, are susceptible to problems such as intermittent connections, shorts and corrosion. However, there are proven ways to assess passive components to keep vehicles operating at peak performance. Here are our three considerations for designing MIL-STD-1553B components.
Bus Coupler Considerations
To protect network integrity, MIL-STD-1553B requires that data bus couplers, aka stub couplers, be placed between the main data bus and the vehicle subsystems, computer system or terminal. Each stub coupler must also be equipped with fault isolation resistors and a step-up transformer.
It is important to balance the number of couplers and the distance between them. Bus couplers inevitably add a degree of mismatch. The main bus has a consistent impedance along the transmission line until a stub causes an abrupt change in impedance, resulting in reflections and loss. MIL-STD-1553B specifies that a transformer-coupled stub be no longer than 20 feet to minimize the impedance load on the main bus.
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. A high impedance of the coupling stub can minimize signal distortion but since this impedance is reflected back to the main bus, the impedance must be kept below a certain threshold.
In-line stub couplers, spliced into the main bus cable, have the benefit of high reliability and space savings with the tradeoff of flexibility during installations and overhauls. Box stub couplers allow for a highly modular configuration in aircraft but the increase in connections can decrease their reliability compared to in-line stubs.
Cable Harness Considerations
It is important to consider cable length, twinaxial vs. triaxial cables, and jacketing/insulation.
In a 1553B installation, the maximum attenuation specified for the cables is 1.5 dB per 100 feet at 1 MHz; for longer cable runs a lower attenuation is necessary. In other words, the maximum length of the bus is directly correlated to the gauge of the inner conductors and time delay of the transmitted signal.
Twinaxial cables are normally used for the main data bus in a 1553B network. They have balanced conductors that leverage differential signaling where any external common-mode noise can easily get canceled. They also require a minimum of 90% coverage for increased noise immunity.
Triaxial cables are used for specialized high frequency applications where the second layer of shielding is chassis ground while the inner braid functions as the return path for the signal. This provides protection from common-mode interference.
The cable harnesses used with avionics equipment require extra care to combat signal interruption and external noise. Twisted shielded pairs provide certain benefits and added shielding offers EMI protection.
Cable jacketing is the first defense against harsh environments. Normal jacketing materials swell, crack or deform when in extreme temperatures. For added protection, look for products with abbreviations such as PTFE for its strength and elasticity, PFA for cable jacketing and FEP for insulation.
Other than requiring the polarity of a concentric connector, MIL-STD-1553B does not specify the type of connector that must be used for cabling. Two common types are concentric threaded and concentric bayonet. Threaded connectors are versatile but do not hold as tightly as bayonet mounts and present a disadvantage in tight spaces, where there is not enough leverage to torque a connector. A bayonet-type connector is simpler to attach and better in tight spaces.
Choosing the Product That Is Best for You
To summarize, data bus couplers are necessary but can introduce challenges by changing transmission line impedance. It’s important to balance the number of couplers and the distance between them. Second, the maximum length of the bus is directly correlated to the gauge of the inner conductors and time delay of the transmitted signal. Third, you can use either threaded or bayonet connectors, but the bayonet style has a couple of distinct advantages.
MilesTek offers a wide range of MIL-STD-1553 products that are in-stock and ready to ship. For additional help or advice on choosing what product is best for you, contact the MilesTek sales team at 1-877-924-1134 or firstname.lastname@example.org.