What is the legacy of the MIL-STD-1553?

Posted on June 10, 2009
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The digital data bus MIL-STD-1553 was published in 1973 to replace analog point-to-point wire bundles between electronic instrumentation. Revision A to the standard was issued in 1975 as a tri-service standard.  After two years effort by a SAE task group, Revision B was published as a tri-service/NATO standard in 1978.  Various notices have been published to update the standard.  After 30 years of familiarity and reliable products, the data bus continues to be the most popular militarized network.

What are the Physical Characteristics of MIL-STD-1553B?

Posted on June 10, 2009
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The MIL-STD-1553B bus has four main elements: (1) a bus controller that manages the information flow; (2) remote terminals that interface one or more simple subsystems to the data bus and respond to commands from the bus controller; (3) the bus monitor that is used for data bus testing; and (4) data bus components (bus couplers, cabling, terminators and connectors). Data is sequentially transmitted and received in a multiplexing scheme over two copper wires from computer to computer at a rate of 1 megabit per second. In most vehicle applications, redundant buses are employed.

What are the Integral Hardware Pieces of a Typical Data Bus?

Posted on June 10, 2009
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The data bus LAN topology encompasses:

• Bus couplers (coupling transformers with fault-isolation resistors)
• Bus terminators
• Twinax cable (shielded twisted pair)
• Concentric twinax connectors (with a center contact and an
intermediate cylindrical contact)

Are Bus Couplers Necessary?

Posted on June 10, 2009
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If a terminal device has internal isolation resistors and transformers, then direct coupling is possible to the bus cabling without the use of bus couplers. Direct coupling connections must only be used with stub lengths of less than 1 foot.  However, MIL-STD-1553B cautions the employment of direct coupling because a terminal short could disable the entire bus. Direct stubs can also cause significant impedance mismatches on the bus. Therefore, bus couplers are recommended because their isolation transformers and resistors are external to the terminal device.  Compared to direct coupling, bus couplers extend the stub length to 20 feet and provide electrical isolation, better impedance matching and higher noise rejection characteristics. The electrical isolation prevents a terminal fault or stub impedance mismatch from affecting bus performance.  All devices, including the bus controller, bus monitor and remote terminal, must be connected to the stub ends of the coupler.

What Type of Bus Couplers are Available?

Posted on June 10, 2009
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Bus couplers are available in various stub configurations including:

• Box type (used in lab or vehicle applications)
• In-line type (used in vehicles where light weight and small size are important).

Why Does the Bus Need Termination?

Posted on June 10, 2009
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According to MIL-STD-1553B, the bus must be terminated at both ends of the cable with resistors equal to the cable characteristic impedance.  The purpose of termination is to minimize the effects of signal reflections that can cause waveform distortion.  If termination is not used, the communications signal can be compromised causing disruption or intermittent communications failures.  Furthermore, both ends of the bus, whether it includes one coupler or a series of couplers connected together, must be terminated. Since typically 78 ohm cable is utilized, the terminators should contain 78 ohm resistors.  Some couplers have built-in terminators and are generally used at the ends of the bus in multi-coupler applications. These types of couplers are mainly for vehicle applications as they limit the flexibility of test lab set-ups.

Should the Stub Ports be Terminated?

Posted on June 10, 2009
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In a lab application, unused stub ports on the coupler need not be terminated since the stubs have higher impedance than the bus.  A high-impedance terminator (1000 to 3000 ohms) is sometimes used in vehicle applications to simulate a future load from an unspecified device. In both cases, an RFI cap over the unused stub is a deterrent to interference and/or dust.

What is the Characteristic Impedance of the Bus?

Posted on June 10, 2009
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Although MIL-STD-1553B specifies that the data bus should have a characteristic impedance between 70 and 85 ohms, industry has standardized on 78 ohms.

What Type of Cable is Recommended?

Posted on June 10, 2009
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Having a characteristic impedance of 78 ohms at 1 MHz, a 2-conductor twisted-pair cable known as twinax is used to connect the bus and stub devices.  The insulated pairs are balanced and have an overall shielding braid around the pairs.  The twisting of the signal-carrying pairs theoretically cancels any random induced noise caused by the pair.  The two internal dielectric fillers separate the braid from the pairs to minimize the leakage capacitance to ground. The fillers also assist in uniform twisting of the pairs.  The 90% braid coverage protects the pair from external noise.  PVC outer jacket cable is suitable for lab use while high-temperature rated outer jacket cable is applicable for vehicle use.

What is the Maximum Length of the Bus?

Posted on June 10, 2009
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MIL-STD-1553B does not specify the length of the bus.  However, the maximum length of bus is directly related to the gauge of the cable conductor and time delay of the transmitted signal. A smaller conductor attenuates the signal more than a larger conductor. Typical propagation delay for a 1553B cable is 1.6 nanoseconds per foot.  Thus, the end-to-end 100-ft. bus would have a 160 nanosecond propagation delay, which is equal to the average rise time of a 1553B signal. According to MIL-HDBK-1553A, when a signal’s propagation delay time is more than 50% of the rise or fall time, it is necessary to consider transmission line effects. This delay time is proportional to the distance propagated.  Also, consideration must be given to the actual distance between the transmitter and receiver, and the individual waveform characteristics of the transmitters and receivers.

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