Additional Information - MilesTek MIL-STD1553B Concepts and Considerations
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Additional Information - MilesTek MIL-STD1553B Concepts and Considerations |
The digital data bus MIL-STD-1553B was designed in the early 1970's to replace analog point-to-point wire bundles between electronic instrumentation. The latest version of the serial local area network (LAN) for military avionics known as MIL-STD-1553B was issued 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. |
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. |
The data bus LAN topology encompasses:
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Bus couplers are available in various stub configurations including:
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The purpose of the coupler is to reduce reflections and maintain signal impedance levels. Since direct coupled devices (without couplers) provide no DC isolation or common mode rejection, direct connection to the bus should be avoided. Without couplers, any shorting fault between the device's internal isolation resistors (usually found on the circuit board) and the main bus will cause failure of the entire bus because the device's internal isolation resistors are not sufficient to ensure against shorting out the bus. In addition to transformers, the bus couplers have built-in fault isolation resistors providing protection for the main bus in the event of a short circuit in the stub. All devices, including the bus controller, bus monitor and remote terminal, must be connected to the stub ends of the coupler. |
Both ends of the bus, whether it includes one coupler or a series of couplers connected together, must be terminated (in accordance with MIL-STD-1553B) with 78 ohm terminators. 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. Some couplers have built-in terminators and are generally used at the end 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. In a lab application, unused stub ports on the coupler need not be terminated since the stubs have a higher impedance than the bus. A high-impedance terminator (1000 to 3000 ohms) may be 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. |
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