Are Bus Couplers Necessary?
Posted on June 10, 2009
Filed Under FAQ |
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.
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