If you are considering Bus Couplers, you may want to take into account a few MIL-STD 1553B specifics.
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.
How to Balance the Number of Couplers and the Distance Between Them
While bus couplers are necessary to protect the internal wiring and circuitry, they inevitably
add a degree of mismatch. The main bus (often 78 Ω twinax) has a consistent impedance along
the transmission line until a stub 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.
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