APPENDIX A: COUPLING AND TERMINATION
Coupling and termination are important considerations for MIL-STD-1553
databuses. This appendix introduces concepts relating to coupling and
termination. Actual interfaces, cabling, and part numbers will vary from product
to product. All figures included in this appendix are used generically for
illustration of these concepts.
A.1 Bus
Termination
The main databus consists of a pair of twisted, shielded wires with a
characteristic impedance in the range of 70 to 85 ohms. The databus must be
terminated at both ends with a resistor to provide proper loading and to minimize
signal reflection and degradation on the bus. The resistor value should be close to
the characteristic impedance of the databus. The resulting total load on the
databus is the two terminating resistors in parallel (about 39 ohms). Even with a
very short databus, the load from the terminating resistors is still required. Notice
how the resistors terminate the databuses in Figure A.1 and Figure A.2. Note that
some Ballard products have on-board termination resistors that can be switched
in manually or under software control.
Note: The most common problem in a new system is an improperly
terminated databus.
A.2 Transformer versus Direct Coupling
MIL-STD-1553 can be either direct or transformer coupled. Most military 1553
systems are transformer coupled.
Both coupling methods have a transformer as part of the terminal’s interface, but
transformer coupling has an additional external transformer coupler that isolates
the stub from the main databus and reduces signal reflections. The signal level on
the main bus is the same for both direct and transformer coupling. Though it is
rarely done, systems can mix the use of direct and transformer coupling.
A terminal must be properly configured for either direct or transformer coupling.
There is a difference between the terminal’s internal interface circuit for direct
and transformer coupling:
1.
The transformer-coupled terminal has a lower turns ratio and no isolation
resistors, but this is made up for in the external coupler, which has a step-up
transformer and isolation resistors (see Figure A.1).
2.
The direct-coupled terminal has a higher turns ratio and has isolation
resistors that are connected directly to the main databus. Direct coupled stubs
should be kept as short as possible (see Figure A.2).
Lx1553-5 User’s Manual
A-1
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