Astronics OMNIBUS II NI PXIe, User Manual

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OmniBus II NI PXIe User Manual A-1
APPENDIX A: COUPLING AND TERMINATION
Coupling and termination only apply to OB2 modules for MIL-STD-1553.
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 Ω. Terminate the databus 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 Ω). 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—Transformer Coupling to a Dual-
Redundant Databus and Figure A.2—Direct Connection to a Dual-Redundant
Databus.
Note: Some Ballard products have onboard termination resistors that you can
switch-in manually or under software control. 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 MIL-
STD-1553 systems are transformer coupled.
Both coupling methods have a transformer as part of the terminal’s interface, but
MIL-STD-1553 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. In rare cases, 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:
•
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—Transformer Coupling to a Dual-Redundant
Databus).
•
The direct-coupled terminal has a higher turns ratio and has isolation resistors that you
can connect directly to the main databus. Keep direct-coupled stubs should as short as
possible (see Figure A.2—Direct Connection to a Dual-Redundant Databus).