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APPENDIX A COUPLING AND TERMINATION
Coupling and termination only apply to OmniBus modules for MIL-STD-1553
and ARINC 708. Electrically, these databuses have similar characteristics. Ex-
cept where a protocol is specified, the following discussion applies to both.
A.1 Bus Termination
The main databus consists of a pair of twisted, shielded wires with a characteris-
tic 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 reflec-
tion and degradation on the bus. The resistor value should be close to the char-
acteristic 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 MIL-
STD-1553 systems are transformer coupled. ARINC 708, however, is normally
direct coupled.
Both protocols 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.
Though it is rarely done, systems can mix the use of direct and transformer cou-
pling.
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 re-
sistors, but this is made up for in the external coupler, which has a step-up
transformer and isolation resistors (see Figure A.1).
•
The direct-coupled terminal has a higher turns ratio and has isolation resis-
tors that are connected directly to the main databus. Direct coupled stubs
should be kept as short as possible (see Figure A.2).
OmniBus II PCIe/PXIe User’s Manual A-1