What is the significance of Primary and Secondary buses?
Typically, a Bus Controller, a Remote Terminal, and a Bus Monitor have two bus connectors, labeled variously "A, B", "Primary, Secondary", "Primary, Backup", "0, 1" etc.
How should these connectors be wired?
Most 1553 hardware which is produced "off the shelf" contains provisions for supporting a "dual redundant" 1553 bus. Dual Redundant means two complete 1553 buses, with only one of the buses active at a time. The bus controller chooses which bus to transact a given message on and the addressed remote terminal responds on that bus with the required data or status. The next message may be on the same bus, or on the other bus, as determined by the bus controller.
This means that both buses must be connected to each of the terminals in the system. And that the two buses must not be connected to each other. This is illustrated in the following figure for transformer coupling.
This normally requires that the bus wiring described above be duplicated for the two buses; there are two of everything. This is, of course, the definition of dual redundant. If the bus controller never accesses one of the buses, the wiring for that bus need not be supplied. This is sometimes done in some testing environments to reduce the cost of the test equipment. The disadvantage is that the second bus cannot be tested easily.
When adding terminals to the above systems simply add more of the same parts.
To add a terminal using transformer coupling you need to add another transformer coupler in series with the bus (or use a coupler containing more than one transformer). Connect the stub connection on the transformer to the terminal with a cable. Then set the terminal to "Transformer" coupling. Do the same thing for the "B" bus and the job is complete.
Please see the MIL-STD-1553 Tutorial for more information about connecting a bus.