The purpose of a MIL-STD-1553 bus coupler is to maintain signal impedance levels and reduce reflections. Couplers are connected between the MIL-STD-1553 bus and 1553 stubs. Stubs can be up to 20 feet long and connect to Bus controllers, Remote terminals, and Monitors.

They include transformers with a fixed turns stepped-down ratio of 1.4 to 1.0 from the bus to the stub. Additionally, fault-isolation resistors with values of 0.75*Z0 are placed on the bus side of the transformers. These resistors are included to protect the bus from short circuit faults on stubs or in terminals. This prevents a short circuit on a single stub or terminal from shorting out the main bus, which could cause the entire bus to fail.

Couplers also provide enhanced DC isolation, common mode rejection, and lightning protection for connected terminals. This is in addition to the protection offered by the terminals’ internal isolation transformers.