Simplex signaling (SX) is signaling in which two conductors are used for a single telecommunication circuit, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors. The return path for the current is through ground.
It is distinct from a phantom circuit in which the return current path for power or signaling is provided through different signal conductors. SX signaling may be one-way, for intra-central-office use, or the simplex legs may be connected to form full-duplex signaling circuits that function like composite (CX) signaling circuits with E&M lead control.
Simplex is also used to describe a powering method where one or more signal conductors carries direct current to power a remote device, which sends its output signal back on the same conductor. Phantom powering as used in audio is a form of simplex powering, as the return current flows through the ground or shield conductor.
References
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
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