Simplex signaling
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Simplex signaling (SX): Signaling in which two conductors are used for a single channel, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors.
Note: 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.
Source: From Federal Standard 1037C and from MIL-STD-188