The Siemens 3RK1901-2NR21 is an AS-Interface M12 branch that transitions the AS-i flat cable to an M12 connector — specifically a 1-meter cable with an angled M12 cable socket. It uses an insulation piercing method to tap into the flat cable, so no stripping or cutting the backbone is needed; the cable simply lays into the housing and the screw fixing clamps it down. The branch carries both the AS-i data/power pair and the auxiliary power (AUX) on the same M12 connector, with pin assignment: 1 = ASI+, 2 = AUX-, 3 = ASI-, 4 = AUX+. The contact rating is 4 A maximum, with a rated operational current of 3 A. Rated for the full AS-i voltage range of 26.5 to 31.6 V, it operates from -25 to +75 °C and stores from -25 to +85 °C.
This is a field-mount accessory for AS-Interface networks — it creates a local M12 drop point for a sensor or actuator without cutting the flat cable. Mounting is by screw fixing; the tightening torque for the fixing screw is 1.65 N·m. The manufacturer note states that the cable must not end in the feeder — meaning the branch is designed for a pass-through topology, not a dead-end termination.