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SHIMANO - Junction B - E-TUBE - Internal DI2 Wire Routing - 4-Port For internal DI2 wire routing, SHIMANO's SM-JC41 Junction B is the central connector for DI2 systems. Wires from the front and rear derailleurs, cockpit area, and battery all plug into the 4-port SM-JC41 junction, which is housed inside the bike frame.