SHIMANO DI2 SM-JC40 JUNCTION B BOXShimano's Di2 electronic drivetrain is a little more complicated than a mechanical drivetrain. Instead of having cables and housing, it uses wires, batteries, and junction boxes to control the entire setup. The Shimano Di2 SM-JC40 Junction B Box can be viewed as the brains of the operation. All of the wires and battery plug into the junction box. Features Designed for external frame wire routing Manufacturer's part number: ISMJC40 | Shimano | Di2 Sm-Jc40 Junction B Box External Frame E-Tube Junction Box B