Cable-hiding frames made every remaining exposed piece of Di2 hardware stick out, and the old under-stem Junction A was the last obvious offender. The Shimano EW-RS910 Internal Junction Box moves that connection point either into the end of your drop handlebar or into a frame port designed for it, leaving only a small charging button visible from the outside. It handles the same jobs the older SM-EW90 did—connecting the shifters to the rest of the system, charging the battery, checking battery status, and running derailleur micro-adjustments—just without the clutter under the stem.

This is a 2-port junction, which changes how you route wires compared to the 3-port and 5-port boxes most Di2 owners are used to. One port runs into the handlebar to meet the shifter wires, and the second port runs to the frame down to the battery and derailleurs. Because there are only two ports, an EW-JC130 Y-connector inside the handlebar is the usual way to merge the left and right shifter wires before they reach the junction. If your bar has internal routing holes near the levers, the whole cockpit hides cleanly; if it doesn't, you can still run wires externally under the bar tape and get most of the benefit.

This is an 11-speed Di2 part, so it works with Dura-Ace R9150, Ultegra R8050, and older 6870 and 9070 systems using SD50 E-Tube wiring. It is not compatible with 12-speed Di2, which uses a different wiring standard entirely. Frames like the Trek Madone, Pinarello Dogma, and various Giant and Specialized models include a built-in frame port for this junction; on other bikes it lives in the bar end. The box ships with a matching blank end cap, holders, dummy port covers, and the frame mount, so you have the parts for either installation. As always with Di2, keep firmware matched across every connected component or shifting behavior can get strange.