Every wire in a Di2 system has to connect somewhere, and Junction A is where the cockpit side of the network comes together. The SM-EW90-A mounts beneath your stem using a rubber strap that wraps around the brake cable housing, keeping it accessible but out of the way. Two of its three E-Tube ports connect to the left and right shifters, while the third port runs a wire down through the frame to Junction B, which then distributes signals to the derailleurs and battery.

The junction box puts system status right where you can see it. A button on the housing activates the LED display: green means the battery holds more than 50% charge, solid red indicates below 50%, and flashing red warns that you're running low and should charge soon. The same button handles derailleur adjustment mode for indexing and rear derailleur crash protection reset after the bike takes a hit. When paired with a BT-DN110 internal battery, pressing the button also initiates Bluetooth pairing for wireless connectivity to the E-Tube Project app on your phone or tablet.

The charging port built into the SM-EW90-A accepts the SM-BCR2 charger, so you can top off the battery without accessing the battery itself. Plug in the charger, and power flows through the E-Tube wiring to reach wherever the battery sits in the frame or seatpost. This same port serves as the connection point for firmware updates when you link the system to a PC running E-Tube Project software, letting you update all connected components from one access point.... Read More