Shimano Dura-Ace Di2 SM-EW90 3-Port Junction Box

Shimano Dura-Ace Di2 SM-EW90 3-Port Junction Box Info

When the Shimano Di2 E-Tube wires exit the Di2 brake/shift levers or the Di2 bar end shifters, they need to go somewhere to attach to the Di2 system.  That somewhere is the SM-EW90 Junction Box. 

Sometimes called Junction A, or seen as the connection box sitting under the stem, this is where the wires need to be plugged in to give directions to the front and rear derailleurs and get electricity from the battery.  It also is how you charge Di2 batteries while leaving them on the bike.

There are two versions of the Junction Box.  SM-EW90-A is the three-port version.  SM-EW90-B is the five-port version. 

Three ports is the minimum.  This represents a wire coming from one shifter, a wire coming from a second shifter, and a wire going to the junction box, aka junction B, by the bottom bracket.  With three ports, you can either run shifting from the brake levers or the bar ends (aka aero bars).  You can't do both.  If you want the option to expand your system at all, the five-port box is probably the better choice.  That, or you'll upgrade to the five-port box when you expand your system.  However, if you're looking to add Sprinter Switches (SW-R610) or Climber Switches (SW-R600), these plug into the ports on the drop bar integrated levers, so you don't need the five-port box if you think you'll go this route.

The Junction Box comes with mounting plate as well as two rubber straps to affix the box to your bike.  The box also has a port for on-bike charging of Di2 batteries and reprogramming and updating your system.

The Shimano Di2 SM-EW90 Junction Box gathers the shift wires before they communicate with the derailleurs.