Internal Junction B

The junction box EW-JC304 is used inside the frame to connect 4 EW-SD300 E-Tube wires to each other - it is the 12-speed equivalent of the SM-JC41 internal junction B.

This comes in use in several situations:

For one, it is a pretty good method to wire up 12-speed shifters, and connect them to the battery.

You can also use this junction to (temporarily) connect your wireless shifters to the battery using the front derailleur wire. At the time of writing, this is the only- way to update 12-speed wireless shifters.

Finally, I like to use this junction when connecting the SM-PCE02 to my 12-speed bike. When used with an additional EW-SD300 wire, it lets you connect the PCE02 to the entire bike, rather than just to one component.

Just like most other two-port and four-port junctions, the EW-JC304 is just a 'dumb' junction. There is no logic, no firmware. All it does is connect two wires.