If you're wiring up a 12-speed Di2 groupset, the junction box hiding inside the frame is doing more work than most riders realize—it's the connection point that lets two shifter wires and a battery wire share the same electrical network without any signal loss. Shimano's EW-JC304 is the 4-port junction built specifically for the SD300 wire system used across current Dura-Ace R9200, Ultegra R8100, and 105 R7100 Di2 groupsets. It sits inside the frame, typically in the downtube or head tube area, and handles the handoff between your shifters and the seatpost battery.

The most common wiring configuration runs one SD300 wire from each shifter down to the EW-JC304, then a single wire out the other end to the DN300 battery in the seatpost. That leaves one of the four ports unused, which is where the included dummy plug comes in—every unoccupied SD300 port in the E-Tube system needs a dummy plug installed to maintain the seal and keep the electrical circuit intact. Skip it and you'll chase phantom shifting issues that have nothing to do with the derailleurs themselves.

The 4-port layout also gives you the option to splice two pairs of SD300 wires together when standard wire lengths won't reach on larger frames or unusual routing paths. Because the junction is passive—no firmware, no logic, just an electrical bridge—it passes full power and data between connected wires without any performance compromise. One important install note: the SD300 connectors are noticeably thinner and more fragile than the older SD50 style, and the TL-EW300 plug tool is required for clean insertion and removal. Trying to press them in by hand is how ports get damaged.