Di2 With Only One Junction Box

[X11]

In a Di2 (Ultegra 6870) setup consisting of left and right brake/shift levers,
front and rear derailleurs, and a battery, could a single (5-port) junction box be used?

[pdlpsher1]

I don't see why you can't. You just need to get fairly long cables assuming your junction box will be near the bars.


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[11.4]

We've played with a number of variant layouts. You definitely can get rid of the internal junction box as long as you can make all the connections. You can use any length of Di2 wire from the levers to the box and same from the two derailleurs and the battery. I ran one setup, for example, where the one junction box hangs right below the seat post battery, and you have to remove the post to charge the system.

You'll run into two issues:

1. Where to put the junction box and whether to have it accessible for charging, programming, low power and crash light, etc. You do get shifting signals if your power gets low so you don't need the box for that.

2. The bigger issue was how to route the wires. The frame I was using was equipped for Di2 only, with one port at the top of the down tube, one port at the front derailleur, and one at the rear derailleur. When you want to route two wires from the two shifters down to the inside of the frame, you don't have a ready hole for two wires, and nothing is gained by running them externally. I would have used a B junction box to reduce that down to one wire, but that doesn't accomplish much if you then have an external B box hanging around.

It's a completely different story if you have (or are willing to drill for) internal routing on your bars. You have a hidden place to make various connections, including the 5-port box. This is actually what DA 9170 is going to offer as an option, which has to deal with the same problem of finding bars with cable ports or drilling your bar.

In the end, the neatest setup was simply to have the five-port box under a Garmin mount or under the stem, in either case mounted with very fine hardware so no rubber band was needed. Seriously, it's a nice clean look and I wouldn't sweat it until I had bars that allowed alternative layouts.

[Gearhead65]

Keep in mind you can use your shifters as a junction box of sorts. 3 ports per shifter. At minimum, run LS to RS, then back to whereever you put your A.

[Junior7]

For it is better to use 3 ports shifter.

[jaypee]

if I read this correctly the new bar end JB ew-rs910 is only 2 port so those wanting to use this may well have to go this route anyways

[X11]

The Di2 wiring diagrams I've seen all show 2 junction boxes, A and B.
It wasn’t clear whether each of the two types performed separate,
distinct functions. From the replies, it seems that A and B are (essentially) the same.

Keep in mind you can use your shifters as a junction box of sorts. 3 ports per shifter. At minimum, run LS to RS, then back to whereever you put your A.

I was not aware of that. A single 4-port box should be sufficient, then.

[goodboyr]

Junction a is smart, junction b is dumb.

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[Nefarious86]

Left shifter to right shifter. Right shifter to A junction 5 port mounted under BB. Rear wiring as usual, route RD FD and battery to the A junction.

[ms6073]

A junction 5 port mounted under BB

I am guessing the frame design necessitated that approach as given the amount of grit and grime I clean off the rear brake of on my Scott Solace, I would not want the A junction also catching that stuff under the bottom bracket. I have seen a couple images of one of the WW bikes with a 3-port A junction mounted just below the seat post mounted bottle cage. While not very 'aero', I like having the A Junction under the stem, since it allows the rider to use the diagnostic function in case I need to make a derailleur adjustment while riding.

[11.4]

Junction a is smart, junction b is dumb.

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Exactly. The B junction box is simply a connector with no other purpose. There is circuitry and logic inside the A box.

[11.4]

Left shifter to right shifter. Right shifter to A junction 5 port mounted under BB. Rear wiring as usual, route RD FD and battery to the A junction.

We played with such setups but what has one really gained except the minuscule weight savings of the B junction box, offset by a little extra wire? Installing the A junction under the bottom bracket exposes it to the weather.

We actually tried a setup where we used a B junction inside the frame and installed the A junction under the saddle with a single obscure hole in the top of the seat post to feed a single wire to the A box. It actually worked. The single wire goes down the seat post next to the battery and connects to a B box in the seat tube. Then you have the A junction accessible for charging, programming, crash resets, and so on, and essentially no wiring visible. However, the under-stem A-box is scarcely obtrusive or problematic. Mount it with double-stick tape so you don't need to use the rubber band or zip ties and it looks very clean. A bigger issue is how to run the wire from the bars down to the port in the frame. We've seen all kinds of versions of this, ranging from the very neat Cannondale approach to prominent custom frame builders who put the Di2 port in all kinds of wacky locations that prevent the cable from running right with the rear brake cable housing.

[X11]

Junction a is smart, junction b is dumb.

Exactly. The B junction box is simply a connector with no other purpose. There is circuitry and logic inside the A box.

Are shifting changes saved in junction A, the shifters, or the derailleurs?

[mattyNor]

I'm looking into doing something similar as above since I'll be doing a 1x build. Plan is to do the junction A on top on the seatpost route with one cable going from shifter to junction, one cable going battery to junction, then the last cable going junction to derailleur. I'm planning to use the 2 port A junction so for charging I'll have to unplug something, my question is what would I unplug? Oh and a second question since I'm going 1x I don't need internals for the left shifter, what's the lightest/cheapest option I could go with for a disc dropbar lever?

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[Tifosiphil]

Quick question as we have a lot of DI2 knowledge in here. If I'm using the EW67A cable between shifters and an external battery do I require a seperate A junction?

Understand I actually can't charge the system without removing the battery from the bike this way