Below-the-stem Junction A
The External junction box A SM-EW90 used to be the junction box everyone had on their bikes. DURA-ACE 9150 introduced the EW-RS910 junction A, which has been the go-to junction box for any new bike since then.
While it is available in a 3 and 5-port version, the latter is rarely used and most people seem to opt for the 3-port version of the junction box.
Time Trial bikes tend to feature the 5-port junction a lot, due to the lack of traditional shift levers and the need to connect 4 shifting units. An alternative way to allow for more connected shift units on TT bikes is through the use of multiple SM-JC41 junctions B. These can usually be tucked away in the base bar.
You could replace this junction with the mountain bike System Information Display (XT SC-M800 | XTR SC-M9051), if you wanted to. This Information Display functions as a Junction A with three e-Tube ports, a display and features Bluetooth/ANT+ connectivity.
What does the button do?
Depending on the battery and other components installed on your bike the junction box button behaviour changes. As described in the Di2 Beginner's guide, pressing the button for 0.5 seconds will turn the LED red and enter adjustment mode, allowing you to index the rear derailleur or adjust the front derailleur top limit.
You can trigger rear derailleur crash protection reset mode by presssing-and-holding the button for 3 seconds, until the red LED starts to flash. Make sure you turn the cranks, because the rear derailleur will then go through all the gears.
If you have the BT-DN110 battery or BM-DN100 external battery mount though, the button behaviour is:
- Press-and-hold the button for 0.5 seconds to enter Bluetooth LE connection mode (red and green LEDs will flash)
- Press-and-hold the button for 2 seconds or more to enter adjustment mode
- Press-and-hold the button for 5 seconds or more to trigger rear derailleur crash protection reset
Note that if you have a modern battery you'll probably also have a synchronized shift capable bike and pressing the button twice will let you change the current shift mode.
Besides the button, the junction also has two LEDs. Besides indicating whether the bike is in adjustment mode, Bluetooth pairing mode or triggering rear derailleur crash reset, they can also indicate the battery charge level and selected shift mode