The Shimano SM-EW90-A [paid link] Junction A is an essential part of the Di2 (Digital Integrated Intelligence) electronic shifting system, serving as the central cockpit interface between shifters, derailleurs, and the main battery. Despite its small size, it plays a critical role in ensuring smooth, reliable electronic shifting. Understanding its compatibility with other Di2 components is vital for achieving flawless performance and preventing costly setup errors.
Understanding the Role of the SM-EW90-A
The SM-EW90-A [paid link] acts as a communication hub, linking the rider’s controls to the derailleurs and system battery. It distributes power and data through Shimano’s E-Tube wiring network and provides a port for system diagnostics and charging. As part of the E-Tube ecosystem, it supports both internal and external battery configurations and integrates with Shimano’s E-Tube Project software for firmware updates and customization.
The unit features three E-Tube ports—making it suitable for single or dual-control lever setups. A five-port version, the SM-EW90-B [paid link], is available for more complex configurations. This distinction is central to understanding the compatibility limitations and requirements of the SM-EW90-A.
Compatible Drivetrain Systems
The SM-EW90-A is compatible with Shimano Di2 11-speed and 12-speed road systems that use E-Tube wiring. This includes:
- Dura-Ace Di2 (9070, R9150, and R9250 series)
- Ultegra Di2 (6870, R8050, and R8150 series)
- 105 Di2 (R7150 series)
It is not compatible with first-generation 10-speed Di2 (7970) components, which used a different wiring standard and junction design. For riders upgrading from older systems, understanding this incompatibility is essential, as mixing E-Tube and non–E-Tube components can lead to communication failures or total system malfunction.
Wiring and Port Compatibility
The SM-EW90-A uses E-Tube SD50 connectors, which are standard for most 11-speed Di2 systems. Each of its three ports can connect to:
- Left and right shifters (ST-9070, ST-R8050, etc.)
- E-Tube wires leading to Junction B (SM-JC41 [paid link]) or wireless units (EW-WU111)
However, with the introduction of 12-speed Di2 systems (R9250, R8150), Shimano transitioned to E-Tube SD300 connectors, which are smaller and more compact. This difference makes direct connection impossible between SM-EW90-A (SD50) and new-generation SD300 components without adapter cables (EW-AD305).
When integrating SM-EW90-A into mixed-generation setups, the EW-AD305 adapter ensures secure data transfer between SD50 and SD300 systems. Still, riders should minimize mixed wiring whenever possible, as signal reliability can slightly degrade when bridging connector types.
Battery and Power Unit Compatibility
The SM-EW90-A interfaces seamlessly with both external batteries (SM-BTR1) and internal seatpost batteries (SM-BTR2 and BT-DN110), provided the rest of the E-Tube wiring supports these units. However, optimal performance requires the BT-DN110 or later battery models to enable Bluetooth or ANT+ communication via a compatible wireless unit (EW-WU111).
If used with older batteries, the SM-EW90-A will still function for shifting but will not support wireless connectivity or advanced E-Tube Project tuning features. Riders upgrading to modern systems should ensure that both their battery and wireless modules are compatible for full functionality.
Shifter and Derailleur Integration
For shifters, the SM-EW90-A is designed for Di2 electronic levers such as ST-9070, ST-6870, ST-R9150, ST-R8050, and ST-R7150. These units connect directly via E-Tube SD50 cables. For riders using time trial or triathlon setups, it also supports SW-R671 or SW-R9160 bar-end shifters through the same interface.
On the derailleur side, compatibility depends primarily on the drivetrain series. It connects indirectly to both the front (FD-R9150 [paid link], FD-R8050 [paid link], etc.) and rear (RD-R9150 [paid link], RD-R8050 [paid link], etc.) derailleurs through Junction B (SM-JC41 [paid link]) and E-Tube wiring. However, 12-speed derailleurs using SD300 connectors require adapter integration, making direct use less practical without modernizing the entire system.
Firmware and Software Compatibility
The SM-EW90-A operates within Shimano’s E-Tube Project software environment for setup, firmware updates, and customization. However, riders should be aware that older firmware versions may not communicate correctly with newer Di2 components or the latest E-Tube mobile app.
Before installation, it’s recommended to:
- Update all Di2 components via the E-Tube PC software.
- Connect the SM-EW90-A using a SM-BCR2 USB charging interface for diagnostics.
- Ensure firmware versions across all components match the E-Tube compatibility chart published by Shimano.
Neglecting firmware consistency can cause incomplete shifting functions or even total communication failure between units.
What Works Well with SM-EW90-A
The SM-EW90-A performs best when paired with:
- Fully 11-speed Di2 E-Tube systems (Dura-Ace 9070 / Ultegra 6870 and later).
- Internal seatpost batteries (BT-DN110) with full system support.
- EW-WU111 wireless units for Bluetooth connectivity.
- SM-JC41 Junction B units to simplify cable routing in frames.
This configuration ensures robust performance, clean wiring, and complete functionality for both adjustment and firmware management.
What to Avoid When Using SM-EW90-A
Certain configurations and practices can lead to compatibility issues or unreliable performance:
- Mixing SD50 and SD300 connectors without adapters – causes signal loss or non-responsive shifting.
- Using non–E-Tube components (7970 generation) – completely incompatible with the SM-EW90-A.
- Outdated firmware – leads to inconsistent system behavior and communication errors.
- Overloading the 3-port unit – for setups requiring multiple shifters or accessories, the SM-EW90-B [paid link] (5-port) is the better choice.
- Incorrect cable routing – excessive bends or tension in E-Tube wires can lead to poor contact and intermittent functionality.
Following Shimano’s E-Tube routing guidelines ensures secure and long-term reliability of the SM-EW90-A within any Di2 system.
Optimizing Compatibility for Performance
To maximize performance and compatibility, cyclists should maintain a uniform E-Tube generation throughout their Di2 setup. Upgrading to newer components like the BT-DN110 battery and EW-WU111 wireless module ensures better diagnostics and connectivity.
Additionally, regular firmware checks through Shimano’s E-Tube Project software maintain communication stability across all components. By keeping the wiring layout clean and balanced, the SM-EW90-A can deliver precise electronic shifting across all conditions.
Conclusion
The Shimano SM-EW90-A Junction A remains one of the most dependable and adaptable control units for 11-speed Di2 systems. However, its compatibility depends heavily on maintaining uniform E-Tube connections, proper firmware management, and matching component generations. Riders who understand these compatibility secrets can prevent system failures, optimize their shifting precision, and ensure the longevity of their Di2 setup.