SINAMICS S120 Control Units – The Brains of the Drive System

When you think about the power and precision of a modern motion control system, it’s easy to picture the motors and drives doing the heavy lifting. But behind every successful move, there’s a control unit making hundreds of decisions per second. In the SINAMICS S120 family, the control unit—or CU—is where the magic happens. These Siemens drive controllers manage everything from motion sequences to safety functions, ensuring your machine runs exactly the way it should.

Whether you’re controlling a single-axis pick-and-place robot or an entire production line with dozens of coordinated axes, the right control unit makes the difference between smooth, efficient operation and constant troubleshooting. Let’s look at the three main options—CU320, CU310, and CU305—and see how they fit into different applications.

Overview – CU320, CU310, CU305 Roles

Each model in the S120 control unit lineup is built for a specific purpose. Understanding these roles up front helps you choose the right controller for your project and avoid unnecessary complexity or cost.

Model	Primary role	Typical use	Notes
CU320-2	Central, multi-axis Siemens drive controller	Complex machines with shared DC bus and many axes	High performance; variants for PROFINET or PROFIBUS; flexible topology via DRIVE-CLiQ
CU310-2	Single-axis control unit mounted on a power/motor module	Standalone axes or smaller machines; fast commissioning	Compact; reduces panel space; available in DP/PN variants
CU305	Integrated single-axis controller for servo applications	High-precision motion on one axis (pick-and-place, winding, indexing)	Focus on servo performance and tight integration

The CU320-2 is your go-to for large, interconnected systems where multiple axes must share data and operate in perfect sync. The CU310-2 is a space-saving option ideal for individual drive control, while the CU305 specializes in high-speed, high-accuracy servo applications where tight control loops matter most.

Part Numbers for Easy Sourcing

For engineers and maintenance teams, knowing the correct part number ensures you get the right variant for your network and application.

Control Unit	PROFIBUS DP	PROFINET	Notes
CU320-2	6SL3040-1MA00-0AA0	6SL3040-1MA01-0AA0	Multi-axis control
CU310-2	6SL3040-1LA00-0AA0	6SL3040-1LA01-0AA0	Single-axis control
CU305	6SL3040-0JA00-0AA0	6SL3040-0JA01-0AA0	Compact, economical

Having these part numbers on hand speeds up procurement and reduces the risk of ordering the wrong device—especially when integrating into mixed PROFINET/PROFIBUS environments.

Connectivity – PROFINET, PROFIBUS, EtherNet/IP

Communication is at the heart of every modern automation setup, and SINAMICS S120 control units excel at talking to the rest of your system. The choice of CU variant determines your native interface, but even the base options give you plenty of flexibility:

• PROFINET – Ideal for new Siemens-based systems, offering deterministic control and easy diagnostics.
• PROFIBUS DP – Still widely used in established plants, ensuring backward compatibility.
• EtherNet/IP – Possible via communication options or gateways for integration with Rockwell Automation networks.
• DRIVE-CLiQ – A Siemens interface that automatically identifies modules and feedback devices, speeding up commissioning.

This flexibility means you can integrate S120 control units into mixed environments, bridging the gap between older machinery and the latest Industry 4.0-ready systems.

Integration with TIA Portal – Benefits

Pairing your SINAMICS S120 with Siemens’ TIA Portal environment unlocks powerful engineering and maintenance benefits. Using Startdrive within TIA Portal allows you to configure drives alongside PLCs and HMIs in a single, unified workspace:

• Save time with prebuilt templates and parameter sets.
• Monitor drive status in real time without separate tools.
• Document changes for faster troubleshooting in the future.

The result is faster commissioning, fewer errors, and a more maintainable system—key advantages in any high-pressure production environment.

Firmware Management – Updates for Security & Performance

Like any intelligent device, a SINAMICS S120 control unit benefits from regular firmware updates. These updates aren’t just about new features—they also address security vulnerabilities, improve stability, and resolve bugs that could impact uptime.

• Keep all CUs in a system on the same firmware version to avoid mismatches.
• Back up parameters before applying updates to prevent configuration loss.
• Test updates on a single unit before deploying across an entire plant.

Consistent firmware management is a simple but often overlooked part of long-term drive reliability.

Troubleshooting – Common Issues & Fixes

Even the most reliable systems can run into problems. Knowing the most common symptoms and their fixes can cut hours off your downtime:

1. Network/PLC can’t detect the drive – Check basic connections, ensure the correct GSDML file is loaded in your PLC project, and verify that device names are unique on the network.
2. DRIVE-CLiQ topology alarms – Loose connectors or missing modules can trigger these alarms. Reseat cables, verify module power, and compare the physical layout to the topology view in diagnostics.
3. Axis won’t enable or trips on safety – Review Safety Integrated wiring, confirm telegram settings match the application, and check for active limit switches or interlocks.
4. Performance issues – Slow response, overshoot, or hunting often points to tuning issues. Recheck motor data, encoder configuration, and loop tuning parameters.

Which SINAMICS S120 Control Unit Should You Choose?

Ultimately, the choice comes down to your application:

• CU320-2 – Multi-axis machines with centralized control.
• CU310-2 – Compact single-axis setups where space is limited.
• CU305 – High-precision servo tasks on a single axis.

Matching the CU’s communication capabilities to your PLC’s network ensures the smoothest integration possible.

Conclusion

SINAMICS S120 control units are the nerve centers of high-performance drive systems. By selecting the right model, integrating it effectively, maintaining its firmware, and staying ahead of common troubleshooting scenarios, you can maximize machine uptime and efficiency. Whether you need a powerful multi-axis brain or a nimble single-axis controller, Siemens has a CU ready for the job.