Resolving SINAMICS CU310-2DP RDY/COM 2 Hz Red LED Booting Failure

1. Problem Overview

On a SINAMICS S120 drive system, a Control Unit CU310-2DP (MLFB 6SL3040-1LA00-0AA0) fails to leave the boot phase after power-up. Both the RDY (ready) and COM (communication) LEDs on the front of the Control Unit flash red at 2 Hz (250 ms on, 250 ms off). The BOP20 Basic Operator Panel stays dark because it does not receive internal 24 V from the unit, and the engineering tool STARTER (or Startdrive in TIA Portal) cannot establish an online connection over PROFIBUS, PROFINET, or the service interface (X127).

The same cabinet contains an older CU310 (non-2 variant) with an identical CompactFlash card (CFC) that boots without issue. When the suspect card is inserted into the older CU310, the older unit also boots and STARTER connects normally. When that same card is moved back to the CU310-2DP, the 2 Hz red blink returns. Swapping the BOP20 does not change the behavior, confirming that the panel and the CU310-2DP are not the cause. No DRIVE-CLiQ components (Motor Modules, Line Modules, encoders, Terminal Modules) are connected at the time of the fault; the unit is in a stand-alone state on the bench.

Per Siemens field service guidance, this is a Control Unit / CompactFlash card compatibility problem, not a hardware failure of the CU310-2DP itself. The card shipped with the unit (or the card carried over from a CU310) contains a firmware image that is below the minimum version required to boot the -2 hardware platform. The fix is a card with a current 1 GB SINAMICS BootLoader and firmware ≥ 4.5, sourced from the official Siemens spare-parts channel.

2. Affected Hardware and MLFB Cross-Reference

3. LED State Decode and Behavior Matrix

The CU310-2DP has a single combined RDY/COM LED on the front bezel plus an OPT (option) LED that indicates the state of the communication board. Under normal operation, during ramp-up, and during fault states, the RDY/COM LED is a state indicator for the bootloader, the operating system, and the fieldbus. The 2 Hz red flash is one of the standardized fault states and is not serviceable from STARTER or the BOP20 because the unit never reaches the operating system that exposes the parameter interface.

Inline timing representation of the 2 Hz red blink:

4. Root Cause Analysis

The CU310-2 platform is a hardware revision of the original CU310. The bootloader and the on-board operating system were rewritten to support:

• Larger parameter image (Functional Safety extended, more drive objects, expanded PROFIdrive profile V4.2)
• Web server with HTML5 interface (only available from firmware 4.4 onward)
• PROFIenergy profile, cross-traffic, and isochronous PROFIBUS extensions
• Support for the 1 GB card with the SINAMICS BootLoader signature
• Improved security: signed firmware, signed licenses, anti-tamper checks on the card

Firmware 2.6 SP2 (originally released for the legacy CU310) does not contain the boot descriptors for the -2 hardware ID. When the bootloader on the -2 platform interrogates the card, the firmware version check fails and the unit falls back to the 2 Hz red fault blink. Because the BOP20 is fed from the operating system, it never receives initialization, which is why it stays dark. The same card works in the legacy CU310 because the legacy bootloader accepts the 2.6.x image; the -2 bootloader does not.

The compatibility constraint can be expressed as:

boot_ok = (fw_major >= 4) AND (card_size >= 1GB) AND (bootloader_signed == TRUE)

If any of these three conditions is false, the -2 unit will not leave the bootloader. The card size check is not optional: even a properly signed 512 MB card with the correct firmware will be rejected if it lacks the 1 GB identifier in the partition table.

5. Pre-Diagnostic Checklist

Before declaring a card-compatibility issue, eliminate the cheap-to-test failure modes on the bench:

1. Verify the 24 V supply at X124 pins 1, 2 (P24, M). Voltage must be 20.4 V to 28.8 V DC. Use a calibrated DMM at the terminal block, not at the power supply output.
2. Measure the current drawn by the Control Unit. A CU310-2DP with no DRIVE-CLiQ loads draws approximately 0.6 A at 24 V. If the current exceeds 1.5 A with no load, the unit has an internal short and must be replaced.
3. Inspect the CFC slot for bent pins, contamination, or a partially inserted card. The card must click into the ejector and sit flush with the bezel.
4. Confirm that the card you are about to test is actually the one mounted. Field engineers have been bitten by cards left in an enclosure from a previous unit.
5. Confirm that no DRIVE-CLiQ cables are connected. A shorted DRIVE-CLiQ cable can also hold the unit in a fault state. Rule this out before swapping cards.
6. Try a known-good, recently manufactured 1 GB card with a known firmware ≥ 4.5 if one is available on the bench.
7. Check the ambient temperature. The CU310-2 is rated for 0 °C to +55 °C at the air intake. Below 0 °C the bootloader can stall; above 55 °C the unit will throttle and may also fail to boot.
8. Verify the firmware variant on the card label. Some cards are sold with a "Safety" or "High Feature" license. Either is compatible, but a card with a "Basic" license on a cabinet with Safety components will boot but will refuse to enable Safety Integrated after commissioning.

6. Required Firmware and CF Card Specifications

How to read the firmware version printed on the card label:

SINAMICS S120
FW V4.5 HF6
BootLoader V4.5.1
MLFB 6SL3054-0CG0x-1AAx
S DSC  E/xx/xx

If the "FW V" line starts with 2.6 or 2.5, the card belongs to the legacy CU310 family and will not boot a CU310-2. The "BootLoader" line tells you whether the card is from the new card family at all; old cards may not print a BootLoader version at all.

7. Step-by-Step Resolution Procedure

1. Power down the cabinet. Lock out the 24 V supply and tag out per local safety rules. Wait for the DC bus of any connected Power Module to drop below 50 V (typical 5 min discharge time for a 200 V class PM, 10 min for a 400 V class PM, 15 min for a 690 V class PM).
2. Open the cover of the CU310-2DP and eject the suspect CFC by pressing the ejector lever. Place the card in an ESD-safe bag; do not touch the contacts.
3. Insert a 1 GB SINAMICS CompactFlash card that has firmware ≥ 4.5 (and the SINAMICS BootLoader) loaded. The card should click home and sit flush with the bezel.
4. Restore 24 V at X124. Observe the RDY/COM LED:
   • 0 to 5 s: LED off, then yellow flashing 2 Hz → bootloader is reading the card
   • 5 to 30 s: LED green flashing 0.5 Hz → loading firmware from card
   • 30 to 60 s: LED green steady → unit is ready and cyclic operation has started
5. Connect a service PC to the service interface X127 (RJ45) with a standard Ethernet cable. The service interface assigns an IP address via DHCP fallback to 169.254.11.22 / 255.255.0.0 if the host does not have a DHCP server. Open the Web Server by entering the configured IP in the browser (default 169.254.11.22 if untouched) and confirm the unit shows the SINAMICS S120 CU310-2 DP landing page with firmware 4.5.x.
6. Launch STARTER (or TIA Portal with Startdrive) and go online. The unit should report "Online — connection to target system established" and display the drive objects of the connected components.
7. If the cabinet already contains a connected Power Module and DRIVE-CLiQ topology, perform an automatic configuration with the "Configure DDS" / "Automatic configuration" wizard in STARTER. The CU310-2 will read the topology from the connected Motor Modules, Line Modules, and Terminal Modules.
8. Save the project to the new card via the "Copy RAM to ROM" / "Load to target device — Save to card" function in STARTER. This protects the parameter set against power loss. The save is incremental: only changed parameters are written.
9. Cycle power to confirm that the project boots from the card without intervention. The unit must come up to RDY green steady in the same 60 s window.

8. Verification Sequence

1. LED check: RDY/COM must be green steady after the first 60 s of power-up. No red or yellow blink should remain.
2. BOP20 check: BOP20 should display the drive state (e.g. oP = operate-ready, S_ = standby) and respond to key presses. If the BOP20 is still dark, the card was not recognized — re-seat and re-power.
3. Service interface check: Ping the service IP from the service PC. Round-trip time should be < 1 ms on a point-to-point cable.
4. STARTER online check: Right-click the drive in the project tree → "Online → Connect to target system". The status bar must read "Online" and the diagnostic buffer must be queryable.
5. Firmware version check: In STARTER, read parameter r0018 (Control Unit firmware version) and r0019 (BootLoader version). Both must report 4.5.x or higher. The parameter identifier format is x.yy.zz where x is the major, yy the minor, and zz the patch.
6. Diagnostic buffer check: Read the diagnostic buffer (r0947[0...63]) and confirm no entries with the same fault code you started with. The buffer should be either empty or contain only commissioning messages.
7. PROFIBUS / PROFINET check: From the PLC side, confirm the controller is in data exchange with the drive. The SFB / SFC status must read "OK" and the cyclic I/O must be updating.
8. Functional Safety check (if applicable): With Safety Integrated enabled, read r9771 (SI diagnostics) and confirm that the safety signature is consistent with the safety program on the PLC. A mismatch will not stop the boot but will refuse to enable motion.

9. Field-Commissioning Notes and Edge Cases

9.1 Old card works in the old CU310, fails in the CU310-2

This is the most common confusion in the field. The card is not broken; the firmware generation is wrong. Engineers who migrate a project from a CU310 to a CU310-2 and assume the card can be carried over will see the 2 Hz red blink every time. Always commission with a fresh card supplied from Siemens for the new platform.

9.2 Firmware 4.5 HF6 is on the card but the unit still blinks

If the unit still produces the 2 Hz red blink after the swap, the next most likely cause is that the card was formatted on a non-Siemens tool (e.g. Windows formatter) and is missing the SINAMICS BootLoader signature. The card must be re-flashed using the Siemens Web Server (under "Save / Restore") or STARTER (under "Card → Functions → Format card"). Do not use Windows format. A secondary cause is a card that has been re-licensed with a non-Siemens tool and lost the signature: in that case the card is effectively bricked and must be replaced.

9.3 Multiple units, mixed cards

In retrofit programs with mixed CU310 and CU310-2 stock, label every card with the unit it was commissioned on. Use color-coded ESD labels to differentiate legacy (<2.6 SP2) cards from new (≥4.5) cards. The 1-second cost of labeling prevents a 4-hour diagnostic loop the next time someone swaps a card. A reasonable convention: red label for legacy, green label for the -2 platform, blue label for Safety license.

9.4 Mixed firmware in a multi-axis line

If a line is upgraded one axis at a time, the new CU310-2 with firmware 4.5 can communicate on PROFIBUS with older units on firmware 2.6.x as long as the PROFIdrive profile version is set to V3.1 (default) in the project. Mixing V4 (PROFIdrive on PROFINET) and V3.1 in the same bus is allowed but requires consistent telegrams. Confirm with the project planner before commissioning. The PROFIdrive profile version is read from the GSD file revision; V3.1 is the default for the 2.6 SP2 cards and V4.2 is the default for the 4.5 cards.

9.5 DRIVE-CLiQ topology after the card swap

After a card swap, the drive object list and the DRIVE-CLiQ topology are read from the new firmware. If the cabinet has been wired and the components are connected, perform an automatic device configuration: STARTER → drive → "Configuration" → "Automatic configuration". This populates the drive objects based on the connected components, regardless of what was on the previous card. The DRIVE-CLiQ cable length and topology are auto-detected; the only manual entry required is the motor code from the motor nameplate.

9.6 Cold start below 0 °C

The CU310-2 is rated for 0 °C to +55 °C. Below 0 °C the bootloader can stall and present a 2 Hz red blink that is NOT a card issue. Warm the cabinet to above +5 °C and retry. Outdoor cabinets in cold climates should use the cold-climate variant of the card or a thermostatically controlled heater.

9.7 EMC-related boot stalls

If the unit is mounted near a variable-frequency drive output cable or a welding supply, conducted and radiated EMC can corrupt the bootloader read. Route signal cables separately from power cables, use shielded PROFIBUS, and ground the shield at both ends through a low-impedance bond. Symptoms include a 2 Hz red blink that is intermittent and correlated with the operation of nearby equipment.

10. Related Fault Codes on the CU310-2

After the unit is online, the following fault codes are useful for the surrounding commissioning steps. They are not the cause of the 2 Hz red blink (the unit never reaches the runtime to report them), but they are the next class of faults you will see during commissioning and should be familiar with:

11. Boot Sequence State Diagram

The boot sequence of the CU310-2 follows a fixed state machine. Understanding it helps interpret the LED state at any point in time.

12. Compatibility Matrix (CU310 vs CU310-2)

13. Replacement and Spare-Part Sourcing

Order the replacement card through the Siemens regional sales channel with the following decision inputs:

• Target firmware version: 4.5 HF6 minimum, or the latest released version (e.g. 4.6 / 4.7 / 5.x) as advised by Siemens product management at the time of the order.
• Card capacity: 1 GB (mandatory for the -2 platform).
• MLFB family: 6SL3054-0CG0x-1AAx. The trailing digit (0/1/2) encodes the firmware generation; confirm with the Siemens product configurator or your regional sales contact.
• License scope: confirm whether the cabinet needs Safety, High Feature, or Basic licensing. The card carries the license key; an undersized card will boot the unit but will not enable licensed features.

Do not order a card from third-party memory vendors. The SINAMICS BootLoader and the license storage area are signed; a non-Siemens card will not be recognized. Spare stock should be kept in an ESD bag in a dry cabinet at 15–35 °C. Track each spare card in a register that records: MLFB, firmware version, bootloader version, date received, and the unit it has been issued to.

14. Preventive Measures and Lessons Learned

• Mark every CFC with the firmware generation and the commissioning date. The 30 s of discipline at commissioning saves 4 hours the next time a unit fails to boot.
• Store the project backup (the *.par file and the *.dnc archive from STARTER) in a versioned location tied to the unit serial number. The card image is recoverable from the backup if the card is lost.
• Train field engineers to read the firmware version on the card label before insertion. The "FW V" line is the single piece of information that distinguishes a working card from a 2 Hz red-blink card.
• When replacing a CU310 with a CU310-2 in a retrofit, treat the new platform as a new commissioning: new card, new firmware, new project backup, new license check.
• Document the migration in the cabinet's electrical drawing set. A redline in the schematic that points to a "CU310-2DP, FW ≥ 4.5" saves the next engineer 30 minutes of searching.

15. Spare Card Inventory Practice

For a service organization supporting a fleet of SINAMICS S120 drives, the spare card inventory should be managed as follows:

1. Maintain a minimum of two 1 GB cards per site, both loaded with the current released firmware. Rotate them into service every six months to keep the firmware current.
2. Keep at least one legacy 512 MB card on hand for troubleshooting on a legacy CU310. Label it "LEGACY ONLY — do not insert in CU310-2".
3. Log every card issue in a register: card MLFB, firmware version, date issued, unit it was issued to, technician initials.
4. When a card is returned from the field, do not re-shelve it without re-imaging. A card that has been written to a non-standard partition table (e.g. by a third-party tool) is suspect.
5. Discard cards that have been in service for more than 5 years. The flash endurance of consumer-grade CF cards is rated for 100 000 write cycles per sector; in a write-heavy commissioning environment, this corresponds to roughly 5 years of typical use.

16. Related Documentation and References

• Siemens Industry Online Support — firmware downloads, release notes, FAQs
• SINAMICS S120 product support page — product manuals, function manuals, list manuals
• SINAMICS S120 product family page — hardware catalog, selection guides

FAQ