Resolving Sinumerik 840D Alarm 120202 NC-PLC Connection Failure

1. Problem Statement: Alarm 120202 on Sinumerik 840D NCU

The Sinumerik 840D HMI displays alarm 120202 "Waiting for connection to the NC/PLC" whenever the operator panel cannot establish its expected communication link with the Numerical Control Unit (NCU) and the integrated PLC. The HMI top bar collapses to "MASTER" followed by question marks, and the message "Server Init' Communication Lost" is rendered by the machine tool runtime. The NCK 2-digit 7-segment display reads '6', with both POK and PR LEDs illuminated green, and every module to the right of the NCU in the 611D drive stack shows red fault LEDs.

This signature is reproduced across multiple field reports: it begins as an intermittent fault that is cleared by a hard reboot (power off, 10-second pause, power on), and then becomes permanent after an uncontrolled power outage. Once it becomes permanent, simple reboots no longer recover the link and the production cell is stopped.

2. Affected Hardware Identification

Confirm exact catalog (MLFB) numbers before ordering replacement parts so the firmware variant and hardware revision match.

The 6FC5357-0BB25-0AA0 corresponds to an NCU 561.5 / NCU 57x class module used with Sinumerik 840D sl. The 6FC5210-0DF33-2AA0 identifies a PCU-class operator panel running the HMI runtime. Both parts are documented in the Siemens Equipment for Machine Tools catalog and the relevant Siemens Industry Online Support product pages.

3. LED and 7-Segment Code Interpretation

Every NCU 840D exposes a 2-digit 7-segment display plus status LEDs that reveal its boot phase and communication state. Reading these correctly is the fastest path to localizing a 120202 root cause.

3.1 NCK 7-Segment Codes

3.2 Status LEDs on the NCU Front Face

When POK and PR are green and BF is red, the bus fault is the prime suspect. When POK is red or off, the problem is upstream (24 V supply, NCU power section, or backplane).

4. Root Cause Analysis

Alarm 120202 with a stuck 7-segment '6' is most commonly produced by one of the following failure modes, ordered by field frequency:

1. Damaged PROFIBUS / OPI cable or connector between NCU and HMI or between NCU and the segment carrying HMI/MCP.
2. Terminating resistor switch in the wrong position on a PROFIBUS DP or OPI connector, causing reflections that prevent a stable link.
3. Loss of the 611D drive bus - red LEDs on every drive module can halt the NCK handshake if the drive enable chain is broken.
4. Corrupt firmware or NV-RAM contents after uncontrolled shutdown (power loss mid-write).
5. Failed NCU module - rare but possible after a surge event coincident with the power failure.
6. Failed HMI/PCU module or its backplane connector.

The reported history (intermittent failure escalating after a power outage) is a textbook signature of a partially broken bus connection that survives enough reboots to occasionally reconnect, then gives up after the surge event damages a connector or corrupts the retain image.

5. Safety and Pre-Troubleshooting Checklist

1. Capture exact catalog numbers of NCU, HMI, and every 611D module to the right of the NCU.
2. Document every LED color on the NCU face and every drive module face.
3. Note the 7-segment code at power-on and whether it changes during the boot sequence.
4. Measure battery voltage under load: 3.0 V is at the lower functional bound; replace any battery that has been in service more than 5 years or measures below 3.0 V.
5. Confirm no recent mechanical work on the cabinet that may have stressed PROFIBUS / OPI cables.
6. Photograph the cabinet before and after any re-termination to preserve evidence of the original wiring.

6. Step-by-Step Troubleshooting Procedure

Step 1 - Power-Down and Visual Inspection

1. Open the main disconnect and lock-out / tag-out.
2. Wait the manufacturer-specified discharge time and verify with a multimeter that the 611D DC bus is below 5 V DC.
3. Open the cabinet and inspect visually: burnt connectors, broken retaining screws, pinched PROFIBUS cables, coolant or oil ingress, rodent damage.
4. Replace any connector with visible discoloration, carbon tracking, or oxidized pins.

Step 2 - Verify the OPI Link (NCU ↔ HMI)

The OPI bus is the dedicated PROFIBUS-derived link that carries HMI ↔ NC traffic. On NCU 561 / 571 modules the OPI port is on the top-right of the module.

1. Locate the OPI port - typically the right-most PROFIBUS-style D-sub on the top edge of the NCU.
2. Trace the cable directly to the HMI/PCU. Confirm it is the OPI cable and not the PROFIBUS DP cable by following the wire path; the OPI cable lands on the X1/X2 connector of the PCU.
3. Disconnect the OPI cable at both ends. Measure each conductor for end-to-end continuity. Reading should be below 50 Ω for the data pair and below 1 Ω for the shield.
4. Measure conductor-to-shield insulation. Reading below 1 MΩ indicates damaged insulation, usually from a pinch or from coolant attack.
5. Replace the OPI cable with a known-good shielded twin-axial PROFIBUS cable (Siemens 6XV1830-0AH10 or equivalent).

Step 3 - PROFIBUS DP Cable and Connector Verification

The PROFIBUS DP bus typically runs from the NCU out to I/O stations, drives, and possibly the MCP. Connector faults are the single most common cause of 120202.

1. Inspect each PROFIBUS DP connector for bent pins, loose retaining screws, missing strain relief, or a crushed cable jacket.
2. Confirm the ON/OFF terminating-resistor switch on every connector:

3. A frequent field mistake is leaving the terminator ON at a middle NCU port when the OPI cable also terminates at that node.
4. Re-make any connector showing wear. Use only Siemens 6ES7972 style PROFIBUS connectors with integrated termination and a diagnostic LED.
5. After re-termination, observe the diagnostic LED on each connector: it should pulse steadily when traffic is present, and remain off only at unpowered or unconnected nodes.

Step 4 - Verify the 611D Drive Bus

A red fault LED on every module to the right of the NCU indicates the drive bus is not communicating. The drive bus is a proprietary bus that connects the NCU to the 611D power modules via ribbon cable (older systems) or fiber (newer systems), depending on configuration.

1. Identify every 611D module (611D infeed, axis modules, spindle modules) and note its fault LED state.
2. Power down. Remove and reseat the drive-bus ribbon cable between the NCU and the first 611D module.
3. Inspect the ribbon cable for cracked insulation at the bend, oxidized connector pins, or misaligned insertion (offset by one row is a frequent field error).
4. Verify the 24 V supply to the 611D infeed module. Loss of 24 V will latch every downstream module into a red fault state.
5. With the drive bus restored, power up and observe whether the 611D red LEDs clear during the NCU boot sequence.

Step 5 - Bypass Test of MCP / Keyboard Bus

The reported failure history mentioned the keyboard LEDs flashing together, which implicates the MCP bus.

1. Disconnect the MCP at the bus connector closest to the NCU.
2. Power up and observe the 7-segment code. If the code advances past '6' the MCP bus or MCP itself is the fault.
3. Try a direct (jumper) OPI cable from the NCU to the HMI without any intermediate PROFIBUS hardware. If this does not change the symptom, the fault is in the NCU, its OPI port, the HMI port, or in firmware state.

Step 6 - Use SIMATIC Manager for Online Diagnostics

When physical-layer checks pass but 120202 persists, online diagnostics from a service PG can pinpoint the failure.

1. Connect a service PG with SIMATIC Manager (Step 7 V5.5 SPx) or TIA Portal V15+ with the Sinumerik add-in to the NCU X127 service port.
2. Configure the PG Ethernet interface for the default 192.168.214.0/24 subnet; target the NCU at 192.168.214.1.
3. Open the online view and check Accessible Nodes. The NCU should appear with its PROFIBUS DP and OPI nodes listed.
4. Open the PLC online diagnostic buffer and record every entry with timestamp. Typical signatures are: "Peripheral fault", "OB not loaded", "DP slave failure", "Station failure".
5. Open the NCK online diagnostic buffer (from HMI or via the service PG). Look for "120202", "120200", "120201", "380001", or PROFIBUS framing errors.
6. Compare the OPI node count and the DP node count to the expected configuration; a missing node is the fault location.

Step 7 - Re-Image NCK and PLC Firmware

If bus diagnostics pass but 120202 persists, the NCK or PLC retain image may be corrupted by the power failure.

1. From the service PG, issue an NCK reset followed by a PLC stop sequence from the online menu.
2. Re-load the NCK firmware from a backup archive on the machine maker's CF card.
3. Re-load the PLC project (.s7p or TIA archive).
4. Cold restart (NCK power-on reset) and observe the 7-segment transition through 1 → 2 → 3 → 4 → 5 → 6 → 8.
5. Confirm all axes reference and that the part program executes once before returning the machine to production.

Step 8 - Replace Suspect Components

1. If the OPI port on the NCU is faulty (verified by direct cable swap with a known-good NCU), replace the NCU.
2. If the HMI port is faulty (verified by trying a known-good HMI), replace the HMI or its backplane.
3. Always replace the buffer battery at the same time as any NCU replacement, and date-mark the new battery.
4. For 611D modules showing red LEDs that do not clear after reseating the drive bus, replace the infeed module first; axis modules will often recover once the infeed is healthy.

7. PROFIBUS Physical-Layer Reference

8. Battery and Retain-Data Verification

The NCU 561.5 uses a lithium buffer battery to retain SRAM contents across power cycles. The reported +3.0 V is at the lower functional bound.

9. Related Fault Signatures

10. Verification After Repair

Apply the following checks after every repair step to confirm a stable recovery:

1. 7-segment transitions 1 → 2 → 3 → 4 → 5 → 6 → 8 during boot and stops on '8'.
2. Alarm 120202 does not reappear within 10 minutes of operation.
3. HMI displays the operating-area screen (not "MASTER" + question marks).
4. All 611D drive LEDs are green.
5. MCP LEDs respond to mode selection.
6. Perform an NCK reset from the HMI and confirm normal ramp-up.
7. Cycle power two more times to confirm the link is stable.
8. Run a small part program in single block and verify axis motion before returning the machine to production.

11. Spare Parts and Documentation Pointers

• NCU: 6FC5357-0BB25-0AA0 - keep one cold spare if the machine is critical.
• HMI: 6FC5210-0DF33-2AA0 - confirm CF card content is cloned to the spare.
• PROFIBUS connectors: 6ES7972-0BA12-0XA0 (with diagnostic LED) - stock at least four per cabinet.
• OPI cable: 6XV1830-0AH10 - stock one pre-terminated spare.
• Battery: 6FC5247-0AA00-0AA0 - stock two, replace every 5 years.

Always verify the active firmware version on the spare NCU before installing it. The firmware version on a replacement NCU must match the project that the machine builder commissioned, or the retain data will not load. Use Siemens Industry Online Support to confirm firmware compatibility for the 6FC5357-0BB25-0AA0 hardware revision.