Resolving SINUMERIK 840D NCU PF LED 8-Blink PLC Watchdog Fault

1. Problem Definition: PF LED 8-Blink on SINUMERIK 840D NCU

The SINUMERIK 840D Numerical Control Unit (NCU) on machine tools relies on a tightly integrated triad of subsystems: the NCK (NC Kernel) for path and axis interpolation, the integrated SIMATIC S7 PLC for machine logic, and the HMI for operator interface. The status LED field on the front panel of the NCU is the first diagnostic surface an automation engineer or service technician observes when the controller fails to bring a machine out of STOP or READY state.

An eight-blink pulse train on the red PF (Power Fault) LED, while the green POK LED remains lit and the seven-segment H3 display shows the digit "6", indicates a PLC watchdog fault inside the integrated PLC submodule of the NCU. This code corresponds to a fatal PLC stop condition in which the integrated S7 CPU has either tripped its hardware watchdog because the OB1 scan time exceeded the configured watchdog time, or the PLC submodule hardware itself has failed and is no longer responding to the NCK over the internal backplane.

The PLC watchdog condition must be cleared before any further diagnostic communication can occur with the integrated PLC. Because the fault originates below the NCK/HMI layer, online access by STEP 7, HMI Advanced, or commissioning tools is blocked until the PLC resets cleanly or the offending submodule is replaced.

2. NCU 840D Powerline LED Status Indicator Reference

The NCU 573.3 front panel exposes a structured indicator field. Engineering each LED state against the operator components documentation is the first step in fault isolation. Reference: Siemens Industry Online Support search term "SINUMERIK 840D NCU Operator Components".

The PF LED is a coded fault indicator: each count of blinks between long pauses maps to a specific fault class. For the NCU 840D Powerline family the most common blink codes are summarized below.

3. H3 Numeric Display Interpretation

The two-digit H3 seven-segment display on the NCU front panel complements the LED fault field. It distinguishes NCK from PLC subsystem events even when the PF LED pattern is identical.

An H3 reading of "6" combined with PF blink 8 is the unambiguous signature of a PLC watchdog trip. A reading of "8" with PF blink 8 instead indicates PLC cleanly stopped – a different remediation path (program re-download without submodule replacement).

4. Alarm 120202: Communication Failure Analysis

When the integrated PLC is in watchdog trip state, every higher-layer communication attempt fails with consistent behavior on the SINUMERIK 840D side:

• Alarm 120202 – HMI/NCK channels report failure to establish the expected handshake with the PLC. The HMI Advanced screen shows this as an unacknowledgable error banner.
• MPI 187.5 kbit/s on CP5611 (X122 PG port) – the Field PG running STEP 7 / SIMATIC Manager v5.5 cannot enter RUN mode; an "Online: error – impossible" message appears, or the connection stays in STOP.
• MPI 1.5 Mbit/s on X101 – the HMI Advanced software likewise cannot place either NCK or PLC online.

Reference: SINUMERIK 840D Diagnostics Manual, alarm number 120202 chapter; Siemens Industry Online Support entry 109476350 (840D sl NCU state displays – analogous definitions apply to the NCU 573 Powerline).

5. Root Cause: PLC Watchdog and Submodule Failure

Two failure classes can trigger the 8-blink PF + H3="6" signature:

1. Software / scan-time watchdog trip: the PLC user program contains an OB1 scan that exceeds the configured OB1 watchdog time, or a non-maskable interrupt (OB82, OB121, OB122) was triggered by an I/O fault and the CPU went STOP with watchdog. Recovery: MRES reset, reload PLC user program, re-establish communication.
2. Hardware failure on the PLC submodule: the integrated PLC module itself is damaged. In the NCU 573.3 Powerline architecture, the PLC is implemented as a discrete submodule (a separate PCB carrying the S7 CPU and its RAM/Flash) that plugs into the NCU backplane. If this submodule has failed electrically, no MRES sequence will recover it.

Field-experienced root cause in this case study: the NCU enclosure had been contaminated by a failed backup battery that vented electrolyte and produced conductive corrosion across multiple PCB traces – including the battery-backed SRAM area and the PLC submodule connector. After complete cleaning (capacitors, fans, battery holder, edge connectors, traces), the PLC still would not come out of watchdog. Cross-swapping the PLC submodule into a known-good NCU 573.3 PII confirmed the PLC module 570572.9202 as the failed component.

6. Diagnostic Procedure Before Replacement

Before assuming hardware failure, perform a deterministic reset sequence. This rules out scan-time or program-corruption causes and confirms whether a submodule swap is required.

6.1 Preconditions

• Validated NCK backup file (PLC and NCK series startup files)
• Validated PLC backup file (S7 project archive)
• Field PG with STEP 7 V5.5 + SIMATIC Manager, CP5611 MPI cable
• External HMI Advanced PC with MPI cable to X101, OR an HMI Advanced USB back-up
• Anti-static workstation, replacement PLC submodule on hand

6.2 Visual / Mechanical Inspection

1. Disconnect the NCU from mains and wait 5 minutes for DC-link capacitors to discharge before opening the housing. Confirm with a calibrated multimeter that DC bus is below 30 V DC.
2. Inspect the backup battery holder (typically a 3.6 V lithium unit on the NCU carrier board). Look for white/green crusty corrosion, electrolyte residue, or PCB discoloration.
3. Inspect all electrolytic capacitors for domed tops, leak paths, and PCB staining.
4. Inspect the PLC submodule connector pins for tarnish, bending, or residue. Reseat once, then re-test before any replacement.
5. Inspect all internal fans; a stalled fan on a running NCU can elevate local temperature and provoke intermittent PLC faults. Replace any fan not spinning freely.
6. Vacuum and IPA-clean all PCBs; do not use aggressive solvents on plastic retainers or labels.

6.3 Connect Field PG and Acquire Fault State

1. Plug CP5611 into the -X122 PG MPI port. Configure STEP 7 interface to CP5611 (MPI) 187.5 kbit/s, slot 2 default for NCU 573 PLC.
2. Attempt "Accessible Nodes" (PLC → Editor → Accessible Nodes). The PLC should appear but show RUN/STOP mismatch or "faulty module".
3. Attempt Online → Module Information. The diagnostic buffer should display entries for OB1 watchdog, OB82 / OB121 / OB122, or "PI service refused".
4. Capture screenshot of the diagnostic buffer for the service report.

7. PLC Stop / MRES Reset Sequence

The PLC reset key sequence on NCU 573.3 is initiated from the front panel. Use only the documented procedure below.

1. Set mode selector to STOP.
2. Hold MRES for at least 3 seconds until the PS / PR LED pattern indicates "request to reset".
3. Release MRES, then within 1 second press MRES again briefly, then again. The standard Siemens S7 "STOP → MRES → STOP (2-3-2)" reset is performed for the integrated PLC. The exact timing pattern reference: 2 sec – 3 sec – 2 sec between button events.
4. Observe the LEDs. PS should remain solid green, PF should remain off and stay off if the PLC recovers. PR will toggle as NCK re-initializes.
5. If the watchdog returns immediately (PF blink 8 within 10 s of stop exit), the fault is hardware-class and the submodule must be replaced.

8. Solution: PLC Submodule Identification and Replacement

When MRES does not clear the watchdog, isolate the PLC submodule.

8.1 Confirmation by Cross-Swap

1. Acquire or borrow a known-good NCU 573.3 power-line unit (PII test stand).
2. Power down both NCUs.
3. Extract the suspect PLC submodule from the failed NCU; insert it into the test NCU. Power up.
4. Power down again; insert the test NCU's PLC submodule into the original NCU. Power up.
5. If the original NCU now runs without PF blink 8, the original PLC submodule is the failed component.

8.2 Replacement Part

The PLC submodule identified in this case is Siemens order number 6FC5703-0AF21-0AA0 / internal designation 570572.9202. Because this is a legacy SINUMERIK 840D Powerline component, stock is constrained. Procurement channels:

• Authoritative spare-part request via Siemens Industry Online Support Spares & Repairs service – quote the 6FC machine serial number.
• Authorized Siemens CNC distributor spare-part desk (search under "SINUMERIK 840D Powerline spares").
• Be aware that 840D Powerline spares have transition-class lifecycle – confirm current availability through Siemens directly rather than relying on third-party catalogs.

8.3 Replacement Procedure

1. Power down NCU. Wait 5 minutes for DC-link discharge. Verify with multimeter.
2. Open the housing; locate the PLC submodule slot (typically marked "PLC" on the carrier).
3. Remove the failed submodule, place it in an ESD-safe bag. Do not stack or apply mechanical force to the connector.
4. Insert the replacement PLC submodule; verify seating on both connector pin rows. Torque any retaining screws to the documented value.
5. Reconnect the battery, fans, and any cables removed earlier.
6. Close the housing. Power up.

9. Comprehensive Cleaning and Restoration

If the NCU enclosure showed battery-electrolyte corrosion, the cleaning protocol is non-negotiable. Skipping it leaves a high-impedance leakage path that will destabilize the replacement PLC module within weeks.

1. Mechanical removal: Use a soft ESD-safe brush and vacuum with an antistatic hose. Remove loose crystalline deposit first.
2. Isopropyl alcohol wash: 99% IPA on lint-free swabs. Work in a single direction to lift residue, do not smear.
3. Neutralization: For alkaline battery residue, lightly apply a mild acid neutralizer (e.g., a few percent citric-acid solution), then re-flush with IPA. Avoid acid on the battery holder itself.
4. Trace inspection: Under 10× magnification, inspect each PCB trace in the corrosion path for green / black discoloration that has etched into the copper. If a trace is broken, jumper with 30 AWG Kynar-insulated wire following the original topology.
5. Connector pins: Polish pins with a fiberglass brush intended for electronics (e.g., MG Chemicals 824-1). Do not use steel wool.
6. Replace the backup battery with a new 3.6 V lithium unit rated for the NCU (typically a Saft LS 14250 or Siemens equivalent).
7. Replace all cooling fans in the housing with the correct orientation (blower + extractor) and confirmed rotation.
8. Burn-in test: Run the NCU with the new PLC module for at least 4 hours under load with the HMI connected, verifying PS / PR / POK stay green and PF stays off.

10. Backup Restoration and Verification

Once the replaced NCU is running the PLC RUN-status, restore the machine backups.

10.1 Restore Sequence

1. Connect Field PG via MPI at 187.5 kbit/s.
2. SIMATIC Manager → PLC → Download to Target Station → select archived S7 project. Confirm STOP during download.
3. Restart PLC to RUN.
4. Switch to HMI Advanced (X101, 1.5 Mbit/s) and perform NCK series startup file restore.
5. Verify axis reference-point homing before running any G-code program.

10.2 LED Verification Matrix

10.3 Final Verification

• Perform dry-run of axis JOG in both directions on every axis.
• Verify spindle speed command and feedback at 0, 50, 100 % in override.
• Issue the M00 (program stop) and M30 (program end) to verify PLC logic paths.
• Run a circularity / positioning test.
• Document the new PLC submodule serial number and hour-meter reading on the machine log.

11. Preventive Maintenance Checklist

12. Frequently Asked Questions