Sinumerik 840D All PLC LEDs Lit: NCU Diagnostic Procedure

1. Problem Definition

A Sinumerik 840D CNC machine is switched on and every PLC status LED on every module in the PLC rack lights simultaneously, while the NCK (Numerical Control Kernel) LEDs behave as expected for normal operation. The seven-segment status indicator on the NCU (Numerical Control Unit) alternates between 0 and b. The fault is reproducible from a cold start and survives a reset of the NCU. The question is binary: is this a hardware failure (NCU, PLC CPU, I/O module, power supply) or a software/configuration problem (PLC program, firmware, PROFIBUS/PROFINET topology, distributed I/O addressing)?

This article defines the diagnostic procedure to isolate the root cause, separates power-supply faults from module faults, and documents the verification steps that confirm a healthy system once the offending module or wiring is replaced.

2. Sinumerik 840D System Architecture Relevant to This Fault

The integrated PLC inside the 840D is based on the SIMATIC S7-300 family. Understanding which LED belongs to which subsystem is the first step in diagnosis. The relevant building blocks are:

• NCU (e.g. NCU 710.1, NCU 720.1, NCU 730.1, NCU 751, NCU 760). Houses the NCK and the PLC CPU in the sl line, or interfaces to an external PLC CPU in older 840D (powerline) configurations. NCU LEDs indicate NCK state; PLC LEDs are on the integrated CPU or on a separate S7-300 CPU plugged next to the NCU.
• S7-300 PLC CPU (CPU 314, 315, 317, 317F, 319F). In an 840D sl the PLC is integrated into the NCU module. In an 840D powerline the CPU is a separate S7-300 module with its own LED bar (SF, BF, DC5V, FRCE, RUN, STOP).
• IM 360 / IM 361 / IM 365 interface modules. Connect the CPU rail to the expansion racks that host the digital I/O.
• PP 72/48 or PP 72/48D I/O modules. 72 digital inputs / 48 digital outputs on a single module, designed for spindle and axis drives on the machine. Each PP has per-channel status LEDs.
• SM 321 / SM 322 / SM 323 / SM 331 / SM 332 S7-300 modules. Standard SIMATIC I/O if the OEM chose not to use the PP family.
• MCP (Machine Control Panel), MPP, HHU. Hand-held units on PROFIBUS or PROFINET with their own LEDs.
• OP (Operator Panel) 010 / OP 012 / OP 015 / OP 019 / TCU (Thin Client Unit). HMI; runs HMI Advanced or SINUMERIK Operate.
• SITOP power supply (6EP1334-3BA10 or similar 24 VDC). Feeds the PLC backplane and the I/O modules.

Reference: Siemens Sinumerik 840D sl Function Manual (Basics) and the Sinumerik 840D sl System Manual.

3. LED Status Map: What Each LED Should and Should Not Do

When every LED on every module lights at the same time, the most common cause is not that the PLC is reporting one fault per module. It is that all of the LEDs share a single supply rail or a single reference (0 V) that has shifted in voltage, energising optocouplers and LED drivers across the entire rack.

4. Seven-Segment Display Codes on the NCU

The two-digit 7-segment display on the front of the NCU 7x0 reports the combined NCK/PLC boot state. The codes below are taken from the Sinumerik 840D sl Function Manual diagnostics chapter.

An alternating 0 and b pattern means the NCK is healthy (state 0) but the PLC is repeatedly being held in STOP or cycling between RUN and STOP because of a recurring fault or because the PLC has not been started yet from the HMI. The LED "all lit" symptom is consistent with a PLC that is permanently in STOP: every input is read as high-impedance false, every output remains de-energised, but the LED drivers on the I/O modules share the 24 V bus, so without 0 V referenced correctly the LEDs can be partially lit due to leakage.

5. Diagnostic Methodology - Order of Elimination

Do not start swapping modules. The elimination order below has been field-proven to identify the root cause in the fewest steps.

1. Capture all alarm messages on the OP (top-of-screen red banner) and the active alarm history (Diagnostics → Alarm Log). Save to a USB stick if possible.
2. Read the NCU seven-segment code before and after a power cycle. Note whether the code is stable or oscillating.
3. Check the PLC operating state via the HMI: Commissioning → PLC → Status (HMI Advanced) or Menu Select → Commissioning → PLC (SINUMERIK Operate). The state should be RUN; a value of STOP confirms the PLC CPU is not executing OB 1.
4. Verify the 24 VDC supply at the SITOP (or equivalent) power supply: measure between +24V and 0V with a calibrated multimeter. The tolerance is 20.4 V to 28.8 V (per Siemens SITOP 6EP1334 spec). Anything outside that band is suspect.
5. Measure the 0 V referenced back to the cabinet PE bar. There must be no measurable AC voltage between 0 V and PE; more than 0.5 VAC indicates a ground loop or a broken 0 V conductor.
6. Inspect every I/O module's 0V terminal. A loose terminal screw is the single most common cause of "all LEDs on" on a single rack.
7. With the machine de-energised, isolate each PP 72/48 or SM module in turn by unplugging its front connector, and re-power. If the symptom clears on a specific module when disconnected, that module is the fault source.
8. Remove the IM 361 to disconnect expansion racks. If the symptom clears, the fault is downstream of the IM.
9. Finally, suspect the NCU itself or the PLC CPU. With every external connector removed and only the backplane powered, if the LEDs are still lit, the NCU/CPU module or its backplane is faulty.

6. Root Cause 1 - Lifted 0 V Reference (Leakage Current)

The dominant root cause for "every LED lit on every module" is a missing or high-impedance 0 V reference on the I/O modules. The PLC input optocouplers need a return path through 0 V to sink the input current. When 0 V is open:

• Each input optocoupler's internal LED is biased through parasitic paths and through the LED driver circuitry of neighbouring channels.
• Because the input LEDs are wired with a common anode to 24 V and common cathode to 0 V, when 0 V is open they are left floating; leakage from the 24 V bus through the LED's reverse-bias capacitance, or through the PLC CPU's input buffer pull-ups, can pull enough current through the LEDs to light them dimly.
• The output LEDs on PP 72/48 and SM 322 modules share the same 0 V return for the status indicator. If 0 V is lifted, every status indicator can light even when the corresponding bit is 0.

Diagnostic test (machine off, locked out):

1. Disconnect the front connectors of every I/O module.
2. Re-apply 24 VDC only to the PLC backplane.
3. Measure 0 V to PE at the I/O terminal block. Reading must be < 1 VAC and 0 VDC.
4. Re-connect connectors one by one and re-measure after each step. The moment 0 V rises above 0.5 V AC or 0.5 V DC against PE, the last connector added has a 0 V break.

Reference: S7-300 Signal Modules data sheet (input circuit diagram).

7. Root Cause 2 - PLC CPU in STOP, PLC Program Missing or Corrupt

If the PLC CPU is in STOP, the seven-segment on the NCU will display 1, 5, 6, flashing b, or F. In this state, outputs are forced to their safe/initial state (typically 0), but the LED indicator on each output module is wired to the output driver, not to the bit. Therefore, if the PLC is in STOP and the output is being forced high, every output LED stays on.

Causes of PLC STOP on an 840D:

• PLC project on the CF card is corrupt or missing.
• OB 100 (restart) or OB 1 errored and called SFC 46 "STP" (STOP).
• Diagnostic interrupt from a smart I/O module (OB 82 not loaded) causes CPU to go to STOP.
• The PLC was stopped from the HMI and not started again.
• PROFIBUS/PROFINET slave failure pulled the CPU into STOP via OB 86 (not loaded).

Read the PLC diagnostic buffer through the SINUMERIK Operate menu: Commissioning → PLC → Diagnostic Buffer. The most recent event will name the OB that faulted and the address that triggered it. Cross-reference the address against the hardware configuration in TIA Portal or STEP 7.

To start the PLC manually:

1. From HMI Advanced or SINUMERIK Operate, navigate to Commissioning → PLC → Operating Mode.
2. Select Start PLC (warm restart). The seven-segment should transition from 1/6/b to 0 or 7.
3. If the CPU immediately returns to STOP, open the diagnostic buffer and read the cause.

Reference: Sinumerik 840D sl System Manual - PLC Integration.

8. Root Cause 3 - I/O Module Internal Fault

A single shorted I/O module can drag the rack's 0 V or 24 V down to a level that lights every other module's LEDs. Specifically:

• PP 72/48 (6FC1111-0CA00-0AA0 family): a failed output transistor on any channel can short 24 V to the diagnostic LED rail. Every LED on the rack will light.
• SM 321 digital input modules: a damaged input optocoupler can latch the input bus high. Combined with a software fault, every input bit can appear TRUE simultaneously.
• SM 322 digital output modules: a shorted high-side driver keeps the corresponding output and LED on. If multiple outputs share a common driver IC, the whole module's LEDs light.

Test by hot-swapping the modules on a known-good spare. Power down, swap, power up. Repeat until the suspect module is isolated.

Reference: Sinumerik 840D sl PP 72/48 Manual.

9. Root Cause 4 - NCU / PLC CPU Hardware Failure

If all external wiring is removed, the I/O modules are unplugged, the IM 361 is out, and the symptom persists with the seven-segment oscillating between 0 and b, the NCU's integrated PLC CPU is the prime suspect. Failure modes documented in Siemens field service notes include:

• Defective backplane connector on the NCU causing the PLC CPU to repeatedly reset.
• Corrupt firmware on the NCU CF card (card replaced with wrong image; license binding fault).
• ESD damage to the PLC CPU ASIC.
• Failed 24 VDC input on the NCU causing brown-out resets.

For NCU 710.1 / 720.1 / 730.1: the part number is e.g. 6FC5357-0BB10-0AA0 (NCU 720.1 PN). For NCU 751/760: 6FC5356-.... Replacement NCUs must be loaded with the same firmware version as the original (check via Commissioning → System Info) and the same PLC project, otherwise the PLC will fail to start.

Reference: NCU Replacement Procedure (Siemens KB).

10. NCK LED Normal vs PLC LED Abnormal - What It Tells You

If the NCK LEDs on the NCU behave normally (steady green, seven-segment reaches 0) but the PLC LEDs on every module light, the NCU's NCK subsystem is healthy and the PLC CPU subsystem is the fault zone. This split is the single most useful piece of information at the start of diagnosis.

The seven-segment alternating 0/b pattern in the reported case is the NCK reaching the ready state and the PLC repeatedly being stopped or held in STOP. The reason every LED is lit is the symptom of the PLC being unable to scan, combined with the lifted 0 V and/or stopped CPU.

11. Diagnostic Flowchart

12. Software/Firmware Considerations

If the diagnostic procedure above points at the PLC program (CPU returns to STOP immediately after start), the following firmware/project combinations are the most common offenders on 840D sl installations:

Reference: SINUMERIK Operate Commissioning Manual and 840D sl Upgrade/Upgrade Instructions.

If the NCU was recently replaced, confirm:

1. The CF card firmware matches the original NCU model (e.g. NCU 720.1 PN card 6FC5800-0AS00-0YB0 for FW 4.5 SP4).
2. The PLC project archive (.zip) was reloaded with the original program ID.
3. PROFINET device names were re-assigned (Commissioning → PROFINET → Topology).

13. Verification and Re-Commissioning

Once the offending module or wiring has been corrected, perform the following verification before returning the machine to production:

1. Power down, lock out, re-energise.
2. Confirm seven-segment reads 0 or 7 within 60 seconds.
3. Confirm PLC CPU RUN LED is steady green, STOP LED is OFF.
4. Confirm exactly the LEDs that should be on (driven by active signals) are on; all others are OFF.
5. From HMI, open Commissioning → System Info and verify the NCK and PLC versions match the backup records.
6. Open the alarm log and confirm no active alarms (or only known cosmetic alarms).
7. Jog each axis in JOG mode at 5% rapid; verify drive enable LEDs illuminate only when commanded.
8. Test E-Stop chain (axis drives disable, feed-hold asserted, PLC input E-Stop reads TRUE).
9. Run a part program with the safety door closed and verify the program executes to M30.
10. Sign off the commissioning checklist and back up the NCU CF card image.

14. Troubleshooting Matrix

15. Preventive Measures for Future Occurrences

• Tighten every I/O terminal screw to 0.6 Nm during every scheduled maintenance.
• Use Phoenix or Weidmuller terminal blocks with screw retention; vibration is the primary cause of terminal loosening on milling and turning centers.
• Install the PLC and I/O rack on anti-vibration mounts if the cabinet is mounted on the machine column.
• Schedule quarterly backup of the NCU CF card image and the PLC project archive.
• Monitor SITOP output voltage and 24 V bus with a Siemens SITOP PSE200U or LOGO! CMR for remote diagnostics.
• Keep one spare PP 72/48, one spare SITOP, and (for critical machines) one spare NCU on the shelf with the matching firmware pre-loaded.
• Document the PLC diagnostic buffer routine check into the weekly maintenance procedure.

16. Frequently Asked Questions