Troubleshooting Siemens 810D PLC Stuck on 8 with SIMODRIVE 611D

1. Problem Statement: 810D Boot Failure with PLC Stopped at Status 8

A ROMI CNC lathe equipped with a Siemens SINUMERIK 810D controller (typically paired with SIMODRIVE 611 digital power sections) developed a hard boot fault after a spindle warm-up ramp. The observed sequence is diagnostic in itself:

1. Power-on, NC reference, spindle ramped to 500 RPM for warm-up.
2. Approximately five minutes into warm-up, the spindle decelerated and stopped without operator input.
3. The HMI subsequently reported "NCK no sign of life" (alarm area cleared, communication to the NCK software lost).
4. A subsequent cold start yields no boot: the HMI remains dark or stalls, and the integrated PLC status display remains frozen on 8.
5. Battery replacement and NCK general reset have been attempted; PLC general reset does not advance past status 8.

The OEM's conclusion is that the SIMODRIVE 611D is faulty. This guide walks through the engineering analysis that leads to that conclusion, the verification checks that should confirm it, and the reset procedures that must be executed before declaring the drive module dead.

2. SINUMERIK 810D System Architecture

The 810D is a compact CNC integrating the NCK (NC kernel), PLC 314/315, and operator panel interfaces into a single NCU (Numerical Control Unit) module, typically the NCU 571.2 or 572.2, mounted on the back of the operator panel or in a separate control cabinet slot. Drive communication runs over PROFIBUS DP to the SIMODRIVE 611D line.

Critical 810D components for this fault class:

3. Why the Spindle Ramps Down Before the NCK Dies

The sequence "spindle ramped to 500 RPM, decelerated, stopped, then NCK lost" is a classic power-section failure signature:

• At 500 RPM, the spindle module is drawing continuous current but the IGBT thermal envelope is not stressed. The 611D monitors the IGBT junction temperature, heatsink temperature, DC bus voltage, and motor temperature continuously.
• Deceleration that begins without a programmed M-code points to a fault transition - either the drive tripped on F008 (DC bus undervoltage), F006 (DC bus overvoltage), F011 (heatsink overtemperature), or F023 (motor overtemperature), or the PROFIBUS telegram from the NCU dropped mid-cycle.
• If the 611D power section shorted a phase leg or the I/R (line) module lost control, the DC link collapses to 0 V, the NCU loses its 24 V / 5 V supply rail from the system PSU, and the NCK resets while the PLC catches the last communication before losing the host link.

4. Decoding "PLC Stuck on Status 8"

The integrated PLC 314/315 of the 810D cycles through numbered status states during boot and during normal operation. The display appears either on the HMI status bar (under Diagnosis > PLC Status) or on the seven-segment indicator on the NCU front panel on later 810D hardware. Status 8 is one of the most common "stuck" states.

Status 8 on an 810D PLC means: PLC is in STOP and the boot sequence has reached the point of NCK link handshake, but the NCK is not responding. In other words, the PLC has finished loading, the OS is up, the user program has been parsed, and the I/O bus has been scanned, but the PLC cannot complete its host handshake with the NCK because the NCK is dead.

This state is fundamentally different from:

5. Why "NCK No Sign of Life" with the 611D as Suspect

The NCK and PLC are co-resident on the same NCU module. The 611D does not directly host the NCK. The suspect relationship is therefore indirect, but it is well known in field service:

1. The 611D I/R (line infeed) module and UE (power) modules share the same 600 V DC bus.
2. The NCU 24 V supply is derived from the system power supply, which is often a 6SN1 122 module or a SITOP power supply fed from the same mains.
3. If the 611D shorts internally, the upstream circuit breaker trips, and the entire cabinet loses mains, taking NCU power with it.
4. If the 611D I/R module's pre-charge resistors or DC bus monitoring fail, the DC link can spike above 700 V during regen, which the NCU 24 V supply rejects, causing NCU brown-out. The NCK restarts; the PLC does not because the PCMCIA I/O is interrupted before the PLC state can be re-established.
5. A 611D module that pulls excessive current on the PROFIBUS DP segment can also corrupt NCU communication, leaving the NCK in a "no sign of life" state even when the NCU hardware is intact.

6. Diagnostic Decision Tree

7. Pre-Power Diagnostic Procedure (Power OFF)

With the machine locked out, mains isolated, and the DC bus confirmed discharged, perform the following before powering on again:

1. Visual inspection of every 611D module: open the front covers, look for blown IGBT housings (black residue, cracked plastic), discolored PCB, popped capacitors, or burnt smell. Identify each module by its 6SN1118-0xxx-xx-xx order number and serial plate.
2. Check the I/R module pre-charge resistors: with the module removed, measure between the AC input and the DC bus +. Open pre-charge resistors are a common cause of "DC bus 0 V on power-up" symptoms.
3. Insulation test each motor: with motor leads disconnected from the 611D terminal block, megger at 500 V from each phase to ground. Reading < 1 MΩ indicates winding-to-ground short. For a 1PH6/1PL6 spindle, readings of 100 MΩ or more are normal.
4. Check the motor power cable shield and shield grounding: shield break or ground loop can drive high dv/dt current into the 611D current sensors and force a fault.
5. Inspect the PCMCIA card on the NCU: remove, examine contacts for oxidation, re-seat firmly. Verify the card is the correct one (NCK, PLC, drives, and TOOLBOX are typically four cards or one multi-image card depending on configuration).
6. Verify the 24 V battery: measure open-circuit voltage of the 3.6 V lithium backup battery. Replace if below 3.2 V. Note that battery replacement alone does not fix a corrupt SRAM if the previous battery had been low for a long period - the NCK general reset is then mandatory.

8. Power-On Diagnostic Procedure

1. Reconnect mains, close cabinet, do not start spindle drive enable yet.
2. Power the system. Observe the 7-segment PLC status on the NCU front panel and the HMI status bar.
3. If the PLC still stops at 8, immediately check the NCU LEDs. The NCU 571/572 has multiple status LEDs: SF (red, group fault), BF (red, bus fault), RUN (green, NCK running), STOP (yellow, NCK in stop).
4. If RUN is off and SF is on, the NCK is reporting a startup fault. Connect a PG (programming device) via MPI to the NCU and read the NCK startup with SimoComu or via MMC diagnostic menu.
5. Run a PROFIBUS diagnostics: with the 611D line isolated, does the NCK boot? If yes, the 611D module is dragging the bus. Use an oscilloscope on the PROFIBUS A/B lines to look for a node shorted to 0 V.

8.1 Expected LED Behavior on a Healthy 810D Boot

9. Reset Procedure: NCK General Reset

An NCK general reset clears all NCK RAM (tool data, zero offsets, settable frames, R parameters) and re-initializes the NCK from the PCMCIA image. It does not change the PLC program or the drive configuration.

1. Power the system on. Wait for the HMI to come up far enough to show the menu (or use a PG on MPI).
2. Navigate to Start-up > NCK > NCK general reset. On the OP010 with MMC 103, the path is: MENU SELECT > Start-up > NC > NCK Reset > Yes.
3. Alternative if HMI is unavailable: on the NCU, set the rotary mode switch to position 1 (general reset), power-cycle the NCU, wait 30 seconds, return switch to position 0, and power-cycle again.
4. Wait for the NCK to re-initialize. The PLC will cycle to status 8 then to 9 when NCK is ready.

10. Reset Procedure: PLC General Reset

A PLC general reset clears the PLC work memory and reloads the user program from PCMCIA. It does not clear the PCMCIA user program itself.

1. On HMI: Start-up > PLC > PLC General Reset. Confirm with Yes.
2. On the NCU 571/572 front panel: rotate the PLC mode switch to 3 (MRES = Memory Reset). Hold for 3 seconds until the status display blanks. Release, then within 3 seconds turn again to MRES and hold. The PLC will cycle to status 0 and reload.
3. If the PLC gets stuck at status 8 even after a clean MRES, the NCK is not coming up, and the PLC will not advance. The PLC is not the problem; it is correctly waiting.

11. SIMODRIVE 611D Internal Fault Diagnostics

Once the NCK is brought up by other means (replacement or repair of the 611D line), connect a PG to the 611D module via RS-232 and run SimoCom U or SimoCoder to read the fault memory. The 611D stores the last 8 faults in a non-volatile buffer on the module itself.

11.1 Common 611D Fault Codes and Meanings

11.2 611D Current Sizing Sanity Check

For a 1PH6 / 1PL6 spindle motor, the rated continuous current I_N and rated power P_N are on the motor nameplate. The selected 611D module must have an I_N rating at least equal to motor I_N, and the module's peak current (1.5-2x I_N for 30 s) must exceed the spindle's required acceleration current.

For a 4-pole 1PH6 spindle rated 22 kW at 1500 RPM:

I_line = P / (sqrt(3) * V_LL * eff * pf)
I_line ≈ 22000 / (1.732 * 400 * 0.92 * 0.85) ≈ 40.6 A line current

Module 6SN1118-0DG21-0AA1 (50 A rated continuous, 75 A peak) is a typical match. If the running current at 500 RPM was abnormally high, an IGBT gate-driver fault on the 6SN1118 module would have triggered F001 (overcurrent), and the drive would have latched. Confirm the fault log to determine which module is the actual hardware failure.

12. SIMODRIVE 611D Repair and Replacement Options

If the diagnostic procedure points to a failed 611D module, the engineer has three paths:

12.1 OEM Repair via Siemens

Siemens A&D provides depot repair for 6SN1 1xx power modules. The procedure:

1. Identify the part number from the module's nameplate (e.g. 6SN1118-0DG21-0AA1).
2. Open a support case on Siemens Industry Online Support and request an RMA (Return Material Authorization).
3. Ship to the regional Siemens service center. Standard lead time is 5-15 working days for repair, 2-4 weeks for a reconditioned exchange unit.
4. Cost is typically 40-60% of a new unit for repair, 60-80% for an exchange (with a one-year warranty).

Reference Siemens manual: SINUMERIK 810D / 840D Commissioning Manual and SIMODRIVE 611D Function Manual for module identification and replacement procedures.

12.2 Authorized Siemens Channel Partner

Many local Siemens channel partners stock exchange units and can provide next-day on-site module replacement. This is often the fastest field-service path. List of authorized partners is available on the Siemens regional website.

12.3 Independent Industrial Drive Repair Houses

Independent industrial drive repair houses service 6SN1118 modules and typically offer:

• 24-48 hour turnaround on IGBT and gate-driver repairs.
• Load testing on a back-to-back dynamometer to verify repair quality.
• Warranty of 6-12 months.

13. Bringing the System Back Online (Verification Procedure)

Once the suspected 611D module has been repaired or replaced, follow this verification sequence to confirm the complete boot path:

1. With machine still isolated, measure DC bus voltage: P500 to M500 should be 540-620 V DC (for 400 V mains) within 5 seconds of power-on after the pre-charge cycle.
2. Power on, monitor the NCU seven-segment PLC display. Expected sequence: 1, 2, 3, 4, 5, 6, 7, 8, 9. If status 8 is held for more than 60 seconds, re-check the NCK boot (status LED on the NCU).
3. Confirm HMI is up and the alarm banner shows no active alarms.
4. Navigate to Diagnosis > Bus > PROFIBUS. All configured 611D slaves should appear with status "OK."
5. Jog each axis in JOG mode at 10% feed, then 50%, then 100%. Verify no alarms.
6. Run a spindle warm-up ramp: 100 RPM, 500 RPM, 1000 RPM, 1500 RPM, each step 60 seconds. The 611D module temperature should stabilize below 65°C on the heatsink.
7. Run a part program with 50% rapid, verify position accuracy within 5 microns using a test indicator.
8. Restore all tool data, zero offsets, and settable frames from the backup image.

14. Preventive Recommendations

• Annual battery replacement: the 3.6 V lithium battery has a typical life of 5-7 years in a warm cabinet, 3-4 years in a hot environment. Replace annually as preventive maintenance to avoid the SRAM corruption path that often triggers these symptoms.
• Cabinet cooling: cabinet temperature above 40°C cuts IGBT life in half. Install a cabinet air conditioner if ambient is borderline.
• Spindle warm-up: the operator's choice to ramp to 500 RPM for 5 minutes is good practice. However, ensure the 611D has had at least 3-5 minutes of pre-warmup at zero speed to dry out any humidity in the IGBT module. Cold-start at high torque demand is a known cause of thermal stress failure on 611D spindles.
• PROFIBUS shield: verify shield ground at both ends of the cable with a 1 MΩ resistor in parallel. Missing shield ground is a top cause of intermittent PROFIBUS faults that can corrupt the NCK link.
• PCMCIA backup: maintain a known-good PCMCIA image of the NCK, PLC, drives, and TOOLBOX as a site backup. This is the single fastest path to recovery if the original PCMCIA fails.

15. Summary Fault Matrix