Troubleshooting SINUMERIK PLC Reset Failure and Alarm 300300

1. Problem Overview

On a SINUMERIK 840D sl controller (NCU 7x0.3 / NCU 730.x / NCU 710.3 families) the operator attempts to clear the PLC after a corrupted NCK/PLC reload. The system rejects the procedure with the message "No PLC memory reset possible due to insufficient protection level" and the front-panel PLC general reset (via rotary switch S4) fails to complete: the PS LED lights steady, the PF LED never lights, and the PR LED does not transition to green. The HMI is unreachable, the MCP (machine control panel) LEDs blink synchronously, and the alarm stack is populated with five distinct alarms that must be cleared in dependency order before the controller will return to cyclic operation.

The alarm set commonly reported with this failure mode is:

• Alarm 300300 Axis <name> drive <slot> boot error, error code 5 (reported for multiple axes simultaneously, typically every drive object in the system).
• Alarm 2000 PLC sign-of-life monitoring.
• Alarm 810004 Stop/Abort event, error analysis via STEP 7 required.
• Alarm 27100 At least one axis is not safely referenced.
• Alarm 3000 Emergency stop.

Because the alarms form a dependency chain (drive boot failure → PLC sign-of-life loss → STEP 7 stop event → axes not safely referenced → emergency stop), each layer must be resolved before the next can be cleared. This reference documents the recovery sequence, the S4 PLC general reset procedure, the NCU LED interpretation, and the STEP 7 diagnostic workflow.

2. Alarm Set Description and Parameters

The following table summarizes each alarm, its significance, and the official Siemens definition. Refer to the SINUMERIK 840D sl Diagnostics Manual for the full parameter list of each alarm.

2.1 Alarm 300300 — Drive Boot Error Code 5

According to the official Siemens SINUMERIK 840D sl Diagnostics Manual, alarm 300300 is generated when the NCK cannot establish a usable cyclic link to a SINAMICS drive object during ramp-up. Error code 5 is the most common code seen in the field, and its meaning depends on whether Safety Integrated is active:

• Without Safety Integrated: Timeout while the NCK waits for the drive to acknowledge the configured state (typically the transition from power-on to ready).
• With Safety Integrated active ($MA_SAFE_FUNCTION_ENABLE ≠ 0 on at least one axis): the timeout in the drive startup state machine is identical, but the underlying cause is almost always that the PLC has not reached cyclic operation. The drive and the PLC are mutually synchronous during cyclic mode; if the PLC never reaches RUN, every drive object that uses the Safety Integrated slot will time out with error code 5 simultaneously. This is exactly the symptom reported in the source case (alarms on drive 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 23, 30 — i.e., the entire drive lineup).

The official Siemens recommendation for error code 5 is to first verify PLC run-up before chasing drive hardware. The error code itself is intended for Siemens development diagnostics, not for the field technician — the displayed status code is always needed for deeper root-cause analysis.

2.2 Alarm 2000 — PLC Sign-of-Life

The PLC sign-of-life (German: Lebenszeichen) is a counter that the PLC increments in OB1 and that the NCK reads and checks. The check is configured by machine data MD10050 $NC_PLC_RUN_TIMEUP_TIMEOUT (default value 10 s in SINUMERIK 840D sl). If the sign-of-life counter does not change within the configured window, alarm 2000 is raised and the NCK holds the drives in the boot state — which is precisely what produces the cascade of 300300 errors.

2.3 Alarm 810004 — Stop / Abort Event

Alarm 810004 is the generic "PLC has stopped" alarm. It is always paired with detail in the STEP 7 / TIA Portal diagnostic buffer. The buffer entry will identify the OB that failed to execute (e.g., OB82 = diagnostic interrupt, OB85 = priority class error, OB121 = programming error). The PLC general reset via S4 is the canonical recovery path when this alarm is present.

3. NCU LED Status Interpretation

Every SINUMERIK 840D sl NCU provides a five-LED front-panel indicator that reports the internal state of the controller. The indicator layout is:

On the affected machine the LED state was:

Before reset attempt:
  POK = green
  PS  = red
  PF  = red
  CB  = yellow blinking
  DB  = yellow blinking

After S4 toggle 2-3-2 within 3 s:
  POK = green
  PR  = green      <-- only briefly
After X111 plugged:
  POK = green
  PR  = off
  PS  = red
  PF  = red

The PR-LED transition (green → off) immediately after X111 is connected is the most diagnostic observation in the entire case: the PLC successfully reached RUN, but as soon as the drives came online through X111, the drive bus failed to synchronize and pulled the PLC back into STOP. The PLC general reset itself was not the failure — the failure is downstream of the reset, in the drive startup handshake.

4. Root Cause Analysis

The reported symptom is a cascade, not a single fault. The cascade order is:

1. PLC is not in RUN (root cause — corrupted PLC project, hardware mismatch, or PLC general reset not completed).
2. NCK sign-of-life timeout (alarm 2000) — NCK stops driving the cyclic PLC counter.
3. STEP 7 detects a stop event (alarm 810004) — diagnostic buffer holds the OB-level cause.
4. Drive startup handshake times out (alarm 300300, error code 5) — drive ramp-up waits for the PLC to reach cyclic operation; on Safety Integrated systems this wait is enforced by PLC_RUNNINGUP_TIMEOUT.
5. Axes cannot safely reference (alarm 27100) — axes are held in "not safely referenced" because the drive never reached ready.
6. Emergency stop (alarm 3000) — drive-not-ready propagates into the PLC axis interface and forces ESTOP.

For the SINUMERIK 840D sl family, the most common underlying causes for this cascade are:

• PLC project / hardware configuration mismatch. A reload of the PLC project from STEP 7 or TIA Portal that does not match the actual hardware (HW Config) will put the PLC in STOP with OB82/OB85/OB122. Verify that the HW Config in the STEP 7 project matches the installed module list.
• Defective or mis-seated SINAMICS drive module. A drive that fails to come out of power-on will hold the bus startup, and the NCK will eventually time out.
• X101 / X111 / I/R module wiring fault. The I/R module flat cable carries the digital inputs and the safety-relevant signals; a missing or reversed cable will prevent the PLC from seeing ESTOP, drives-on, and reference-point acknowledge. This is consistent with the source case, where the operator observed different LED states before and after plugging X111.
• Protection level insufficient to perform the reset. The error message "No PLC memory reset possible due to insufficient protection level" indicates that the access level in HMI / SINUMERIK Operate is set above the level required to reset PLC memory. The PLC service switch S4 bypasses the access level, but if the PLC is not in STOP the access-level check still fails.

5. PLC General Reset Procedure via S4 Switch

The S4 rotary switch on the SINUMERIK NCU performs a hardware-level PLC general reset that bypasses HMI access-level protection. The procedure below is the official Siemens sequence published in the SINUMERIK 840D sl Function Manual and the Commissioning Manual. Execute the steps in order; do not hold S4 in any position longer than necessary because S4 is a spring-return detent switch.

1. Place the controller in a safe state (no axes in motion, ESTOP chain closed, drives powered off if the situation requires it).
2. Turn S4 to position 2 (STOP). Wait until the PS LED lights.
3. Turn S4 to position 3 (MRES — request general reset). Hold in this position for at least 3 s. The PS LED will go out briefly and then light again.
4. Within 3 s of the PS LED re-lighting, turn S4 through the sequence 2 → 3 → 2 (STOP-MRES-STOP). The PS LED will first flash at approximately 2 Hz and then light steady. The PF LED will also light to confirm the reset was registered.
5. Once both the PS and PF LEDs are lit, turn S4 to position 0. The PS and PF LEDs go out and the PR LED lights green. The PLC is now in cyclic mode and the general reset is complete.

6. Drive Synchronization After PLC General Reset

Once the PLC PR LED is steady green, the drives must be re-paired. On SINAMICS S120 (used with SINUMERIK 840D sl) the drive startup follows this handshake:

1. NCK powers the drive bus (X120 / X130 / X140 on the NCU depending on variant).
2. NCK requests configuration from drive object 0 (Control Unit). CU must respond within PLC_RUNNINGUP_TIMEOUT (default 10 s, configurable in MD10050).
3. Each drive object is brought up in slot order. The NCK waits for the drive to acknowledge the configured state — this is the source of error code 5 in alarm 300300.
4. Safety Integrated axes require the safety slot to be acknowledged by the PLC, which is why error code 5 with Safety Integrated active is a strong indicator that the PLC never reached cyclic operation.

If the drive startup is the failing layer, perform the following checks in order:

1. Confirm DRIVE-CLiQ topology is intact: every drive must have a green DRIVE-CLiQ LED. A red or dark DRIVE-CLiQ LED on any drive will hold the bus startup.
2. Confirm that the drive firmware version stored in the project matches the version on the CF card. Use the SINUMERIK commissioning tool SINUMERIK Commissioning Tool or the HMI menu Commissioning → Drive system → Version to compare.
3. Confirm that no axis machine data is referenced as Safety Integrated ($MA_SAFE_FUNCTION_ENABLE ≠ 0) on a drive that has not been configured for SI. This is a frequent cause of alarm 300300 after a partial restore.

7. PLC Sign-of-Life Recovery (Alarm 2000)

Alarm 2000 is a symptom alarm — it clears automatically once the PLC reaches RUN. To force a clean run-up:

1. Perform the S4 general reset described in Section 5.
2. Open the STEP 7 / TIA Portal project that matches the installed hardware.
3. Select PLC → Clear/Reset from the online menu, then Download the hardware configuration and the software (blocks) to the PLC.
4. From the online menu, select PLC → Operating Mode → RUN.
5. Watch the sign-of-life counter in the STEP 7 variable table (default name OB1_cyclic_time or the project-specific tag — Siemens example project uses PLC_sign_of_life). It should increment at OB1 cycle rate.

If alarm 2000 returns after a clean RUN, suspect hardware: a defective IM 153 / IM 154 (PROFINET IO), a defective SINAMICS drive module on the same PROFINET segment, or a damaged X120 / X130 / X140 port on the NCU.

8. Safety Referencing (Alarm 27100)

Alarm 27100 is the next-to-last alarm in the cascade. It is cleared by the operator confirming that the Safety Integrated axes have been physically referenced. After the PLC is in RUN and the drives report ready:

1. Switch the controller to JOG mode.
2. On the HMI, navigate to Commissioning → Safety Integrated → Machine Data or — in SINUMERIK Operate — accept the User Agreement dialog that lists each SI axis.
3. For each SI axis, perform a controlled traverse in JOG+ and JOG- to cross the reference marker. The axis will be marked safely referenced in the axis display.
4. Once all SI axes are referenced, alarm 27100 clears automatically.

9. STEP 7 Diagnostic Workflow for Alarm 810004

Alarm 810004 is the canonical "open STEP 7 to find out why the PLC stopped" alarm. Use the following workflow:

1. Connect the programming PC to the X127 service port on the NCU, or to the corporate Ethernet if the NCU is networked.
2. Open SIMATIC Manager / TIA Portal and go online with the PLC (target: the SINUMERIK PLC, accessible as PN/IE node or via the S7 routing path through the NCU).
3. Open PLC → Diagnostic / Setting → Diagnostic Buffer. The most recent entry holds the OB stop reason. Common entries are:
   • OB82 (diagnostic interrupt) — distributed I/O module has reported a fault.
   • OB85 (priority class error) — distributed I/O failure with process image update in progress.
   • OB121 (programming error) — illegal instruction, divide by zero, or invalid DB access in the user program.
   • OB122 (I/O access error) — direct I/O access to a slot that has failed.
4. From PLC → Module Information, walk the module list and read the diagnostic data of any module with a non-green status indicator.
5. Rectify the underlying cause (replace module, fix wiring, correct the STL error) and re-download the project to the PLC.

Refer to the STEP 7 / TIA Portal help topic "Diagnostic buffer of the CPU" for the interpretation of each entry code. The official reference is the SIMATIC S7-1500 / S7-300 / S7-400 system manuals on the Siemens Industry Online Support portal.

10. X101 / X111 / I/R Module Verification

The SINUMERIK 840D sl uses a dedicated I/R module for the digital inputs and outputs that are not part of the standard MCP / handheld terminal. The I/R module flat cable connects to the NCU at the connector labelled X111. The connector labelled X101 is the power supply and PROFIsafe connection.

In the source case the operator observed a PR-LED transition (green → off) when X111 was connected. This is consistent with an I/R module problem: the PLC reaches RUN before the I/R module is online, and then the I/R module either fails to come up (causing OB82 in the PLC) or presents a mismatched PROFIsafe signature (causing the PLC to drop back to STOP). The diagnostic workflow in Section 9 will reveal the OB that caused the stop.

11. Verification and Commissioning Checks

After the recovery sequence is complete, perform the following checks before returning the machine to production:

1. All five NCU LEDs match the healthy state: POK green, PR green, PS off, PF off, CB off.
2. The alarm stack is empty (or contains only "clearable" alarms such as axis-not-referenced, which is expected after a power-on).
3. From STEP 7, PLC → Diagnostic/Setting → Operating Mode shows RUN.
4. All SINAMICS drive objects report state ready in the HMI Commissioning → Drive system view.
5. All Safety Integrated axes have been safely referenced and alarm 27100 is clear.
6. Emergency stop chain has been tested: pressing the physical ESTOP button must clear the drives, and releasing it must restore readiness without producing any new alarm.
7. PLC sign-of-life counter increments at the expected OB1 cycle rate (monitor in STEP 7 VAT).
8. Backup the now-healthy PLC project and the SINAMICS drive parameters to the CF card and to an external archive.

12. Summary of Recovery Sequence

1. Resolve the access-level problem by performing the S4 PLC general reset (Section 5) — this is the only path that bypasses the HMI protection level.
2. Re-download the STEP 7 / TIA Portal project that matches the installed hardware. Read the diagnostic buffer to find the OB stop reason (Section 9).
3. Verify the drive bus (DRIVE-CLiQ) topology and firmware compatibility. Resolve alarm 300300 (Section 6).
4. Confirm PLC sign-of-life increments (Section 7).
5. Safely reference all SI axes to clear alarm 27100 (Section 8).
6. Test the ESTOP chain to clear alarm 3000.
7. Run the verification checks in Section 11.