Resolving SINUMERIK 840DE Error 2001 After Battery Loss and NCU Reset

The SINUMERIK 840DE (also written 840D sl) is a CNC platform in which the Numerical Control Unit (NCU) hosts an integrated SIMATIC S7-300 PLC. When the buffer battery is removed or depleted for an extended period, the NCU loses its retentive data: the PLC user program, the NC machine data, the compensation tables, and the tool/magazine data all return to default. After the operator re-inserts a fresh battery and performs a controlled boot through the mode selector switches S3 and S4, the HMI typically shows alarm 2001 ("PLC not in cyclic operation / HMI-PLC connection failure"). This alarm is the HMI's signal that the integrated PLC has no valid cyclic program to run, so the HMI cannot read PLC status, axis enable, or feed/spindle enable from the PLC interface (DB10/DB11 area).

The 2001 alarm is not a hardware fault. It is a commissioning-state alarm that the HMI raises whenever the PLC-HMI coupling has not been established after a total NCU reset. The cure is to reload the PLC base program (the SIEMENS-supplied basic program plus the machine builder's user program) and then reload the NC archives from the PCMCIA card or from a backup. This article walks through every step required, including the hardware prerequisites, the S3/S4 boot sequence, the X121 power-supply check, the PLC program load over MPI/PROFIBUS, the archive load from PCMCIA, and the HMI-side recovery. It also covers the related 810D panel variant where the SF LED remains lit after a comparable reset.

1. System Architecture of the 840DE NCU

The SINUMERIK 840DE combines three logical controllers on one backplane:

Inside the NCU 6FC5 357-x (840DE), the integrated PLC is functionally a CPU 317-2 DP equivalent with PROFIBUS-DP master capability. External I/O is connected through SIMATIC S7-300 signal modules or distributed ET 200 stations. The PLC user program is a STEP 7 project that contains two mandatory parts:

1. SIEMENS base program (also called "Grundprogramm" or "PlcBase") — supplies OB1, OB40, OB82, OB100, OB102, the FC/FBs for NCK-PLC interface (FC21, FC22, FC25, FC26, FB1–FB4), and the HMI coupling DBs.
2. Machine builder user program — the application logic in OB1, the safety wiring, axis enables, M-functions, etc.

Both parts are erased from the NCU's SRAM/Flash when the battery is dead long enough to lose the buffered state. Re-inserting a fresh battery alone does not restore the program — it only stabilizes the retentive area for the next load.

2. Root Cause of Alarm 2001

Alarm 2001 in the SINUMERIK alarm database is described in the Siemens DOConWEB documentation as "HMI: PLC not in cyclic operation." Internally, the HMI polls DB10 / DB11 (the standard interface to the PLC) and checks the cyclic-life counter. After an NCU total reset the PLC is in STOP, the cyclic counter is not incremented, and the HMI raises 2001 after the timeout. Common root causes are:

In the original case the operator performed the S3/S4 boot (general reset / power-on reset of the NCU), which clears all PLC and NC data. The 2001 alarm is therefore the expected first state after a total reset and confirms that the PLC has nothing to run.

3. Prerequisites Before the Recovery Procedure

1. New, fresh battery installed in the NCU (lithium 3.6 V, e.g. 6FC5247-0AA18-0AA0 or equivalent) and the holder latch locked.
2. STEP 7 V5.5 (or compatible) installed on a service PG/PC with the SINUMERIK 840D toolbox. STEP 7 is the only tool that can talk to the integrated PLC over MPI/PROFIBUS.
3. PC adapter USB (6ES7 972-0CB20-0XA0) for MPI/DP communication, or a CP 5611/CP 5613 A2 PCI card for faster DP transfers.
4. PLC user project backup (the machine builder's .S7P file), ideally on a USB stick or directly on the PG.
5. NC archive backup (the .ARC file set or the entire PCMCIA card content) from the machine commissioning.
6. Siemens DOConWEB access for the matching SINUMERIK 840D sl Commissioning Manual (IHA) and the base program (S7BPxxxx) installer that matches the NCK software version.
7. 24 V DC power supply (typically 5 A or higher) connected to X121 on the NCU. Without this rail the S7-300 I/O bus stays unpowered and the PLC cannot enter RUN, even with a valid program.

4. Verifying Hardware: X121 Supply and IM361 Interface

The 840DE NCU powers its S7-300 segment from an internal DC/DC converter, but the external 24 V for the I/O bus must be present on connector X121 of the NCU (or on the IM361 interface module for a separate S7-300 line). The pinout is:

Procedure:

1. With the cabinet energized, place a calibrated multimeter between X121 pin 1 and pin 20.
2. Verify a stable 24 V DC. A reading below 21 V indicates a weak supply or excessive current draw on the I/O bus; above 27 V indicates a failing power supply.
3. If an IM 361 interface module (6ES7361-3CA01-0AA0) is used to extend the S7-300 line, confirm that the IM 360 on the NCU side and the IM 361 on the expansion rack side are both seated and that the interconnecting cable is undamaged. The IM 361 also has its own 24 V requirement (X1, pins 1/20).

If the 24 V is absent, the integrated PLC cannot complete its self-test on power-up and stays in STOP with SF (red) LED lit. Restoring the 24 V is a precondition to any software recovery.

5. S3 / S4 Mode-Switch Reference for 840DE

The mode selector on the front of the NCU is a 4-position rotary switch labeled S1. Inside the HMI/NCK context, three software switches named S3, S4, and S5 control the NCU boot behavior. The relevant combinations are:

For the recovery case the switches must be in the S3 = 1 / S4 = 1 position (both ON) during power-up. The LEDs on the NCU confirm the active state. Hold the switches in this position only for the power-up cycle; the NCU reads them once during POST and the switches can be returned to 0/0 once the system boots into the commissioning menu.

6. Step-by-Step Recovery Procedure

6.1 Confirm the post-reset state

1. Insert the fresh battery, close the NCU cover, restore cabinet power.
2. Set S3 = 1, S4 = 1, then power-cycle the NCU.
3. Wait for the HMI to display the SINUMERIK power-up screen. Note any 7-segment code on the NCU front. The HMI will show alarm 2001 because the PLC has no program.
4. Return S3 and S4 to 0/0 to prevent an accidental reset on the next boot.

6.2 Establish STEP 7 → NCU communication

1. Connect the PC adapter (6ES7 972-0CB20-0XA0) to the X122 MPI/DP port on the NCU. The X122 pinout is the standard PROFIBUS 9-pin D-sub.
2. In STEP 7, set the PG/PC interface to PC Adapter (MPI) and set MPI address 0, baud 187.5 kbit/s for the initial connection.
3. Click PLC → Display Accessible Nodes. If the integrated PLC is detected, STEP 7 will list the NCU's MPI address (default 0 for the 840DE integrated PLC, or 1 if a CP is inserted). If nothing appears, double-check the adapter cable, the 24 V supply on X121, and the baud rate.

6.3 Reload the SIEMENS base program (S7BP)

1. Open the base program installer that matches the NCK software version (e.g. S7BP_4_8_SP4.exe for SW 4.8 SP4). The base program installer and the corresponding NCK software are delivered on the same CD and are also available through Siemens Industry Online Support with a service contract.
2. Run the installer and select the target STEP 7 project. The base program supplies the S7 blocks, the system data, the HW Config for the integrated PLC, and the standard interface DBs (DB10, DB11).
3. Compile the project (Station → Consistency Check → Save and Compile).

6.4 Add the machine builder user program

1. Open the machine builder's .S7P file or insert the machine builder's S7 blocks into the STEP 7 project created in 6.3.
2. Re-run Save and Compile. STEP 7 will flag any address conflicts (typically because the machine builder used a different slot for a module). Resolve conflicts to match the actual hardware configuration in the cabinet.
3. From STEP 7, click PLC → Download. The integrated PLC on the NCU transitions from STOP → RUN. The SF LED on the NCU should clear, and the HMI alarm 2001 should clear within a few seconds of the first successful cyclic scan.

6.5 Load NC archives from PCMCIA card

1. Insert the PCMCIA card supplied by the machine builder. The card must contain the directory structure:\pre>

   /siemens/sinumerik/nck
   /siemens/sinumerik/plc
   /siemens/sinumerik/hmi

2. On the HMI, navigate to Start-up → Commissioning → Archive and select Read in NC archive from PC card.
3. Choose the matching .ARC file and confirm. The NCK will import machine data, compensation tables, and tool data.
4. When the HMI prompts for an HMI password and you do not have it, the password can be reset from Start-up → HMI → Password → Reset on the commissioning screen. On a 840D sl with HMI Advanced, the default commissioning password after a total reset is SUNRISE for the operator level and CUSTOMER for the manufacturer level; the service level password EVENING may be required to view all alarms.

6.6 Set the NCK reference point

1. Power the drives on. The HMI shows the machine in commissioning mode.
2. Set the NCK MD20150 (default G-code) and the axis MDs as supplied by the machine builder.
3. Approach each axis's reference point using the JOG + − keys with the appropriate NCK MD34110 (REFP_CAM_DIR_IS_MINUS) and related settings.

7. Verification Checklist

Use this matrix at the end of the recovery to confirm a healthy state:

8. Variant: SINUMERIK 810D Panel with SF LED Still Lit

The 810D is the compact sibling of the 840D. The integrated PLC is identical in concept but the operator panel and the HMI software are different. After a battery-loss reset on an 810D the user may see:

• SF LED lit on the 810D front panel — this is normal during the first 30–60 s after power-up while the PLC is in STOP. If the LED does not clear after the PLC enters RUN, run STEP 7 hardware diagnostics to identify the offending module.
• Empty program directories on the HMI — the 810D also requires the PLC and NC archives to be reloaded from PCMCIA. The procedure is analogous to the 840DE: insert PCMCIA, use the HMI Commissioning → Read in archive dialog.

For the 810D, the base program and the NCK software are bundled as the "BDH" package. The relevant part numbers are 6FC5 250-x for the HMI software and 6FC5 252-x for the PLC toolbox. The DOConWEB path is the same: https://www.automation.siemens.com/doconweb/ with the SINUMERIK 810D/840D/840Di family filter.

9. Common Error Codes After a Total Reset

10. Field-Commissioning Tips

1. Always pull the PLC project out before any battery change. The risk of a multi-day downtime from an accidental reset is high. Keep a current .S7P and .ARC on a service laptop.
2. Verify the NCK software version before installing the base program. Mixing SW 4.4 with S7BP 4.5 causes subtle block-header mismatches and the PLC enters RUN but with a degraded interface. The 840D SW 4.4 series does not support CYCLE800 / CYCLE830 in some post-processors — confirm the SW level with the post-processor chain.
3. Use a known-good PCMCIA card. Siemens NC archives are CRC-checked on read. A failing flash cell on the card yields a partial load and subtle axis-tuning mismatches that look like drive faults.
4. Save the HMI password in a secure location. Without the password, full archive read-out requires a service login that is not always available at the customer site. The default passwords above are reset on every total reset and may differ from the operational passwords the end user set.
5. Re-enter all axis-specific machine data after the NC archive load. The MD files (machine data) and the SD files (setting data) live in the NCK Flash; a total reset wipes the in-RAM copies but the Flash copies are preserved. After a successful archive load the MDs may be in a partially loaded state — always cycle NCK reset (S3=1, S4=0) once after the load to re-initialize.

11. Part Numbers Reference

12. Summary of the Recovery Workflow

The workflow in SVG form (state machine):

13. Frequently Asked Questions