Restoring Sinumerik 840D NC Data from ARC Archive Files

Overview

The SINUMERIK 840D powerline NCU (Numerical Control Unit) is the central control of a 2-axis machine tool. When the on-board lithium buffer battery fails, the NCK (Numerical Control Kernel) loses its retained data: machine data (MD), setting data (SD), tool data, R parameters, part programs, compensation tables, axis data, and PLC retentive flags. Recovery is only possible from an external backup, which on these systems typically carries the file extension .arc — a Siemens-specific packed archive containing the /NC and /PLC card image directories.

This article covers a real field scenario: an NCU Box 6FC5247-0AA00-0AA3 with a dead buffer battery, an OP177B 6AV6642-0DA01-1AX1 operator panel, and a SIMODRIVE 611 drive section controlling two axes. The user has a valid .arc backup file on a service PG/PC and needs to write it back to the NCU. The OP177B runs SINUMERIK HMI Embedded, which does not expose a back-up/restore UI, so the restore must be performed either by PCMCIA card directly on the NCU, or via HMI Advanced on a PCU 50, or by direct transfer to the NCU card image over X127 with RCS Commander / WinSCP.

All Siemens documentation referenced below is available on the Siemens Industry Online Support portal at support.industry.siemens.com.

System Identification and MLFB Decoding

NCU 6FC5247-0AA00-0AA3

The Siemens order code (MLFB) breaks down as follows:

• 6FC5247 — NCU 571 / 572 / 573 family, 840D powerline generation.
• 0AA00 — base configuration (no special add-on features).
• 0AA3 — hardware revision / feature release.

This NCU is delivered without an internal HDD. All persistent storage lives on the PCMCIA flash card inserted in the front slot (slot X5, marked NC-CARD). The card is hot-pluggable only when the NCU is powered down and contains the active /NC, /PLC, /OEM, /SAMPLES, and /HMI-ADV directories. When the battery is dead AND no card is present, the NCU boots with factory defaults and the previous commissioning is lost.

OP177B 6AV6642-0DA01-1AX1

The OP177B PN/DP is a 5.7" color touch operator panel. The suffix 1AX1 identifies the powerline variant configured for the SINUMERIK 840D as a SINUMERIK-specific HMI. The “PN/DP” marking means it carries both PROFINET and PROFIBUS interfaces; when used on 840D powerline it normally connects to the NCU via PROFIBUS (MPI/DP) at the NCU’s X2 connector. The OP runs SINUMERIK HMI Embedded software — a slimmer HMI than HMI Advanced — and only exposes operating screens. There is no service menu for archive up/download.

SIMODRIVE 611 with 2 Axes

The SIMODRIVE 611 drive bus is configured via DIP switches and the SimoCom U / IBF commissioning tool. With 2 axes (typically X and Z on a lathe or X and Y on a mill) and a SIMODRIVE 611D or 611U closed-loop module, the drive data and motor data live inside the NCK, not in the drive module itself. A valid .arc restore therefore repopulates motor data and encoder data automatically as part of the NCK image.

Understanding the Sinumerik .arc Archive Format

The Siemens .arc is a packed file created by SINUMERIK HMI’s archive utilities: the “Series Start-up” tool, the “Commissioning Archive” soft-key on HMI Advanced, or the “Back up / Restore” menu on PCU 50. Internally the file is a serialized container with the following structure:

arc_begin
  version       0x00010000
  date          YYYYMMDD
  time          HHMMSS
  file_count    N
  for each file:
    name        (null-terminated string)
    size        (uint32)
    crc32       (uint32)
    raw bytes   (size bytes)
arc_end

A typical .arc from a 2-axis 840D powerline system contains the following logical partitions:

A partial /NC-only archive restores NC data but leaves the PLC untouched. A combined archive (the default from HMI Advanced) restores both — but the order in which the loader processes them matters, as detailed later in this article.

Prerequisites and Pre-Restoration Checks

Before any restore, verify the following.

Hardware Inventory

• Functional NCU 6FC5247-0AA00-0AA3 with intact PCMCIA card slot.
• PCMCIA flash card of sufficient size (typically 64 MB or larger for a 2-axis system with HMI Embedded; 256 MB or 1 GB if an HMI Advanced partition is present).
• Replacement NCU buffer battery: Siemens 6FC5247-0AA18-0AA0 lithium, 3.6 V, 1.5 Ah, AA form factor.
• Original .arc backup file with verified MD5/SHA1 checksum.
• PG/PC with STEP 7 V5.5 + SPx and (if using HMI Advanced) an HMI Advanced license dongle.
• For network restore: crossover Ethernet cable or a small switch, plus a TCP/IP connection to NCU port X127.

Software Versions

• STEP 7 V5.5 + SP3 or later for PLC restoration.
• SINUCOM / RCS Commander or HMI Advanced v7.x for direct NC restore.
• For OP177B reconfiguration: WinCC flexible 2008 SP3 or later (HMI Embedded).

NCU Buffer Battery Replacement

The NCU buffer battery (3.6 V lithium, AA form factor in the holder on the front of the NCU) maintains NCK RAM and PLC retentive data when the NCU is powered off. Once it drops below ~2.7 V the NCU loses retained data. Replace the battery before powering up with the new card image.

Procedure

1. Power down the NCU and wait at least 30 s for the DC bus to discharge.
2. Open the battery holder on the front of the NCU (slide the plastic tab downward).
3. Remove the spent 6FC5247-0AA18-0AA0 cell.
4. Insert the fresh cell; observe polarity (+ on top, markings inside the holder).
5. Close the holder.
6. Re-apply 24 V to the NCU. The BF (bus fault) LED should remain off within 30 s if the card image is valid.

Identifying the HMI Platform

The OP177B 6AV6642-0DA01-1AX1 in this configuration runs SINUMERIK HMI Embedded. The menu path MENU SELECT > SERVICES > DATA IN > FROM NC CARD that appears in some legacy service notes is only present on PCU 20 / PCU 50 with HMI Advanced, not on HMI Embedded. On HMI Embedded, no operator-level restore screen is available.

Therefore three viable restore paths exist on this hardware:

For a service scenario with a single backup .arc file, Path A is the most reliable and the recommended field procedure for NCU 6FC5247-0AA00-0AA3.

X127 Service Ethernet — Network Configuration

The NCU’s X127 connector is a 10/100 Mbit Ethernet port dedicated to engineering access. Default NCU-side settings on 840D powerline:

Configure the service PG/PC Ethernet interface with a static 192.168.214.2 / 255.255.255.0 address (or set it to DHCP and let the NCU assign a lease). Disable any other active network interfaces on the PG/PC to prevent routing conflicts. A direct crossover cable is sufficient; a small switch also works. Do not connect X127 to a corporate network — it has no firewall and exposes the service interface.

Method 1 — Restore via PCMCIA Card

Tools required

• PCMCIA card reader (e.g. SanDisk PCMCIA adapter or equivalent).
• Blank or freshly formatted PCMCIA flash card (64 MB minimum; 256 MB recommended).
• PG/PC with card reader.
• The original .arc file.

Card preparation

1. Format the PCMCIA card as FAT16. FAT32 will not boot HMI Embedded.
2. Create the directory structure \NC\ and \PLC\ on the card.
3. Copy the .arc file to the root of the card. Do not place it in a subdirectory.
4. Eject the card cleanly from the reader (use Windows “Safely Remove Hardware” or the equivalent).

NCU side procedure

1. Power down the NCU. Wait for all LEDs off.
2. Insert the PCMCIA card into NCU slot X5 (NC-CARD) with the label facing outward.
3. Power up the NCU. Watch the LED sequence:
   • RDY off, then green blinking (CF card access).
   • RDY solid green → boot OK.
   • RDY red or fast blink → card not recognized (re-seat, re-check FAT16).
4. After the OP177B finishes booting (it shows the “SINUMERIK” splash, then either the operating area or an alarm), navigate with the OP keys to the commissioning menu.
5. Because OP177B has no restore UI, the restore is triggered by an NCK soft-key sequence. The standard sequence on HMI Embedded:
   • Press <RECALL> + soft-key “Service” (rightmost soft-key on the operating area screen).
   • Enter password SUNRISE if prompted (default Siemens service password; OEMs may have changed it).
   • Select Commissioning Archive > Read from NC Card.
   • Confirm the file selection of the .arc file.
   • Wait for the progress bar to complete; do not power down during the restore.
6. If the OP177B does not expose the service menu (some firmware versions lock it), proceed to Method 2 or 3.

Post-restore verification

• The RDY LED on the NCU must remain solid green.
• The OP177B must show the operator area with the correct machine designation.
• Trigger a single NCK reset (NCK-Reset soft-key) to re-read the machine data.

Method 2 — Restore via HMI Advanced over X127 / MPI

If the site has a PCU 50 with HMI Advanced (HMI v7.5 or later), or if you can temporarily install HMI Advanced on a PG/PC, the restore is straightforward.

Topology

[PCU 50 / PG] ----- Ethernet crossover ----- [NCU X127]
[PCU 50 / PG] ----- MPI / PROFIBUS ---------- [NCU X2 (MPI)]

Procedure

1. Connect PG/PC to NCU X127 with a crossover cable or via a small switch.
2. Configure the PG/PC Ethernet interface with static 192.168.214.2 / 24.
3. Start HMI Advanced on the PCU 50 (or on the PG with HMI Advanced installed).
4. From the main operating area, soft-key: Start-up > Commissioning > Archive > Read-in Archive.
5. Select the network drive / USB / floppy where the .arc file resides.
6. Confirm. The restore progress shows the file being unpacked into the active card image.
7. When the progress bar reaches 100%, the system prompts: “NCK Reset required for NC data; PLC restart required for PLC data.” Accept both.
8. Wait for the OP177B to reconnect (it may blank for ~30 s during the NCK reset).

Method 3 — Direct Restore via RCS Commander (Service Tool)

For Siemens service engineers and advanced integrators, the RCS (Remote Control Service) Commander can stream the .arc file directly into the NC-CARD image over X127, bypassing both the OP UI and the PCMCIA swap.

Procedure

1. Connect the PG/PC to X127 as in Method 2.
2. Launch RCS Commander.
3. Log in as service user auduser (default password auduser — change per site policy).
4. Select the NCU target and wait for the connection handshake.
5. Transfer the .arc file to /card/user/sinumerik/archive/.
6. Open a service shell and execute the NCK command:

   pi_nck_archive_in(<file>.arc)

7. Wait for the return code 0 (success). Any non-zero return code indicates a CRC or version mismatch.
8. Perform an NCK reset to commit the change.

This method requires a working Ethernet service path and a valid service login, but is the fastest when the PCMCIA card is in a known-good state and only the NC data needs to be replaced.

Required Restore Sequence — NC First, Then PLC

On SINUMERIK 840D, the PLC and NCK share the same retain battery but have separate non-volatile stores on the PCMCIA card:

• /NC/ directory — NCK data.
• /PLC/ directory — PLC S7 project.

A combined .arc archive contains both, and the loader processes the NC portion first and the PLC portion second. The recommended manual order on a green-field restore is:

1. Insert the new card and power up.
2. Load the NC portion only (or load the full archive and let the loader take the NC branch first).
3. Perform an NCK reset.
4. Verify the axes can be referenced in JOG mode.
5. Load the PLC portion.
6. Restart the PLC (STOP → RUN).
7. Verify the PLC HMI connection (interface DBs in DB 10, DB 20, DB 31, DB 32 must update).

Commissioning the SIMODRIVE 611 Drives

After NC and PLC are loaded, the SIMODRIVE 611D / 611U drives must be commissioned. On 840D powerline with a valid .arc restore, the drive data is recovered automatically because it is stored under /NC/ as part of the NCK image. The following checks are still required:

On a 2-axis configuration, the axis naming (AX1, AX2) is determined by MD 20070 (axis name) and MD 20080 (axis assignment to channel). If the axis order is reversed after a restore (e.g. X and Z swapped), swap the motor encoder connectors — not the drive bus — and re-reference.

Sinumerik Alarm Codes Reference

The following NCK alarms are the most common ones encountered immediately after a .arc restore on 6FC5247-0AA00-0AA3. None of these indicate a corrupt archive; they are the standard “reboot required” alarms.

Post-Restore Verification Checklist

Run this checklist before handing the machine back to the operator.

• NCU RDY LED is solid green.
• OP177B shows the correct operating area, no alarm line at the top.
• HMI soft-key Diagnosis > Service Overview shows “Commissioning data valid = yes”.
• Axes can be jogged in JOG mode in both directions.
• Axes reach the reference point without alarms 27008 / 27010.
• Tool list (Tool Management > List) shows all tools from the archive.
• Zero offsets (G54..G599) match the commissioning sheet.
• PLC runs (RUN LED on the PLC section solid green).
• PLC HMI connection live: HMI-Status soft-key shows all interface DBs at 0.
• Safety circuits (Emergency Stop, safety door) tested and OK.
• Handwheel pulse generator responding (if fitted).
• Spindle speed setpoint and actual match across the speed range (if a spindle axis is present).

Troubleshooting Matrix

Preventive Backup Best Practices

To avoid the scenario covered by this article, follow these preventive steps on every 840D powerline system:

1. After every commissioning or major service intervention, perform a Start-up > Commissioning > Archive > Read-out Archive and store the .arc on at least two independent media (e.g. CF card image + network share).
2. Compute and record the SHA1 of every backup at creation time; compare on restore.
3. Replace the NCU buffer battery every 5 years, or sooner on machines with high thermal cycling. Log the replacement in the maintenance log.
4. Keep at least one spare PCMCIA flash card per NCU type on site, pre-formatted FAT16.
5. Record the HMI software version (e.g. HMI Embedded 06.04.20) and NCK software version (e.g. 06.04.20) in the maintenance log. Restores across major versions can require an upgrade in between.
6. Verify after every cold start (power off > 30 s) that the NCU retains the axis position. If positions are lost, the battery is failing.

Frequently Asked Questions