Resetting Siemens FM-NC NCU 570: NCK Switch Procedure

Problem Overview: FM-NC NCU 570 Will Not Connect to MMC

An FM-NC NCU 570 in a SINUMERIK 810D / FM-NC system can enter a state in which the HMI (MMC) reports a connection failure to the NC after the PLC has been replaced, the NCK image is missing, or the battery-backed SRAM has lost its contents. The PCU / MMC shows a dialog such as "An error has occurred" or "Connection to NC error" when the operator attempts to download a backup, and the machine refuses to drive any axis. The PLC side may run normally and accept the project download, but the NCK side never finishes booting because the persistent image on the NCU does not exist or is unreadable.

The field symptom matches a missing or corrupted NCK file system on the NCU's internal storage. To recover, the NCK must be reinitialised through the rotary selector switch on the front of the NCU and a fresh NC backup must be streamed in over the HMI/MMC link. The remainder of this article documents that procedure, the switch semantics, the role of the PLC interface (NEW_CONF), and the verification steps required to confirm a clean recovery.

Identifying the NCU: 570 vs 750 and the Family Line

FM-NC stands for "Flexible Manufacturing Numerical Control." The NCU 570 and NCU 750 are two distinct performance classes within the SINUMERIK 810D / FM-NC family. They are not interchangeable: the NCK image, the boot loader, the maximum axis count, and the HMI commissioning plug-ins differ. If a service event has mixed the spare stock, the system will boot into an undefined state and the HMI will refuse the connection.

Before any reset procedure, confirm the nameplate on the front of the NCU. The MLFB (Siemens order code) starts with 6FC5... and includes the performance class digit. Cross-check the order code against the machine builder's parts list. If the installed unit does not match the original documentation, the "NC not found" behaviour is expected and no switch-position reset will fix it.

Front-Panel Layout: Switch, RESET, and LEDs

The NCU 570 front panel exposes the following controls and indicators relevant to a field reset:

• Rotary selector switch (8-position, marked 0 through 7) - alters the NCK boot behaviour. Position 0 is normal operation.
• RESET button - hardware reset of the NCK only; does not reinitialise persistent data.
• 7-segment display - shows NCK boot state. State "6" with a trailing dot indicates a ready NCK; rapid cycling indicates a boot loop.
• Status LEDs - power good, NCK ready, PLC link active, and CF / PC card access.

The 7-segment display state is the most reliable indicator of the boot phase. Before declaring the unit dead, observe the display for at least 60 seconds from power-up and write down the sequence. This is the same sequence the Siemens support engineer will ask for if the call is escalated.

Rotary Switch Positions: What Each Setting Does

The rotary selector switch is the primary service tool for the NCU. The position is read by the NCK bootloader only at power-up; changing the position while the unit is running has no effect. The standard positions are summarised below; the values are consistent with the Siemens FB2 Function Manual for SINUMERIK 840D / 840Di / 810D / FM-NC.

Prerequisites Before Touching the Switch

1. Confirm the NCU variant is the 570 documented in the machine's parts list. If there is any doubt, read the nameplate MLFB and compare to the parts list before proceeding.
2. Locate a valid NC backup image. The file is typically a *.arc archive produced by the "Series start-up" or "Backup/Restore" tool in MMC/HMI. The image must be from a system with the same NCU 570 variant and the same NCK software version.
3. Confirm the PLC is in a known-good state. Re-download the PLC project if necessary and observe the PLC's SF / BF LEDs - they should be green or off, not flashing red.
4. Verify the MMC / PCU is running and can see the network. The HMI must be able to ping the NCU at its X120 / Industrial Ethernet address.
5. Place the machine in a safe state: drives disabled, E-Stop pressed, all axes referenced away, and main contactor dropped. The reset procedure will momentarily disturb the drive bus.
6. Photograph the current rotary switch position and the 7-segment display state before any change, so the original state can be restored if the reset fails.

NCK Reset Procedure: Step-by-Step

1. Power down the complete SINUMERIK line: 24 VDC control voltage to the NCU, the PLC, and the MMC / PCU. Wait at least 30 seconds for the internal bulk capacitors to discharge. The 7-segment display must be dark before the case is opened.
2. Open the NCU front cover and locate the rotary selector switch. Confirm the current position. If it is not in position 0, note the current value, then rotate to position 2 (NC clear, MMC open for backup, no Jeida card). The detent positions are firm; do not apply force.
3. Restore 24 VDC control power. The NCK will boot into the clear-and-wait state. Observe the 7-segment display. It should stop in a state that indicates "waiting for backup" - typically a slow-blinking single digit, not the normal ready "6".
4. From the MMC / HMI, open the Series Start-up tool (or the Backup/Restore tool, depending on the HMI generation). Select the matching NC backup file (*.arc). Initiate the download to the NCU.
5. During the download, the 7-segment display will animate through several states. Do not power down. The download may take 3 to 15 minutes depending on the backup size and the network speed.
6. When the download finishes, the MMC will report success. At this point the NCK has the new image in volatile memory but not yet on persistent storage.
7. Power down the line again. Rotate the selector switch from position 2 back to position 0 (normal operation). Re-apply 24 VDC.
8. Observe the boot sequence. The 7-segment display should advance to "6" within approximately 60 to 120 seconds, with a trailing dot indicating the NCK is ready and the PLC link is up.
9. From the MMC, open the machine's main operating area. The drive enable and reference-point dialogues should now be reachable. Reference the axes and run the machine in JOG at low override before going to AUTO.

Restoring the NC Backup via MMC: File Format and Path

The NC backup produced by the standard tool is a single archive, not a directory. The extension is .arc and the file is signed by the tool that produced it - the HMI checks the header before sending. The backup is normally stored on a network share, on the PCU's local disk, or on a USB stick plugged into the PCU. The HMI's path dialog must point to the directory that contains the file, not to the file itself.

For a 570-class system, the typical NC backup file is in the order of 30 to 80 MB. A restore of this size over Industrial Ethernet at 100 Mbit/s will saturate the link and is normal; the progress bar in the HMI will reflect the throughput.

If the download stops mid-stream with a "Connection to NC error" message, the most common causes are:

• Backup file is for a different NCU variant (e.g. 750 image into a 570 unit).
• Backup file is for a different NCK software version than the bootloader expects.
• Backup file is truncated or damaged on the source medium.
• Network link between PCU and NCU has dropped or is overloaded by another participant.

PLC Interface Reconfiguration: NEW_CONF

Even when the NCK is fully booted, the PLC and the NCK must agree on the size and layout of the NCK-PLC interface. If the PLC project was rebuilt for a different NCK version, the interface will not match and the PLC will report a bus error to the NCK. The NCK exposes a procedure to re-read the PLC interface from the live PLC, called NEW_CONF. This is documented in the Siemens FB2 Function Manual for SINUMERIK 840D / 840Di / 810D / FM-NC, section "Re-configuring" of the PLC interface function.

Three equivalent methods are available:

• Hardware method: press and hold the RESET key on the front panel of the NC module for approximately 5 seconds, then release. The NCK re-reads the interface definition from the live PLC.
• Software method: from the HMI, navigate to Start-up / NCK / NEW_CONF and trigger the reconfiguration. The result is identical.
• Power method: a clean power off / on of the NC module causes the same re-read on next boot, provided the PLC is online first.

Always run NEW_CONF after replacing a PLC or after changing the PLC project. Without it, the NCK retains the old interface and the drives will not enable even though both sides report "ready".

Verification: How to Confirm a Clean Recovery

1. 7-segment display settles on "6" with a trailing dot within 120 seconds of the final power-up.
2. NCK ready LED on the NCU front panel is solid green.
3. PLC link LED is solid green; PLC SF and BF LEDs are off.
4. HMI / MMC opens the operating area without an alarm banner. The "Connection to NC" indicator is green.
5. Drive enable can be requested from the HMI without an alarm. The drive bus (Profibus / Profinet) reports all slaves as healthy.
6. Axis reference point can be run from JOG in both directions without an alarm.
7. A trial part program in single block with dry-run override runs to completion and matches the expected motion.

If any of the above steps fail, capture the active alarm number from the HMI alarm log and consult the troubleshooting matrix below before repeating the procedure.

Troubleshooting Matrix: Common Errors and Fixes

NCU 570 vs NCU 750: Why This Matters for the Reset

Field reports show that an installed unit described as "FM-NC NCU 570" may in fact be a 750, because the front-panel layout of the two units is nearly identical, the rotary switch positions are the same, and the nameplate is the only reliable differentiator. If the wrong backup image is loaded into the wrong NCU, the NCK will reject it with a header-mismatch error and the MMC will not proceed past the connection check.

Always read the MLFB on the nameplate before sourcing the backup. The 6FC5 order code's last three digits before the suffix identify the performance class. Do not rely on the label on the cabinet door, the spare-parts box, or the operator's manual alone - those have all been observed to disagree with the actual installed unit in the field.

Safety, ESD, and Power-Sequencing Precautions

• The NCU contains ESD-sensitive devices. Use a grounded wrist strap whenever the front cover is open and the rotary switch is being changed.
• Power sequencing: drop the main contactor and the 24 VDC control supply before opening the cabinet. Lock-out / tag-out the upstream disconnect.
• Wait at least 30 seconds after removing 24 VDC before touching the rotary switch. The 7-segment display must be dark to confirm discharge.
• Never change the rotary switch while the unit is powered. The boot loader only reads the switch at power-up; mid-run changes have no effect and can mask a real fault.
• After the reset, verify the E-Stop chain and the safety door interlocks before any automatic motion. The PLC project may have lost safety logic if the battery was discharged for an extended period.
• Confirm the NCU battery voltage with the unit powered (typical reading 3.0 to 3.4 VDC across the battery test points). A weak battery will erase the SRAM on the next power-down and force another full reset.

FAQ