SINUMERIK 840D NCU 120202 Fault: OPI Address Restoration

Problem Overview

On a SINUMERIK 840D system with three NCU 710.1 / NCU 720.x controllers arranged on a shared MPI/DP/OPI backplane, a failed NCU 3 was replaced with a new, factory-fresh unit. The replacement unit received its MPI and PROFIBUS addresses from a previously generated NetPro project, and downstream PROFIBUS communication to the SIMODRIVE / SINAMICS drives was restored immediately. The HMI's active-node scan confirmed addresses 6 and 7 were alive, but the system immediately raised alarm 120202 "NCU3: Wait for connection to NC/PLC" and refused to allow any archive (\*.arc) selection for the new NCU from the operator panel.

The HMI could not list NCU3 in the archive-restore target selection, and any attempt to push the PLC.arc from Simatic Manager directly into the new NCU ended in the integrated S7-300 PLC going to STOP with a diagnostic buffer entry indicating OB121 (Programming Error OB) was missing from the user project. The neighbouring NCU 1 and NCU 2 neither contained OB121 in their projects nor exhibited the alarm, which made the OB121 reading a symptom rather than the root cause.

System Topology Captured Before Failure

The multi-NCU topology is essential to understanding the address map. The original bus layout was:

All three NCUs live on the same MPI/OPI segment and are scanned by the HMI as a single namespace. The HMI only allows archive-restore operations against an NCU whose NCK/PLC handshake is already established, which is why the 120202 alarm effectively blocks archive I/O from the HMI's "Start-up" menu even though the PROFIBUS layer to the drives is healthy.

Alarm 120202 — "NCUx: Wait for Connection to NC/PLC"

Alarm 120202 belongs to the family of HMI / NCK link alarms and is raised when the HMI boot sequence reaches the point where it expects to handshake with the NCK (and through it the integrated PLC) on a configured OPI address, but no NCU responds at the expected OPI node. Typical primary causes are:

1. The new NCU is at the factory default OPI address (13) while the rest of the system expects it at the slot the previous NCU occupied (e.g. 12 for the third slot in this topology).
2. The PLC firmware boot is stuck on a missing system OB (OB100, OB101, OB121), which prevents the NCK from completing the PLC-side handshake.
3. The NCK software version installed at the factory does not match the cross-licensed version expected by the project (CF card / version mismatch).
4. NCK licensing is absent on the new unit, so the NC remains in a permanent "wait for license" state.
5. The PROFIBUS configuration was loaded before the MPI/OPI parameters, leaving the HMI unable to find the NCU on the OPI segment.

Root Cause Analysis

The replacement NCU was delivered with the Siemens factory-default OPI address of 13. The original NCU 3 occupied OPI address 12 in the project's bus configuration. As a result:

• The PROFIBUS DP address (7) loaded correctly because the NetPro project wrote it.
• The OPI address stayed at 13 because OPI parameters are not part of the standard NetPro download; they live in the NCK's startup file (\*.ini on the CF card) and are normally only changed by an explicit commissioning step.
• The HMI, which expects the NCU at OPI 12, polled the slot and found nothing. NCU 3 with OPI 13 was visible to the bus scan but was treated as an unknown / unconfigured node, so the HMI could not bind the archive-restore target to it.
• The PLC STOP with OB121 reported by Simatic Manager was a downstream effect: Simatic Manager's online view was reaching a non-existent or mis-mapped PLC slot, returning a "block missing" diagnostic that was actually an address-resolution failure, not a real OB121 absence in the archive.

Resolution Strategy

Two parallel paths are required to bring the new NCU back into the system. The first brings the new NCU into the HMI's OPI namespace. The second loads the NC and PLC archives onto the now-visible NCU.

Path 1 — Correct the OPI Address with HMI Advanced for PC/PG

SINUMERIK HMI Advanced for PC/PG is a Windows-based commissioning front-end that can connect to a SINUMERIK NCU either across MPI/OPI or across TCP/IP (with the appropriate CP) and lets the engineer access the same Start-up menus that the OP/TP would expose on the machine. It is the only widely supported PC-side tool that can target a specific NCU by OPI address and drive the Start-up > NC > Commissioning > OPI parameters, the Set MD dialog, and the Archive/Restore tree.

For a 840D / 840DE on the affected build, the supported order number is:

HMI Advanced is officially listed as discontinued in the Siemens catalog, but it remains available through Siemens regional spares channels and authorized CNC retrofitters. Always request the part number together with the matching license SLS-CoL media; a copy without the license will boot only into demo mode and will not commit parameter changes.

Procedure — Connect HMI Advanced to the Stand-Alone NCU

1. Disconnect the NCU 3 PROFIBUS DP segment from the machine bus to keep the new unit electrically isolated while commissioning. Leave MPI/OPI connected to the HMI segment so the HMI can still scan.
2. Install HMI Advanced V6.4 on a service laptop. Launch hmi_advanced.exe and select Online > Set Connection.
3. Set the channel to MPI/OPC with the CP5611 / CP5614 / PC-Adapter default bus parameters (187.5 kbps for an 840D OPI segment, profile "MPI").
4. Browse for active nodes. The new NCU should appear at the factory default address (13). Confirm the version banner shows the expected NCU software release.
5. Open Start-up > NC > Commissioning > OPI and edit the OPI node number from 13 to 12. Save and acknowledge. The NCK performs a warm restart automatically.
6. Confirm the change by re-browsing active nodes. The unit now reports at OPI 12. The HMI on the machine will clear alarm 120202 within one NCK cycle.

Procedure — Load the NC and PLC Archives via HMI Advanced

1. Copy the most recent NC.arc and PLC.arc from a known-good backup (in this case the 2022 set) to a USB stick or to a network share reachable by the service laptop.
2. In HMI Advanced, navigate to Start-up > Commissioning > Archive/Restore. With OPI now resolving, the new NCU 3 will be selectable as a target.
3. Select the NC.arc file and execute Restore. The NCK will erase its current non-volatile configuration and overwrite it with the archive. Do not interrupt the restore; an interrupted NC restore bricks the boot sector and requires a CF-card re-image.
4. After the NC restore completes and the NCK reboots, select the PLC.arc and execute Restore. The integrated S7-300 PLC will accept the project blocks — including OB100, OB101, OB121, FB, FC, DB and SDB — and re-enter RUN automatically once the last block is written.
5. Reconnect the PROFIBUS DP segment to the machine. Cycle the NCU once more to force a full NCK + PLC warm start with the drives now reachable.

Path 2 — Simatic Manager Direct Load (When HMI Advanced Is Unavailable)

If HMI Advanced cannot be sourced in time, the alternative is to download the project via Simatic Manager / STEP 7 over MPI. This is the path that originally produced the STOP with OB121-missing in the diagnostic buffer, so two safeguards are required:

1. Before downloading, clear the PG/PC interface to talk to the new NCU 3 only. The wrong slot selection is the single most common cause of the OB121-misread scenario — STEP 7 was reporting a different PLC slot's diagnostic buffer, not the target slot's.
2. Download the entire S7 project with PLC > Download User Program to Memory Card rather than block-by-block. This guarantees OB100, OB101 and OB121 are written before the PLC transitions to RUN.
3. After the download, perform an MRES on the integrated PLC to clear any residual diagnostic entries, then issue a warm restart. Confirm the mode selector returns to RUN with a steady green RUN LED and no SF/BF faults.

Verification

After the OPI address correction and archive restore, verify the following in order:

1. Alarm clear: Alarm 120202 disappears from the HMI's alarm bar within one NCK boot cycle (typically under 60 s). The alarm history records the clear timestamp.
2. Active nodes: The HMI's Start-up > Network > Active Nodes shows the new NCU at OPI 12 and DP 7 with the correct device name.
3. PLC state: The integrated PLC on NCU 3 reports RUN, RUN LED steady green, SF LED off, BF LED off. Diagnostic buffer is clean of OB121 entries.
4. Drives: SIMODRIVE 611-D / SINAMICS S120 modules appear on the PROFIBUS scan with their assigned logical drive numbers and have completed the parameter set transfer from the NCK.
5. Reference / axis enable: Issue an NC reset, then take each axis from follow-up mode to the controlled mode. The HMI's Axes screen should display actual position updates within 1 s.
6. Cold restart test: Power the entire cabinet down, restore mains, and observe a clean NCU 3 boot. The 120202 alarm must not reappear; if it does, the OPI parameter was not committed to the CF card's startup section and a full NC commissioning pass is required.

Backup Hygiene and Future Replacement Strategy

The machine's archive history spanning 2003 through 2022 is unusual but valuable. For any future NCU replacement, follow the table below when picking a restore source.

Troubleshooting Matrix

Field-Proven Caveats

• Do not trust the OB121 entry in Simatic Manager's diagnostic buffer at face value when the target slot is uncertain — the buffer belongs to whichever slot the PG/PC interface is bound to, not necessarily the slot you are trying to commission.
• Loading PROFIBUS parameters before MPI/OPI parameters is the most common cause of a "physically visible, logically invisible" NCU. Always commission OPI first, then PROFIBUS.
• On NCU 710.1 / 720.x hardware, the OPI address is stored in the NCK startup file, not in the PLC project. A PLC download alone will never move the OPI address.
• HMI Advanced for PC/PG is still the lowest-risk way to do a single-NC U restoration on a multi-NCU machine because it scopes its address resolution to the OPI node the operator selects.
• The order number 6FC5253-6BX10-4AF0 is the V6.4 build. Older V6.3 builds (e.g. 6FC5253-6BX10-3AF0) cannot open V6.4-format archive headers and will reject the restore with a CRC error.

FAQ