Resolving 840Di sl PCU 50.3 'Waiting for NC/PLC' Communication

Problem Overview

During new commissioning of a SINUMERIK 840Di sl with PCU 50.3 (PCU Base Software 01.04.02), the system HMI continuously displays the status message "Waiting for NC/PLC" immediately after a programming device (PG) or external HMI is connected to the system Ethernet. The same message appears when the PG/HMI Ethernet cable is removed, indicating that the PCU 50.3 has lost its internal connection to the NCK and PLC runtime that reside on the NCU.

This fault is not a hardware failure of the PCU 50.3 or the NCU. It is a consequence of the system network interface being assigned an IP address outside the SINUMERIK solution line default subnet. The default system network IP of the PCU 50.3 is 192.168.214.241 and that of the NCU is 192.168.214.11. Any change to these defaults without synchronized changes on the NCU side produces a hard partition between HMI and NC/PLC.

840Di sl / 840D sl System Network Architecture

The SINUMERIK 840Di sl uses a distributed control topology in which the PCU 50.3 is the HMI/operator panel host and the NCU is the NCK/PLC host. The two are coupled over an isolated system network, not the plant network. The PCU 50.3 contains two factory-preset Ethernet interfaces (Ethernet 1 and Ethernet 2) per the PCU Base Software documentation, and they are preconfigured for the SINUMERIK solution line system network.

• PCU 50.3 (HMI/operator side): System network interface, default IP 192.168.214.241, subnet 255.255.255.0.
• NCU (NC/PLC side, port X120): System network interface, default IP 192.168.214.11, subnet 255.255.255.0.
• Plant/company network: The second PCU 50.3 Ethernet port is typically routed to the plant LAN, isolated from the X120 system network by a managed or unmanaged switch.
• Thin Client Units (TCU): Connect into the system network switch and obtain their configuration from the PCU 50.3 via DHCP-like configuration frames.

The PCU Base Software Installation Manual (IM8) explicitly states that the two Ethernet interfaces on the PCU 50.3 are suitably preset for the SINUMERIK solution line system network. Modifying these defaults without updating the NCU side and the TCU configuration scripts breaks HMI-to-NCK/PLC name resolution and ARP forwarding, which manifests as the "Waiting for NC/PLC" banner.

Default IP Address Reference

The user's symptom is consistent with assigning the PCU 50.3 system port to 192.168.0.1 while the NCU X120 remained at 192.168.214.11. The two devices are then on different subnets, the PCU cannot ARP-resolve the NCU, and the HMI times out on every MMC/INM connection request.

Root Cause Analysis

Four root causes have been observed in field commissioning of the 840Di sl with PCU Base Software 01.04.02:

1. Misconfigured PCU 50.3 system IP. The system network card has been re-IPed (for example to 192.168.0.1) without updating the NCU X120 side or the TCU configuration. The HMI cannot route to the NCU and falls into the "Waiting for NC/PLC" state.
2. Software installation assumed TCU presence. The default PCU Base Software installation script checks whether the system Ethernet port is active during install. If it sees a link, it assumes a TCU is connected and writes TCU-specific registry/configuration. When the configuration is OP10/OP12 (panel PC) without TCU, the resulting HMI configuration cannot establish a session with the NCK/PLC.
3. NCK reset reverting IP. After an NCK reset (NCK general reset) or power off/on, the NCU system IP can revert to the default 192.168.214.11. The PCU 50.3, which may have been re-IPed to a different subnet, then loses the route. A similar behavior has been observed on the PCU 50.3 side where the IP flips back to the default after a PCU Base Software reinstall.
4. Disabled boot support on the system network card. In Windows network adapter advanced properties (Siemens network configuration tools), the "Boot Support" or equivalent option on the system network card must remain enabled. Disabling it removes the protocol bindings the HMI uses to discover the NCK/PLC.

Diagnostic Procedure

Execute the following checks in order before changing any configuration. Each step isolates one layer of the failure.

1. Verify physical layer. Confirm the link LED on PCU 50.3 Ethernet 1 and NCU X120 is on. The system network cable must run from PCU 50.3 Ethernet 1 to the NCU X120 port (or to a switch that connects to X120). Replace the patch cable if the link is intermittent; a flapping link is enough to keep the HMI in the "Waiting for NC/PLC" state.
2. Verify IP address state on the PCU 50.3. Open a service desktop on the PCU 50.3 (the panel PC or via remote desktop) and run ipconfig /all. The system Ethernet adapter must show 192.168.214.241 with subnet 255.255.255.0. If it shows 192.168.0.1 or any other address, this is the immediate root cause.
3. Verify IP address state on the NCU. From the service menu or via a PG, confirm the NCU X120 is 192.168.214.11 / 255.255.255.0. If the NCU is showing a different address (for example from a previous commissioning), it will not respond to the PCU's ARP requests.
4. Ping both directions. From the PG: ping 192.168.214.241 (PCU) and ping 192.168.214.11 (NCU). Then from the PCU 50.3 itself (remote desktop or local): ping 192.168.214.11. If the PG can ping the PCU but not the NCU, the NCU is either powered down, on the wrong port, or on the wrong IP. If the PCU itself cannot ping the NCU, the system port on the PCU is misconfigured.
5. Inspect boot support setting. On the PCU 50.3, in Windows Network Connections, open the system adapter Properties → Configure → Advanced and verify that any "Boot Support", "PXE Boot", or Siemens-specific option is Enabled. Disabling these breaks the HMI's discovery protocol.
6. Check PCU Base Software install type. If OP10/OP12 panel PCs are in use rather than TCU-based operator stations, the PCU Base Software must be installed with all system Ethernet ports disconnected. Reinstall with the procedure described in the next section.

Step-by-Step Resolution

Option A - Restore the default PCU 50.3 system IP (preferred for new commissioning)

1. Connect a PG to PCU 50.3 Ethernet 1 directly with a crossover or auto-sensing patch cable. The PG should be on 192.168.214.x with mask 255.255.255.0 for this procedure (for example 192.168.214.10).
2. Open the PCU service desktop (OP10/OP12 panel or via remote desktop session).
3. Open Start → Control Panel → Network and Sharing Center → Change adapter settings.
4. Right-click the system Ethernet adapter (typically labeled for the SINUMERIK system network) and select Properties → Internet Protocol Version 4 (TCP/IPv4) → Properties.
5. Set the IP address to 192.168.214.241 and the subnet mask to 255.255.255.0. Leave the default gateway and DNS fields blank on the system port.
6. Click OK and close the adapter properties.
7. Open a command prompt on the PCU 50.3 and run ping 192.168.214.11. The NCU X120 should reply. If not, check the NCK-side IP per the NCU commissioning procedure.
8. Restart the HMI software or reboot the PCU 50.3. The "Waiting for NC/PLC" banner should clear within 30-60 seconds as the HMI establishes the INM/MMC connection to the NCK and PLC.

Option B - Match the PCU to a non-default IP (only if required by the plant network plan)

1. Confirm the NCU X120 IP has been set to match. The NCU's system IP is configured via the NCK startup file or the SINUMERIK commissioning tool. A typical alternative range is 192.168.0.11 for the NCU and 192.168.0.1 for the PCU 50.3, but the pair must be in the same /24 subnet.
2. Set the PCU 50.3 Ethernet 1 to 192.168.0.1 / 255.255.255.0.
3. Update the PCU Base Software registry entries (typically under HKLM\SOFTWARE\Siemens\SINUMERIK and the HMI configuration under System Network Configuration) so that the HMI targets the new NCU IP 192.168.0.11. Refer to the IM8 PCU Base Software manual for the exact registry path for your software version.
4. Reboot the PCU 50.3 and verify the HMI establishes the NCK/PLC session.

Option C - Reinstall PCU Base Software for OP10/OP12 operation

1. Power off the PCU 50.3.
2. Disconnect every Ethernet cable from the PCU 50.3, both Ethernet 1 and Ethernet 2.
3. Boot the PCU 50.3 from the PCU Base Software installation media.
4. During the install, when prompted for the network topology, select the OP10/OP12 panel PC profile (not the TCU profile). With no Ethernet link present, the installer will not write TCU-specific configuration.
5. Complete the install of the PCU Base Software, then the NCU, PLC, and HMI software stacks in the order specified in the IM8 manual.
6. After install, set the system port to 192.168.214.241 per Option A and reconnect the NCU X120 link.
7. Reboot and verify the HMI clears "Waiting for NC/PLC" within 60 seconds.

PCU Base Software 01.04.02 Specific Behavior

PCU Base Software version 01.04.02 has known sensitivities to the network state at install time. The IM8 installation manual records that the two Ethernet interfaces on the PCU 50.3 are preset for the SINUMERIK solution line, but the installer's detection of an active link on the system port triggers a TCU-mode configuration write. The "Waiting for NC/PLC" message on new commissioning with this version is most often a downstream effect of that install-time decision rather than a live network fault.

Verification Procedure

1. From the PCU 50.3 service desktop, open a command prompt and execute ping -t 192.168.214.11 for at least 60 seconds. All replies should be under 1 ms on a direct link, under 5 ms through a switch. Zero packet loss is required.
2. Open the SINUMERIK HMI. The status bar at the top of the HMI must show the NCK and PLC connection state as Connected (or the equivalent green indicator). The "Waiting for NC/PLC" banner must be absent.
3. Navigate to Commissioning → NC/PLC → Connection status on the HMI. Confirm both NCK and PLC show Active.
4. Execute a manual axis traverse from the HMI or via Acknowledgement of a non-axis-moving function (for example M00). A successful acknowledgement round-trips through the NCK and PLC, proving the HMI-to-NC/PLC path is fully operational.
5. If a TCU is present in the system network, repeat the verification from the TCU. A TCU that also shows "Waiting for NC/PLC" indicates a global system network problem (NCU down or wrong IP), while a TCU that works while the OP10/OP12 does not indicates a PCU 50.3-local configuration problem.

NCK Reset and IP Persistence

Both the NCU and the PCU 50.3 can revert to their default system IP after specific events. The behaviors the field engineer must understand are:

• NCK general reset (NCK startup with default machine data): The NCU X120 IP is part of the system image and is reset to 192.168.214.11. If the PCU 50.3 has been moved to a non-default subnet, the HMI will not reconnect after the next NCK reset.
• PCU 50.3 power off/on: On the PCU 50.3 with PCU Base Software 01.04.02, the static IP assigned in the Windows TCP/IP properties is persistent, but a PCU Base Software reinstall returns the system port to the default 192.168.214.241.
• BIOS/UEFI network boot configuration: If the system port is set for network boot (PXE) and the Boot Support option is disabled in the network adapter advanced properties, the HMI binding may not initialize.

Troubleshooting Matrix

Related Configuration Parameters

When the IP plan is intentionally non-default, the following parameters must be coordinated. These are taken from the SINUMERIK 840Di sl/840D sl/840D PCU-Base software manual (IM8) and the NCK machine data documentation.

• MD10010 - Assignment of NCK system IP address (corresponds to NCU X120 default 192.168.214.11).
• MD10020 - HMI system IP on PCU side (corresponds to PCU 50.3 system port default 192.168.214.241).
• MD10030 - Subnet mask for system network (default 255.255.255.0).
• PCU Base Software registry: HKLM\SOFTWARE\Siemens\SINUMERIK\HMISystem\Network - the HMI target NCK address.

The exact machine data numbers and registry paths are version-dependent. Refer to the IM8 manual for the build installed on your PCU 50.3 and the NCK documentation for your NCU firmware build.

Document References

The official Siemens support documentation is the authoritative source for PCU 50.3 system network configuration and PCU Base Software install behavior:

• SINUMERIK 840Di sl / 840D sl / 840D PCU-Base software (IM8) Installation Manual - PCU 50.3 Ethernet interface defaults and PCU Base Software install procedure.