Resolving PCU 50.5 HMI Connection Loss to NCK 720.3PN on 840D sl

Problem Overview

A SINUMERIK 840D sl installation with an NCU 720.3 PN and an external PCU 50.5 loses HMI/NCK connection after the user edits base.ini to open a firewall port and reboots. After the reboot, the PCU's HMI software (SINUMERIK Operate) no longer establishes a connection to the NCK, while:

• ICMP ping from the PCU to the NCU succeeds.
• The MCP (Machine Control Panel) comes online normally on the NCK side.
• Activating the internal HMI on the NCU and viewing the screen through a VNC client works.
• Disabling the internal HMI and pointing the PCU's SINUMERIK Operate back at the NCU still results in a no-connect state.

This article documents the diagnostic logic, the relevant parameters inside base.ini, the network services that must be permitted, the standard recovery workflow, and the verification sequence used to confirm that the HMI/NCK handshake is fully restored.

SINUMERIK 840D sl System Architecture

The 840D sl is built on three logical components that communicate over TCP/IP through factory-defined ports and proprietary protocols:

The PCU boots Windows, loads the HMI services, and then initiates an HMI/NCK connection to the NCU. The MCP and TP are typically networked to the PCU and reach the NCK through the PCU's HMI proxy. When the MCP is online but the HMI is not, the fault is in the PCU-side HMI/NCK handshake, not in the MCP/NCK link or the underlying IP layer.

Affected Components in This Failure Mode

Root Cause Analysis

When base.ini is modified to expose a service through the Windows firewall and the system is rebooted, the HMI/NCK connection can fail for several reasons. Each must be ruled out in order from cheapest to most expensive to verify.

1. Firewall on the PCU now blocks HMI/NCK sockets

Editing base.ini sometimes triggers the PCU to reload the firewall profile, or the operator (intending to open a single port) replaces the rule set with one that is too restrictive. The PCU must allow outbound TCP from the HMI service to the NCU. If the firewall is blocking the HMI/NCK port, the HMI sits in connect-pending and reports Error 150202 "PCU50.5 not connecting to NCK".

2. base.ini corruption or unintended parameter change

base.ini drives the HMI start logic. A stray character, wrong IP, an extra CRLF, or the wrong section key can break the handshake without the HMI service reporting a clear reason. Reverting the file to its last known-good backup is the standard corrective step.

3. Internal HMI vs. external HMI contention

If Internal HMI is enabled on the NCU and an external HMI on the PCU is also starting, both can race for the HMI/NCK session. The internal HMI takes the seat by default. The external HMI must either wait, or the internal HMI must be disabled, or the PCU must connect with a higher priority and displace the internal session. The symptom in the source — internal HMI works, external HMI does not — is consistent with the NCK accepting the internal session first and not handing the seat over.

4. NCK / HMI version mismatch

If the ghost image restored a different HMI runtime version than the NCK SW on the CF card expects, the HMI/NCK connection may fail to complete its protocol negotiation. A version handshake is performed at the start of every HMI/NCK session; mismatch produces a 150202-class error.

5. Network route or gateway change

Although the user reported that ICMP ping from PCU to NCU works, the TCP path used by the HMI service may traverse a different interface or a different route. With the firewall changes in place, the PCU may be reaching the NCU on the management network while the HMI service is bound to the company network interface (or vice versa).

Diagnostic Procedure

Follow this order. Each step takes only minutes and avoids the need for a full PCU re-image if the cause is recoverable.

Step 1 — Capture the current state of base.ini

Open a command prompt on the PCU with administrator rights and back up the current file before changing anything:

The actual path varies by HMI version; the canonical locations are:

• For older HMI Advanced: F:\Base or the corresponding installation drive.
• For SINUMERIK Operate: C:\Program Files (x86)\Siemens\Automation\SCADA\SinumerikHMI or under the configured HMIVersion path.

Step 2 — Verify HMI/NCK network reachability on the application port

Ping is necessary but not sufficient. Confirm the TCP port the HMI/NCK service listens on is reachable from the PCU:

If telnet is not installed, use PowerShell:

The NCU's HMI/NCK listener uses the SINUMERIK-defined TCP port (19212 in current 840D sl software revisions). A successful TCP three-way handshake shows the firewall and IP routing are both healthy. A timeout indicates either a routing problem, a firewall block, or the NCK not having finished booting.

Step 3 — Inspect the base.ini parameters that govern the HMI/NCK link

Open base.ini with a plain-text editor (Notepad, Notepad++) and verify the following sections. Do not use Word, WordPad, or any editor that can insert Unicode characters.

Step 4 — Revert base.ini to the last known-good version

If a backup exists from before the firewall change, copy it back:

If no backup exists, compare the file against a sibling PCU 50.5 at the same SW level, or against the factory default for the same HMI version. A consistent file will have valid section headers, comments starting with ;, and CR/LF line endings only.

Step 5 — Disable Internal HMI on the NCU

With the PCU present, the internal HMI on the NCU must be disabled so the PCU owns the only HMI/NCK session. On the NCU side this is controlled through commissioning (in HMI Advanced: Startup > Operating Area Switchover; in Operate: Commissioning > HMI > Internal HMI), or by setting [internalHMI]Enable=0 in the NCU's mmc.ini / sldgconfig equivalent for the active boot. A reboot of the NCU is required for the change to take effect.

Step 6 — Audit the Windows firewall on the PCU

Open Windows Defender Firewall with Advanced Security and verify the rules governing the HMI service. The minimum required is an outbound allow rule for the HMI executable to the NCU IP, TCP port 19212. A misconfigured rule set that opens one port and implicitly denies the rest is a common cause of 150202.

Step 7 — Reboot in the correct order

NCK first, then PCU. The PCU's HMI service must not start before the NCK has finished booting and the HMI/NCK listener is ready, otherwise the PCU will time out and report 150202. Allow at least 60 s after the NCU's seven-segment display reaches 6 (Ready / Run) before starting the PCU's HMI runtime.

Step 8 — Capture and interpret HMI/NCK log files

If the error persists, pull the HMI startup log:

Search the log for the strings 150202, connection refused, timeout, and version mismatch. Each corresponds to a different root cause:

Step 9 — If the issue cannot be resolved, restore the ghost image

When the user cannot isolate the corrupt parameter or the file system damage, restore the PCU's ghost image. The PCU 50.5 has a dedicated recovery partition and supports a one-key ghost image restore from boot. The procedure is documented in the PCU 50.5 operating manual referenced below.

base.ini Deep Dive

base.ini is the central configuration file for the HMI runtime on the PCU. Errors in this file are the most common cause of 150202 in 840D sl installations. The file is read at HMI start; it is not re-read during runtime. Any change requires a service restart or a reboot.

Encoding and line endings

The file is encoded in ASCII / Windows-1252. UTF-8 with BOM is not accepted by older HMI versions. Line endings must be CR/LF. Editing in tools that convert to LF only (Notepad on macOS, certain Linux editors) will fail to parse, and the HMI start logic silently ignores the entire file, falling back to defaults that may not match the local NCU.

Section order

The HMI service does not require a specific section order, but every section referenced from [HMI] must be present. A missing [HMI_NCK] section produces a no-connect state without a useful error string.

Common mis-edits

• Editing a section header key (e.g., [HMI] → [HMI ] with a trailing space). The section is then ignored.
• Inserting a Unicode character (em dash, smart quote) by accident. The parser stops at the first non-ASCII byte.
• Setting Address to the management IP of the NCU while the HMI service is bound to the company-side interface. The HMI service uses the IP of the local PCU's outgoing interface, not the destination, to determine routing.
• Replacing the file with one from a different HMI SW version. The section keys differ between SW 4.5, 4.7, 4.8, and 5.x.

Network and Firewall Configuration

The HMI/NCK link is a single TCP session from the PCU to the NCU. The NCU listens on a fixed port. The PCU initiates. Therefore, on the PCU the relevant rule is outbound allow to NCU port 19212.

Firewall rule template (PowerShell)

New-NetFirewallRule -DisplayName "SINUMERIK HMI/NCK outbound" `
  -Direction Outbound -Action Allow `
  -Protocol TCP -RemoteAddress <NCU_IP> -RemotePort 19212 `
  -Program "C:\Program Files\Siemens\Automation\HMI\hmiruntime.exe"

This is the narrowest rule that satisfies the HMI/NCK requirement. The wider the rule, the more risk of inadvertently exposing other Windows services on the company network.

Interface binding on the PCU

The PCU 50.5 typically has two Ethernet interfaces:

If base.ini contains a [HMI]Address value on the 192.168.214.x subnet but the PCU's X2 interface is not connected or its metric is higher than X1, the HMI traffic leaves on the wrong interface and the connection times out. Set the interface metric so that the SINUMERIK plant network interface has a lower metric (higher priority) than the corporate network interface:

Set-NetIPInterface -InterfaceAlias "SINUMERIK_Plant" -InterfaceMetric 10
Set-NetIPInterface -InterfaceAlias "Corporate" -InterfaceMetric 20

Commissioning Workflow After Recovery

Once the HMI/NCK link is restored, complete the following checks before returning the machine to production.

1. Confirm SINUMERIK Operate shows the channel status, axes, and tool list (proves NCK variables are streaming).
2. From the HMI, read a known NCK variable via the Diagnosis > NC/PLC Variables area. A non-zero return confirms bidirectional communication.
3. Cycle power on the NCU with the PCU running; the HMI must auto-reconnect within 30 s. If not, Error 150202 returns and a re-image is the most likely cure.
4. Verify PLC online. From SINUMERIK Operate, open Commissioning > PLC > Online. The PLC must be visible and a project upload must succeed.
5. Test the MCP. Move the axis-selection rotary and confirm the active axis indicator updates on the HMI. This proves MCP → HMI → NCK routing.
6. Capture a fresh ghost image of the PCU. Store it on the recovery partition and on an external backup medium. A known-good image saved immediately after commissioning is the most valuable asset for the next incident.

Verification Checklist

Preventive Measures

• Treat base.ini edits as controlled changes. Always back up the file before editing. Maintain a configuration baseline per machine in a versioned repository.
• Apply firewall changes through a single script and test against the verification checklist before placing the machine back in production.
• Keep NCK SW and HMI SW at matched versions. Use the compatibility matrix in the Base Software Commissioning Manual to plan upgrades.
• Take a fresh ghost image of the PCU immediately after commissioning, after every approved change, and after every software update.
• Disable the internal HMI on the NCU when an external PCU is the intended operator interface. Leaving the internal HMI enabled produces intermittent 150202 errors when both sessions contend for the same HMI/NCK seat.
• Document the HMI/NCK port (19212) and the SINUMERIK plant network (192.168.214.0/24 by default) in the plant's network inventory. The plant network should be a separate VLAN, segregated from the corporate network, and reachable only through controlled paths.

Documentation References

• SINUMERIK 840D sl Base Software Commissioning Manual
• SINUMERIK 840D sl Base Software and Operating Software Commissioning Manual
• SINUMERIK 840D sl PCU 50.5 Operating Manual
• SINUMERIK 840D sl Operator Components Manual
• SINUMERIK 840D sl Network Configuration Manual

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