Resolving Siemens TP1200 Sm@rtClient SSL Handshake Failed Error

Resolving Siemens TP1200 Sm@rtClient SSL Handshake Failed Error

The SSL Handshake Failed error returned by the Siemens Sm@rtClient when attempting a remote connection to a TP1200 Comfort Panel is one of the most common VNC-over-TLS issues encountered on the WinCC Comfort/Advanced runtime. Although the message suggests a cryptographic fault, the underlying cause is almost always either a TCP/UDP port conflict on the standard VNC port 5900, a corrupted Panel Images runtime introduced by an automatic TIA Portal update, or a missing port mapping in the customer's routing infrastructure. This document covers the three production-proven fixes, the affected TIA Portal versions, and the verification sequence that engineers can apply in the field without uninstalling the engineering system.

1. Problem Description and Symptoms

Engineers typically observe the following behavior when initiating a Sm@rtClient session:

• The Sm@rtClient connection dialog accepts the panel IP/host name and the configured port (default 5900).
• Authentication succeeds (username/password prompt does not appear, or it does and then fails immediately).
• The client returns: SSL Handshake Failed, Connection terminated, or Secure channel could not be established.
• The same panel can be reached successfully by VNC clients (TigerVNC, RealVNC) on the same port 5900, but only with TLS disabled.
• The error appears consistently, not intermittently, even though it was not present in previous sessions.

Because the error surfaces on a previously working configuration, the root cause is environmental rather than a code defect on the panel.

2. Affected Components, Firmware, and Engineering Versions

If the panel is on a WinCC Comfort V15.1 or earlier runtime, the SSL implementation differs and the fixes below may not apply; verify the firmware version in Start > Settings > OP Properties > System > Device > Firmware Version on the panel.

3. Root Cause Analysis

The SSL Handshake Failed error is a TLS-level failure, but on Siemens Comfort Panels with Sm@rtServer the failure is rarely due to certificate mismatch. In production deployments, three root causes account for the majority of incidents:

3.1 Port 5900 Conflict (Most Common)

VNC's IANA-registered port is 5900. When the customer network implements NAT/port-forwarding to expose the panel to an external client, TCP 5900 is frequently already consumed by another service (a legacy VNC server, a thin-client broker, or a misconfigured VPN concentrator). The TLS handshake fails because the Sm@rtClient negotiates with the wrong backend service, which either drops the ClientHello or terminates the connection without negotiating cipher suites.

3.2 Corrupted Panel Images Delivery (TIA V16 Update 6)

The Panel Images delivery mechanism in TIA Portal V16 downloads the runtime image set into the local Windows Apps catalog. When Update 6 is installed on top of an existing V16 base, the Fit checkbox option can truncate or mis-register the image files. The panel receives a runtime that contains an incomplete TLS stack, and the handshake fails immediately after the ClientHello. The repair procedure is a clean re-installation of Panel Images, with the Fit checkbox left unselected.

3.3 Outbound Port Restriction on the Engineering Station

If the engineer's Windows machine is governed by Group Policy or a corporate firewall that blocks outbound TCP 5900, the Sm@rtClient can establish the TCP socket but cannot complete the TLS handshake. The same machine can usually reach the panel via HTTP/HTTPS on port 80/443, which is why the failure looks "selective". Re-mapping the Sm@rtServer to a non-default port and opening that port in the GPO resolves the issue.

4. Sm@rtServer / Sm@rtClient Architecture Overview

Understanding the protocol stack is essential for the verification steps later in this document.

+---------------------------------------------------------------+
| Sm@rtClient (Windows, TIA add-in or standalone)               |
|     - VNC viewer with TLS                                     |
|     - Reads .tcc configuration from project                   |
+------------------------+--------------------------------------+
                         |  TCP 5900 (default)
                         |  TLS 1.2 handshake (port 5900)
                         v
+---------------------------------------------------------------+
| Sm@rtServer (built into WinCC Comfort runtime on TP1200)      |
|     - Listens on TCP 5900                                     |
|     - TLS 1.2 with X.509 self-signed cert by default         |
|     - Re-uses project user accounts (HMIs users)             |
+---------------------------------------------------------------+
                         ^
                         |  PROFINET / Industrial Ethernet
                         |
                  Comfort Panel TP1200

The Sm@rtServer does not require a public certificate authority. It ships with a self-signed certificate that Sm@rtClient must explicitly trust. The trust relationship is recorded the first time the operator accepts the certificate in the connection dialog. If the panel firmware is re-flashed (for example, after the Panel Images re-installation described in Section 6), the certificate is regenerated and the Sm@rtClient must re-trust it.

The relevant IANA assignments are:

5. Solution 1 - Re-map the Sm@rtServer Port from 5900 to 5910

This is the fastest field fix and the one most often confirmed by Siemens support engineers.

5.1 Procedure

1. Open the TIA Portal project that targets the TP1200.
2. In the project tree, select the TP1200 device and open Properties > Sm@rtServer (sometimes listed under Remote Access depending on TIA version).
3. Enable the Sm@rtServer if it is not already enabled.
4. Change the Port field from 5900 to 5910.
5. Compile (Hardware and Software) and download the project to the panel. A full re-compile is recommended, not a delta download.
6. Cycle power on the TP1200 if the Sm@rtServer does not restart automatically.
7. Update the NAT/port-forwarding rule on the corporate firewall so that the external port also maps to 5910 on the panel side.
8. From the Sm@rtClient, enter the panel IP and port 5910, then reconnect.

5.2 Why 5910?

Port 5910 has substantially less background traffic than 5900 because very few non-VNC services bind there. By keeping the offset to 10 (a small number), the operator does not need to remember an exotic port. The IANA registry lists 5900+n as the dynamic display range for VNC, so any unused display in that range is technically valid. 5910 is the most commonly used alternate in the Siemens ecosystem.

6. Solution 2 - Re-install Panel Images (TIA V16 Update 6 Fix)

If the panel was working before a TIA Portal V16 Update 6 installation and the Sm@rtClient handshake started failing immediately afterward, the corrupted Panel Images delivery is the most likely cause.

6.1 Procedure

1. On the engineering station, open Windows Settings > Apps > Installed apps.
2. Locate the entry SIMATIC Panel Images V16 (the version number is part of the display name).
3. Uninstall it.
4. Open TIA Portal V16 without any installed update (use the base installation media or the V16 base ISO).
5. In Options > Support Packages / Panel Images, download the Panel Images again.
6. During the download, do not enable the Fit checkbox.
7. Continue downloading until the Fit checkbox no longer appears - this usually requires three download iterations.
8. Rebuild the TP1200 project and re-download the runtime to the panel.
9. On the panel, navigate to Start > Settings > OP Properties > System > Transfer and re-enable remote access (Sm@rtServer).
10. Accept the new self-signed certificate in the Sm@rtClient on first reconnect.

This procedure has been reproduced on customer sites and is documented in the Siemens Knowledge Base under the Panel Images V16 Update 6 entry. The repeating Fit-checkbox behavior is the diagnostic fingerprint of the corrupted delivery.

7. Solution 3 - Adjust the Sm@rtServer Service Port and Open It in Policy

When the engineering workstation is governed by a strict Group Policy or perimeter firewall, the fix must be applied on both sides of the connection.

7.1 On the Panel

1. Project > TP1200 > Properties > Sm@rtServer > enable Use custom port.
2. Set the port to any value not used by the host network; 5910 is preferred but 15900, 25900, or any value above the VNC display range is acceptable.
3. Compile and download.

7.2 On the Engineering Workstation

1. Open Windows Defender Firewall with Advanced Security (or the corporate firewall console).
2. Add an Outbound Rule allowing TCP 5910 to the panel's IP/subnet.
3. If the IT department uses GPO-controlled firewall rules, request an exception for the Sm@rtClient executable at %ProgramFiles%\Siemens\Automation\Portal V16\Bin\SmartClient.exe (path varies with TIA version).
4. Reconnect from Sm@rtClient using the new port.

7.3 Verification

Confirm the open path with PowerShell before involving the Sm@rtClient. This isolates network from application faults:

Expected output should show TcpTestSucceeded : True. If it shows False, the issue is network policy, not the Sm@rtServer, and the IT group must be engaged.

8. Certificate Handling and Trust

The Sm@rtServer generates a self-signed X.509 certificate at first boot. The certificate is stored in the runtime image and re-generated on every firmware update. When the panel is re-flashed (for example after the Panel Images re-installation), the certificate changes and the Sm@rtClient must be told to trust it again.

If a public key infrastructure is available, the recommended path is to install a CA-signed certificate into the panel. This is configured under TP1200 > Properties > Security > Certificates in the TIA Portal project. The certificate is exported from the engineering station and downloaded into the panel along with the rest of the runtime. Sm@rtClient then validates the panel against the corporate CA root, eliminating the manual trust prompt on every connection.

9. Verification Sequence

After applying any of the three solutions, walk through the following sequence before declaring the incident resolved.

1. TCP reachability: Test-NetConnection -ComputerName <panel IP> -Port <port> must return TcpTestSucceeded : True.
2. TLS handshake: From PowerShell, attempt (Invoke-WebRequest -Uri https://<panel IP>:<port> -SkipCertificateCheck -Method Head).StatusCode. A 4xx/5xx HTTP error is acceptable; a connection timeout is not.
3. Sm@rtClient connect: Launch Sm@rtClient, enter the panel IP and the configured port, accept the certificate, supply the HMI user credentials.
4. Panel mirror visible: The runtime screen of the TP1200 should render inside Sm@rtClient within 2-3 seconds.
5. Touch forwarding: Click on a button in the Sm@rtClient window and confirm that the action is executed on the panel (e.g., a screen change or a tag toggle).
6. Disconnect/reconnect cycle: Close the Sm@rtClient window and re-open it within 30 seconds. The handshake should succeed without re-accepting the certificate.

10. Troubleshooting Matrix

11. Preventive Measures for New Deployments

• Always set a static IP on the TP1200 or a DHCP reservation. Sm@rtServer sessions break when the IP changes mid-session.
• Document the Sm@rtServer port in the cabinet drawing; treat it as a controlled configuration item.
• When commissioning a fleet of panels, deploy a CA-signed certificate. The one-time effort pays off across every future firmware update.
• Restrict Sm@rtServer access by IP allow-list in the firewall. The HMI user authentication only protects the runtime, not the network.
• Avoid installing TIA Portal updates on the engineering station in the middle of a commissioning campaign. Stage updates on a separate machine first, validate Panel Images delivery, and then promote.
• Keep a clean TIA V16 base image on a backup partition. If Panel Images Update 6 is required for a bug fix, install it on the backup and validate the full download sequence before promoting.

12. Frequently Asked Questions