Resolving S7-1200 G2 TLS Error with MTP1200 Unified HMI in TIA Portal V20

The second-generation SIMATIC S7-1200 G2 controller (CPU firmware V20.0 and higher) introduces the same mandatory secure PG/PC and HMI communication model that has shipped with S7-1500 since firmware V2.9. When the new G2 CPU is paired with a Unified Basic panel such as the MTP1200 Unified Comfort, projects that compile cleanly in TIA Portal V20 still download successfully, but the PLC terminates the OPC UA / S7Comm-Plus TLS session immediately after the HMI requests a resource. The error LED on the CPU lights, the diagnostic buffer fills with certificate-related entries, and the panel shows the connection as "Not connected / Connection interrupted".

This article documents the exact failure mode, the underlying TLS / X.509 trust mechanism Siemens introduced in TIA V19 and enforced by default in V20, the published Siemens KB procedure (entry ID 109800515), and the field-verified steps to restore the S7-1200 G2 ↔ MTP1200 link without downgrading firmware.

1. Problem Description

1.1 Symptom matrix

1.2 Sample diagnostic buffer text

The relevant diagnostic events (firmware V20.0) typically appear as:

Event 1   16:42:08.512  2025-01-12
  TLS connection from 192.168.1.12 aborted:
  certificate of the communication partner is not trusted.

Event 2   16:42:08.498  2025-01-12
  HMI connection (HMI_1) - state change to "Connection interrupted".

Removing the HMI device from the project, recompiling, and downloading clears the event because no TLS handshake is initiated.

2. Affected Hardware, Firmware, and Software

3. Root Cause: Mandatory TLS / X.509 Trust Model

3.1 Why V19 changed the rules

Starting with TIA Portal V19, Siemens extended the S7-1500 secure-communication concept to the new S7-1200 G2 family and to WinCC Unified panels. Each end of an HMI/PG link now presents an X.509 device certificate. The receiving side validates:

1. The certificate chain terminates in a trusted root (self-signed Siemens TIA Engineering root, project-specific root, or the WinCC Unified root that the panel firmware embeds).
2. The certificate's notBefore ≤ current time ≤ notAfter.
3. The certificate subject matches the configured partner (CPU name / HMI name).
4. The certificate purpose allows TLS server / client authentication (Extended Key Usage: serverAuth, clientAuth).

If any check fails, the side initiating the secure session (the panel, in this case) terminates the connection. The PLC records a "certificate of the partner not trusted" event and lights the ERROR LED.

3.2 Why the MTP1200 in particular is impacted

WinCC Unified panels generate their device certificate on the engineering PC during project compile and embed it into the runtime image at download time. S7-1200 G2 CPUs likewise generate a CPU certificate when the project is first compiled. Neither certificate is automatically cross-installed on first contact. The engineer must explicitly export the panel certificate and import it into the PLC's trust list (or vice-versa), or rely on the project root that TIA builds when both devices belong to the same TIA project.

If the panel or PLC clock is wrong (RTC drifted, no NTP, battery-backed date lost during long storage), or if the certificate was generated against a project root that was later regenerated, the certificate fails the time-window check above and is rejected as "not yet valid" or "expired".

3.3 Why removing the HMI "fixes" the LED

No HMI → no TLS handshake → no rejection → no error. The PLC is healthy; only the trust chain is broken.

4. Prerequisites for the Fix

• TIA Portal V20 (or V19 as fallback) installed with the matching HSP for the S7-1200 G2 and for the MTP1200 Unified image.
• Engineering PC must reach both 192.168.1.3 (PLC) and 192.168.1.12 (HMI) on subnet 255.255.255.0; the same PG must be the only active TLS initiator during the trust setup.
• Administrator rights on the PC are required to install certificates to Local Machine → Trusted People if the panel uses a self-signed certificate from a non-default root.
• Read/write access to the TIA project, including the project "Certificate Manager" entries.
• Ability to set date/time on both devices (or a reachable NTP source; S7-1200 G2 supports NTP via the CPU PROFINET port).

5. Step-by-Step Resolution

The procedure follows Siemens KB entry 109800515 ("Fix Certificate Error between HMI Unified Panel and SIMATIC S7-1500/S7-1200 or Software Controller") with the G2-specific adaptations validated in field tests.

Step 1 — Synchronise date and time on both devices

1. In TIA Portal, open Online → Accessible nodes, select the CPU, and choose Online & diagnostics → Time. Click Set time on PG/PC.
2. On the MTP1200 Unified panel, open the Control Panel → Date and Time (or push via WinCC Unified tag @TimeLocal) and apply the same PG time.
3. Confirm both screens show year 2024 or 2025. A clock stuck at factory default (e.g. 2010-01-01) instantly invalidates any newly generated certificate whose notBefore is now.

Step 2 — Open the TIA Certificate Manager

1. In the TIA project tree, select Project → Security → Certificate Manager.
2. Confirm two device certificates exist: one for the S7-1200 G2 CPU and one for the MTP1200 panel. They should reference a common "Project root certificate" that TIA auto-generates when both devices are in the same project.

Step 3 — Re-create the certificate trust chain

1. Right-click the Project root certificate and choose Recreate. This regenerates a fresh root, then re-issues the device certificates for the CPU and the panel.
2. Compile the project (Project → Compile all → Software (rebuild all)). Force a full rebuild so the new certificates are embedded into the generated HMI runtime image.

Step 4 — Re-download CPU and Panel

1. Download to the S7-1200 G2 CPU first. The CPU will install its new device certificate into the secure-communication store.
2. Download to the MTP1200 panel. The panel will install the new runtime image including the new device certificate.
3. Power-cycle the panel (Settings → Reboot) so the new certificate is loaded by the secure-communication service.

Step 5 — Validate trust

1. Re-open Online → Accessible nodes → [CPU] and check Diagnostics → Diagnostic buffer. Confirm no new TLS abort entries appear after the next panel connection attempt.
2. Open the panel runtime connection diagnostics; the HMI connection should reach state "Connected (secure)" within 5–10 seconds of reboot.

Step 6 (fallback) — Manually cross-install certificates

If the project root recreation does not propagate (rare, typically when the CPU was previously programmed on a different PG), manually exchange certificates:

1. In the Certificate Manager, select the CPU, click Export certificate, save as PLC_device.cer.
2. Select the panel, open Trusted certificates → Add, and import PLC_device.cer into the panel trust list.
3. Reverse the process: export the panel certificate and import it into the CPU's trust list via CPU → Properties → Security → Trusted certificates.
4. Recompile and re-download both devices.

6. Alternative: Disable Secure Communication (only for non-production benches)

1. Select the S7-1200 G2 CPU in the project tree.
2. Open Properties → Security → Secure PG/PC and HMI communication.
3. Uncheck Activate secure communication for this CPU (or, in TIA V20, set Connection mechanism to Legacy / Non-secure).
4. Compile and download to the CPU. The HMI will now negotiate a plain S7Comm session, identical to the legacy S7-1200 behaviour.

7. Verification

7.1 Quick checks

7.2 PLC-side diagnostic command

From the TIA Portal online command line (or via PRONETA), read the secure-communication partner list:

Online & diagnostics → Security → Secure communication
  Partner: HMI_1 (192.168.1.12)
  State:   Trusted
  Cert:    Valid from 2025-01-12 to 2027-01-11
  Fingerprint (SHA-256): 3F:7A:9B:...

8. Fault-Code / Diagnostic-Event Mapping

9. Time & NTP Recommendations

To avoid recurrence:

• Configure the S7-1200 G2 CPU as an NTP client: Properties → Time of day → Time synchronization → NTP mode. Point at a plant NTP server (typical stratum-2 source, e.g. ntp.example.com) with 10 s polling.
• Push the same NTP source to the MTP1200 via the WinCC Unified Time synchronization settings or via the panel's Control Panel → Date and Time → NTP.
• Maintain RTC battery on the S7-1200 G2; the CPU retains time for ≈ 4 weeks without power.
• Always re-issue certificates after any clock change larger than the validity window (default 2 years from project creation).

10. Network and PG/PC Interface Settings

The TLS handshake rides on TCP/102 (S7Comm-Plus) and TCP/443 (Unified HTTPS). Ensure the engineering PC and panel both allow outbound on these ports and that no stateful firewall is stripping the TLS ClientHello.

On the PG, set the correct access point in TIA Portal → Options → Set PG/PC interface to the NIC connected to 192.168.1.0/24. The wrong S7 access point is the second most common cause of "connection interrupted" events that masquerade as certificate errors.

11. Cross-Platform Notes

11.1 S7-1500 and Software Controller (S7-1500S / ET200SP CPU)

The KB entry 109800515 covers all three families because the TLS / X.509 trust model is identical. For S7-1500 firmware V2.9 and later, the secure-communication toggle is in the same Properties → Security area. The only difference is that S7-1500 supports an OPC UA server certificate in the same trust store, so the panel certificate may need an additional EKU of clientAuth.

11.2 Legacy KTP / TP panels (S7Comm without TLS)

KTP400 Basic, KTP700 / KTP1000 / KTP1200 Comfort on image versions ≤ V17 use plain S7Comm and are not affected. Mixed fleets (one legacy panel plus one Unified panel) require the G2 CPU to terminate both TLS and plain sessions concurrently — this is supported up to the 16-connection limit.

11.3 Third-party HMI on S7-1200 G2

Non-Siemens HMI packages that speak S7Comm-Plus without TLS will be rejected. They must implement TLS 1.2/1.3 with a Siemens-compatible device certificate or use the documented S7-1200 G2 legacy S7Comm path (which still requires firmware parameter Allow legacy non-secure communication to be enabled in TIA V20 CPU properties).

12. Frequently Asked Questions