Resolving S7-1500 to TP1200 Comfort HMI Communication Failure: Secure Certificate Time Drift

An S7-1500 CPU 1515-2 PN (6ES7 515-2AM02-0AB0, firmware V2.9) paired with a TP1200 Comfort panel (6AV2 124-0MC01-0AX0, image V17) on TIA Portal V18 may fail to establish a live HMI connection in the topology editor even when both devices download without errors, simulation passes, and both nodes are reachable from the engineering station. The root cause is a date/time drift on the HMI relative to the certificate validity window of the secure HMI connection. Setting the HMI clock to match the PLC clock (or a valid NTP source) restores the connection immediately. This article documents the symptom, the diagnostic trail, the cryptographic reason for the failure, and the permanent configuration that prevents recurrence in fielded systems.

1. Affected Hardware, Firmware, and Software Stack

The following part numbers, firmware levels, and software versions reproduce the symptom. Pin exact catalog numbers and firmware strings in any service report so that spare-parts ordering and Siemens support tickets remain unambiguous.

The TP1200 Comfort ships in the SIMATIC HMI Unified/Comfort family and supports TLS-protected HMI connections when the project is configured for "Secure HMI Connection" in TIA Portal. The S7-1500 family supports the same secure-communication profile from firmware V2.6 onward. The combination in this article (PLC V2.9, HMI image V17, TIA V18) is fully within the supported matrix.

2. Symptoms Observed in the Field

All of the following were true simultaneously on the affected system:

• The TP1200 and the CPU 1515-2 PN were both visible in the TIA Portal "Accessible devices" view (online > accessible nodes).
• Project download to both the PLC and the HMI completed without error.
• Hardware compilation of the PLC produced zero errors, three warnings (no protection level on the CPU, no subnet assigned to the CP 1543-1 X1, and an S7-1500 display without password protection).
• Hardware compilation of the HMI produced zero errors, zero warnings; four tags, one PowerTag, image V17.0.0.0.
• Simulation of the HMI in TIA Portal (RT/PLCSIM) ran without fault.
• The S7-1500 online diagnostics for PROFINET port X1 correctly resolved the TP1200 as a neighbor at 2.19 m cable length, but with a 12 ns signal delay annotation.
• The TIA Portal topology view showed the HMI connection not live; the link between the CPU and the TP1200 was drawn as a static cable without the green/blue "connected" overlay.
• Disconnecting the Ethernet cable from the HMI did not produce the expected "Connection to PLC lost" runtime alarm on the panel.

The combination of "accessible, downloadable, simulates fine, but no live topology link and no lost-connection alarm" is the signature of an HMI connection that is configured but never successfully negotiated. For secure HMI connections, that means the TLS handshake is being aborted before any application data (S7 protocol) is exchanged, so the runtime alarm engine never sees a "link up" event to track.

3. Diagnostic Trail: Compiler Logs, Port X1, and Topology

3.1 PLC Compilation Output

PLC_1                            (compile: ok)
Hardware configuration           (compile: ok)
  S7-1500/ET200MP station_1
  Rail_0
    CP 1543-1_1                 (compile: ok)
      Ethernet interface_1
        X1                      -> "The module is not yet connected to any subnet."
  PLC_1                         (compile: ok)
                               -> "PLC_1 does not contain a configured protection level"
  CPU display_1                 -> "The S7-1500 CPU display does not contain any password protection."
System diagnostics
  Compiling finished (errors: 0; warnings: 3)

The three warnings are housekeeping items, not fault conditions:

3.2 HMI Compilation Output

HMI_1
  Time stamp: 2/16/2024 1:58 PM - 854,616 bytes used of 12,582,912 bytes maximum
  Hardware configuration           (compile: ok)
  Software compilation started.
    Number of tags: 4
    Number of PowerTags used: 1
  Software compilation completed (device version: 17.0.0.0).
  Compiling finished (errors: 0; warnings: 0)

Zero HMI compile warnings mean the project is internally consistent: the HMI connection points to a valid PLC, the tag references resolve, and the image version matches the panel. The HMI side is therefore not the source of the failure.

3.3 PROFINET Port X1 Online Diagnostics

The 12 ns signal delay is a cable length × velocity factor artifact, not a fault. PROFINET devices with copper interfaces only report signal delays below 100 ns when the segment is short (under ~20 m) and the cable is healthy. A real cable problem (damaged pair, missing shield, crossed RX/TX) would either produce a "Port down" state, an elevated signal delay, or a "Neighbor not found" entry. None of those apply here.

3.4 Topology View State

The topology editor distinguishes four visual states for a PROFINET connection:

1. Configured, not connected — drawn as a plain cable, no overlay icon.
2. Configured, live — drawn with the blue/green "connected" overlay, indicating LLDP neighbor + application relationship established.
3. Configured, partner missing — drawn with a red exclamation overlay.
4. Configured, diagnostic pending — drawn with a yellow warning overlay.

State 1 (configured, not connected) is the only one in this article. It means the engineering configuration is present, the link is up, LLDP is running, but the application relationship that the HMI connection is supposed to bring up has not been established. For an HMI connection (S7 protocol), the application relationship is the OPC UA / S7-communication channel — for secure HMI connections, that channel is wrapped in TLS and is therefore subject to certificate validation.

4. Root Cause: Secure HMI Connection Certificate Validity Window

The TIA Portal generates a self-signed X.509 certificate for each secure HMI connection at compile time. The certificate's notBefore and notAfter fields are populated with the current date/time of the engineering station when the project is compiled. When the HMI boots and attempts the TLS handshake, it validates the certificate against its own system clock.

If the HMI's RTC battery has been removed during storage, or if the panel was powered for the first time years after the certificate was generated, the HMI clock may be:

• Reset to a default factory date (commonly 2000-01-01 or 2014-01-01 depending on image version).
• Set to the last time the RTC was kept alive — possibly years in the past.
• Drifting because no NTP source is configured and the panel has been on storage shelves.

In all three cases, the HMI clock is outside the certificate's notBefore window. The TLS handshake fails with a certificate-not-yet-valid error, the HMI connection is never brought up, and the runtime alarm engine never sees a transition from "connection up" to "connection down" — which is why pulling the cable did not raise a "Connection to PLC lost" alarm.

5. Why Date/Time Drift Breaks Secure HMI Communication

The "Secure HMI Connection" option in TIA Portal activates the TLS 1.2/1.3 profile on the S7-communication channel. The PLC presents the certificate generated at compile time; the HMI acts as the TLS client and validates that certificate against its own trust store and clock.

The validation steps that depend on the panel's clock are:

1. notBefore check: The current time must be greater than or equal to the certificate's notBefore field. A panel clock in the past fails this check with certificate_not_yet_valid.
2. notAfter check: The current time must be less than or equal to the certificate's notAfter field. A panel clock years in the future fails this check with certificate_expired.
3. Revocation check (optional): If CRL or OCSP is configured, the panel checks the revocation status at the configured endpoint. The HTTP request carries a date header; an invalid clock can also cause the responder to reject the query.

For projects generated on a fresh engineering station today, certificates typically carry a validity of one to five years. A panel that has been in storage for two years and was never powered on will have a clock from 2014 or 2000, failing the notBefore check immediately. A panel that has been running but without NTP can drift by several seconds per day, eventually falling outside the window for short-lived certificates.

Conceptually:

6. Step-by-Step Resolution

6.1 Immediate Workaround (Field-Level)

1. On the TP1200, open Control Panel > Date/Time (or use the operator-side settings menu if the project does not lock it).
2. Set the date, time, and time zone to match the CPU's clock. The CPU's local time is visible on the S7-1500 display (menu > Settings > Date & Time).
3. Press Apply, then exit the Control Panel. The HMI retries the secure connection immediately; the topology overlay in TIA Portal turns blue/green within a few seconds.

6.2 Permanent Fix: NTP Time Synchronization

Manually setting the clock is fine for a one-time recovery. In production, configure both the PLC and the HMI to synchronize against a common NTP source so that future clock drift cannot push either side outside the certificate validity window.

On the CPU 1515-2 PN (TIA Portal, device configuration of PLC_1):

1. Open PLC_1 > Properties > Time of Day > Time synchronization.
2. Enable Synchronize with NTP server.
3. Add one or more NTP servers (e.g., the plant NTP appliance at 10.0.0.123).
4. Set the update interval (default 10 s) and the correction factor for the local time zone.

On the TP1200 Comfort (TIA Portal, device configuration of HMI_1):

1. Open HMI_1 > Properties > Time / Date / Region.
2. Enable Time synchronization via NTP.
3. Point to the same NTP server(s) used by the PLC.
4. Compile and download the HMI project.

Once both devices pull time from the same NTP source, their clocks agree to within the NTP poll jitter (typically < 50 ms on a healthy LAN), which is well inside any certificate's validity window. The secure HMI connection is then self-healing: a panel reboot, a power outage, or a long storage period can no longer push the panel clock outside the validity window because the next NTP poll restores it.

7. Verification Procedure

Use the following checks to confirm the HMI connection is live after the time fix:

1. Topology view in TIA Portal — the HMI connection line should now display the blue/green "connected" overlay. The status text under the device reads Connection established.
2. PLC online diagnostics — Online > Online & diagnostics > Connections should list the HMI connection with state Established and a non-zero Send/Receive operations counter.
3. HMI runtime — Add a system tag (@ConnectionState or the connection name with the .State suffix) to a diagnostics screen. The state should read Connected (1) instead of Disconnected (0).
4. Cable pull test — Disconnect the Ethernet cable at the HMI. The HMI should now raise a "Connection to PLC lost" alarm within the configured timeout (default 5 s). Reconnect the cable; the alarm clears and the connection state returns to Connected. This test was failing before the time fix; passing it is the definitive verification.
5. CP 1543-1 syslog (if used as security gateway) — Look for TLS handshake success entries from the HMI's IP address.

8. Alternative Root Causes to Rule Out

When the cable pull test still does not raise a "Connection lost" alarm after the time fix, the failure has a different cause. The table below maps the original symptom to the most common other root causes and how to differentiate them.

9. Recommended Field Commissioning Sequence

1. Confirm NTP reachability from the cabinet before connecting the HMI to the network. Use a temporary Windows client on the engineering laptop to ping the NTP server (UDP 123).
2. Power the CPU, configure its PROFINET X1 IP, enable NTP, and let it poll the server for at least one full interval (10 s default) before proceeding.
3. Power the TP1200, open the Start Center (not the runtime project), set the panel's IP, and enable NTP to the same server. Verify the panel reports the correct local time.
4. Download the TIA Portal project to the PLC, then to the HMI. Wait for the HMI to boot into the runtime.
5. From TIA Portal, open the topology editor and confirm the HMI connection is live. Run the cable-pull test; the "Connection to PLC lost" alarm must appear within the configured timeout.
6. From the PLC's online diagnostics, open Connections and verify the HMI connection shows Established with a non-zero operation counter.
7. Tag the engineering station's build server (or the source control check-in) with the certificates' notBefore date so that, when the project is migrated years from now, the new certificates' validity windows are known.

10. Diagnostic One-Pager: Secure HMI Connection State Machine

11. Common Misconceptions Cleared Up

"Simulation works, so the project is fine." PLCSIM and the HMI Runtime simulator do not enforce the secure-comm certificate's real-time clock. A simulator that started at the engineering station's wall clock on 2024-02-16 will pass all certificate checks regardless of the panel's RTC, because the simulator does not consult the panel's clock at all. Simulation therefore cannot reproduce a clock-related TLS failure.

"The topology view showing the cable is good enough." A static cable in the topology editor is the configured connection, not the live one. The live-state overlay is the only topology-view evidence that the HMI connection is up.

"I pulled the cable and the panel did not complain, so the connection must be up." The opposite is true. If the runtime never raised a "Connection to PLC lost" alarm on a cable pull, the connection was never up in the first place. The TP1200 only raises the lost-connection alarm when an established S7 connection is torn down.

"The CPU display does not have a password, so the system is unsecured, but that is unrelated." Correct. The "no display password" warning is unrelated to the HMI connection; it is a housekeeping item about the physical CPU display. The secure HMI connection is governed by the certificate, not by the display password.

"The 12 ns signal delay on port X1 is the problem." No. A 12 ns delay on a 2.19 m copper segment is the expected order of magnitude (roughly 5 ns/m). PROFINET cable diagnostics only flag signal delays above 100 ns or neighbor mismatches. The 12 ns value is normal.

12. FAQ