Resolving Faulty 1995 Alarm Timestamps in PCS 7 WinCC Alarm Logging

When process alarms generated by technological blocks in SIMATIC PCS 7 display incorrect timestamps showing the year 1995 in WinCC Alarm Logging, the root cause is an uninitialized or unsynchronized hardware real-time clock in the S7-400 automation controller. Because PCS 7 utilizes chronological, event-driven messaging where timestamps are assigned directly by the controller (ALARM_8P) rather than the operator station, a CPU that has not established network time synchronization defaults to the factory epoch (1994/1995). To resolve this permanently, configure Central Time Synchronization via NTP or SIMATIC Mode in STEP 7 HW Config. For an immediate temporary fix, use PLC -> Set Time of Day in SIMATIC Manager.

Chronological Alarm Timestamping Architecture in PCS 7

In standard factory automation SCADA systems, alarms are timestamped by the supervisory computer when the tag polling routine detects a state transition. In SIMATIC PCS 7, however, alarm annunciation follows a high-resolution, chronological architecture designed for root-cause analysis.

Whenever an analog limit violation or digital trip occurs in Continuous Function Chart (CFC), the technological block (MonAnL, MotL) calls internal system block ALARM_8P. The controller captures the event, stamps it with the CPU's internal real-time clock to 1-millisecond resolution, packages the data into a communication telegram, and transmits it to the WinCC OS Server. If the CPU clock is wrong, the WinCC alarm banner records the incorrect time directly.

Why Unsynchronized S7-400 Controllers Revert to 1995

When an S7-400 CPU (or CPU 410-5H) experiences a complete power loss without an active backup battery, or when a new controller is commissioned out of the box, its internal hardware clock resets to the hardware baseline epoch: January 1, 1994 or 1995.

If the engineer configures communications and downloads logic without establishing automated time synchronization, the CPU increments time from 1995. When ALARM_8P fires, it packages this 1995 timestamp. The WinCC Alarm Logging server receives the packet, trusts the controller's timestamp, and inserts the 1995 record into the historical SQL archive, making recent alarms invisible in active alarm screens.

NTP vs SIMATIC Mode Time Synchronization

PCS 7 provides two standardized industrial mechanisms to maintain microsecond clock synchronization across the Plant Bus:

Network Time Protocol (NTP): Standardized across IT and industrial networks. The S7-400 CPU or CP 443-1 queries a central NTP time server (e.g., SICLOCK TC 100/400, a GPS master clock, or the primary domain controller) at configurable intervals (e.g., every 10 seconds).
SIMATIC Mode Time Synchronization: Proprietary layer-2 broadcast protocol. One node on Industrial Ethernet (typically the Primary WinCC OS Server or a dedicated SICLOCK master) broadcasts periodic time telegrams, and all S7-400 controllers act as time slaves.

Comparison of Time Synchronization Methods

The table below compares methods for establishing correct controller time in SIMATIC PCS 7.

Synchronization MethodAccuracy & ResolutionConfiguration ComplexityPlant-Wide ConsistencyRecommended UseCentral NTP Server (GPS / SICLOCK)High (1 ms accuracy)Low (Enter NTP IP addresses in HW Config)Optimal: Synchronizes AS, OS, and third-party systemsStandard Industry Best PracticeSIMATIC Mode (OS Server Master)High (1 ms on Plant Bus)Low (Configure CP 443-1 as Slave)High across Siemens DCS nodesStandard fallback if no external NTP existsManual Set Time of Day (PLC -> Set)Poor (Drifts minutes per month)Instant (Single dialog in STEP 7)Poor: Drifts; desynchronizes redundant CPUsTemporary commissioning only

Step-by-Step Procedure to Configure Time Synchronization

Follow this procedure to configure robust NTP time synchronization for the S7-400 automation controller:

Configure NTP Synchronization in HW Config:
- In STEP 7, open the hardware configuration of the target S7-400 station.
- Double-click the Industrial Ethernet communication processor (CP 443-1) or onboard CPU PN/IE interface.
- Navigate to the Time-of-Day Synchronization tab.
- Check Enable NTP time-of-day synchronization.
- Enter the IP address of the primary and secondary NTP time servers.
- Set the update interval to 10 seconds.
Configure CPU Time Slave Status:
- Open CPU Object Properties -> Diagnostics/Clock tab.
- Under Clock, set Synchronization mode on MPI: to None, and Synchronization on AS: to As slave.
- Save and compile HW Config, then download to the controller.
Temporary Manual Clock Alignment (Immediate Fix):
- If immediate correction is required before scheduling an outage, select the CPU in SIMATIC Manager.
- Choose PLC -> Set Time of Day.
- Check 'Take from PG/PC' and click Apply to synchronize the CPU clock to the engineering workstation immediately.

Verification

Verify alarm timestamp resolution through the following tests:
1. In SIMATIC Manager, select PLC -> Set Time of Day: confirm that the CPU module time matches the engineering station time within 1 second.
2. In HW Config online view, inspect the CP 443-1 diagnostic buffer: verify event 'NTP synchronization successful'.
3. In CFC online test mode, force a process alarm on a test block (e.g., set MonAnL.AH_Lim below current value).
4. In WinCC Runtime, open the Alarm Logging banner: confirm that the incoming alarm displays the exact current date and time.

Limitations and Operational Precautions

Exercise extreme caution when manually jumping the CPU clock forward or backward by several years on a running plant. Changing the date from 1995 to the current year can cause WinCC Tag Logging and Alarm Logging archives to corrupt current database segments or trigger massive historical compression backlogs in Microsoft SQL Server. If large time shifts are necessary, briefly stop WinCC Runtime, correct the CPU time, purge orphaned temporary archive segments, and restart runtime.

Engineer FAQ

Why do system alarms show the current time but process alarms show 1995?

System alarms originating from the OS computer (e.g., client disconnects) use the Windows OS clock, while process alarms originating from CFC blocks (ALARM_8P) use the S7-400 CPU hardware clock.

What is the maximum time drift allowed between redundant OS Servers and AS?

Siemens specifies that time deviation between automation controllers and OS servers must remain below 1 millisecond on the Plant Bus to ensure accurate sequence-of-events analysis.

Can the WinCC OS Server act as an NTP time server for the AS?

Yes. By enabling the Windows Time (W32Time) service or running the WinCC Time Synchronization option, an OS Server can broadcast NTP time to S7-400 controllers.