Configuring Plant-Wide Time Synchronization in SIMATIC PCS 7 with SICLOCK TC400 - UTC vs Local Time Architecture

In SIMATIC PCS 7, plant-wide time synchronization using an external master clock like SICLOCK TC400 requires setting the entire automation backbone—including S7-400 CPUs, CP 443-1 communication processors, CP 1623 network adapters, and WinCC OS Servers—strictly to Coordinated Universal Time (UTC). Distributing UTC ensures leap-second compliance and prevents non-monotonic timestamp anomalies or database key corruption during daylight saving time (DST) transitions. Local time offsets and daylight saving adjustments are handled strictly at the HMI presentation layer within WinCC OS Clients and Servers via Windows regional settings.

PCS 7 Time Synchronization Principles - UTC vs Local Time

Accurate time stamping of process alarms and historical analog trends is critical for event sequence reconstruction, root-cause failure analysis, and regulatory audit compliance in industrial plants. SIMATIC PCS 7 mandates a strict dual-level time management architecture:
1. The Automation Backbone (UTC Mandatory):
- All central controllers (S7-400 / S7-400H), distributed I/O interface modules (ET 200M, ET 200SP), Industrial Ethernet CPs (CP 443-1), and WinCC archive databases operate exclusively in Coordinated Universal Time (UTC).
- Using UTC guarantees that timestamps are continuous, strictly increasing, and immune to 1-hour backwards or forwards jumps caused by seasonal Daylight Saving Time (DST) transitions.
2. The Operator Presentation Layer (Local Time Conversion):
- WinCC OS Clients and OS Single Stations display local plant time (e.g. EST, CET, or CST) in alarm banners, trend rulers, and graphic headers.
- WinCC computes local time dynamically by querying the operating system's Windows Time Zone and DST configuration. Historical records stored in the Microsoft SQL Server database remain timestamped in UTC.

SICLOCK TC400 Architecture - SIMATIC Mode vs NTP Mode

The Siemens SICLOCK TC400 (or SICLOCK TC100) acts as a high-precision central time server synchronized via GPS, DCF77, or external network time sources:
- Network Time Protocol (NTP):
- Operates over standard TCP/IP.
- Recommended for synchronization across the plant bus and terminal bus involving PC stations (WinCC OS Servers, Maintenance Stations, Process Historian, Engineering Stations, and third-party IT equipment).
- Delivers synchronization accuracy within +/- 1 ms to 10 ms depending on network switch jitter.
- SIMATIC Mode (Ethernet Broadcast / SNAP Protocol):
- Uses proprietary Layer-2 Ethernet broadcast frames.
- Historically used for direct synchronization of S7-300/S7-400 CP cards (CP 443-1) without requiring IP stack routing.
- Both NTP and SIMATIC modes broadcast by SICLOCK TC400 must be configured to broadcast UTC. Local time offsets must never be injected into the SICLOCK transmission telegrams destined for PCS 7 automation stations.

Configuring AS Controllers, CP 443-1, and WinCC OS Stations

Configuring end-to-end time synchronization across a PCS 7 plant involves three distinct configuration domains:

SICLOCK TC400 Central Transmitter:
- Set output telegrams on Ethernet port(s) to UTC. Disable local time conversion on the broadcast interface.
- Enable NTP server service and enter the primary GPS/radio synchronization source.
AS Automation Station and CP 443-1 (STEP 7 HW Config):
- In HW Config, open the object properties of the CP 443-1 communications processor.
- Navigate to the Time-of-Day Synchronization tab:
- Check Activate NTP time-of-day synchronization.
- Enter the IP address of SICLOCK TC400 as NTP Server 1 (and an optional redundant clock as NTP Server 2).
- Check Forward time of day to station.
- Ensure the Time Zone parameter is set to UTC (or offset 00:00).
- Open the object properties of the S7-400 CPU:
- Navigate to the Diagnostics/Clock tab.
- Set Synchronization mode on the MPI/DP or internal bus to As Slave.
WinCC OS Servers and Clients (WinCC Explorer):
- Open WinCC Explorer on each OS Server.
- Double-click the Time Synchronization editor in the navigation tree:
- Check Synchronization via terminal bus (Slave) or configure the server to synchronize with the external NTP server.
- Check Access point 1 and bind to the dedicated network adapter.
- Open Computer Properties > Parameters:
- Under Time basis for time display in runtime, select Local time.
- Under Time basis for time stamping in messages, select Coordinated Universal Time (UTC).
- In Windows OS Date and Time settings, set the appropriate local time zone and check Automatically adjust clock for Daylight Saving Time.

PCS 7 Time Synchronization Subsystem Configuration Table

The following table details the time configuration parameters across all PCS 7 plant nodes:

Subsystem NodeHardware / Software EntityProtocol UsedTime Format ConfiguredRole in Time HierarchySICLOCK TC400Master Clock TransmitterGPS / NTP / BroadcastPure UTCPlant Master Time Server (Stratum 1)Industrial Ethernet CPCP 443-1 (S7-400)NTP (Layer 4)Pure UTC (Offset 00:00)Time Slave to SICLOCK, Master to CPU backplaneController CPUS7-400 / S7-400HS7 Backplane BusPure UTCTime Slave to CP 443-1; tags process alarms in UTCOS Server (WinCC)Industrial PC / IPC647DNTP / Windows TimePure UTC Database StorageSlave to SICLOCK; translates UTC to Local Time for ClientsOS Client (WinCC)IPC427D / IPC477DWinCC Terminal Bus SyncUTC in background, Local on HMIDisplays local time in graphic alarms, trends, and logsProcess HistorianPH 2014 SP2 / PH 2020NTP ClientPure UTC Database ArchivesArchives all historical long-term data in UTC

Verification

To verify that plant-wide time synchronization is working properly without time drift:
1. Open the diagnostics buffer of the S7-400 CPU in SIMATIC Manager. Check the latest entries and verify that timestamps align perfectly with UTC (e.g. 5 hours ahead of EST).
2. In WinCC Explorer on the OS Server, open the Time Synchronization editor and check the diagnostic status field to verify that the station is synchronized to the master clock IP.
3. Open the WinCC Alarm Logging runtime banner:
- Verify that incoming process alarms display the correct local time corresponding to the operator's clock.
- Hover or inspect the alarm properties to confirm that the underlying timestamp matches controller UTC.
4. Verify that during daylight saving time transition tests, archive trends maintain smooth continuity without overwriting existing data or creating negative-interval gaps.

Limitations and Operational Precautions

When deploying time synchronization in SIMATIC PCS 7:
1. Never Configure AS Controllers to Local Time: Setting the CP 443-1 or S7-400 CPU to local time causes severe database faults in WinCC and Process Historian during autumn DST fallback (when 02:00 rolls back to 01:00), leading to duplicate primary keys and discarded historical data.
2. Avoid Multiple Independent NTP Masters: Ensure all plant subnets (Plant Bus and Terminal Bus) reference the same SICLOCK master clock. Running unsynchronized domain controllers alongside a GPS clock causes oscillating time adjustments (hunting) on WinCC servers.
3. CP 1623 Time Stamping Mode: When high-precision 1 ms SOE (Sequence of Events) time stamping is required for DI modules, ensure the CP 1623 adapter operates in time slave mode receiving broadcast telegrams over the industrial plant bus.

Engineer FAQ

Why does WinCC show alarms one hour off after daylight saving time changes?

This occurs if the Windows time zone on the OS Client is misconfigured or if the OS Server was parameterized to broadcast local time instead of UTC. Ensure Windows 'Automatically adjust clock for Daylight Saving Time' is enabled on all OS PCs.

Can S7-400H controllers synchronize via standard Windows NTP domain controllers?

While CP 443-1 supports NTP, standard Windows domain controllers do not meet the precision requirements of PCS 7 (jitter can exceed 100 ms). Siemens mandates a hardware master clock like SICLOCK TC400 or an industrial-grade IEEE 1588 / NTP time server.

What is the difference between SIMATIC Time Mode and NTP Time Mode on CP 443-1?

SIMATIC Time Mode uses Layer-2 Ethernet broadcast telegrams (MAC address based) and cannot pass through IP routers. NTP operates via standard UDP port 123 over IP, allowing synchronization across routed subnets and managed switches.

Does changing time in the CPU cause bump in PID controllers or SFC sequencers?

Small continuous time corrections (time slewing) do not disrupt PID blocks or SFC steps. However, sudden manual step changes of several minutes can corrupt integration times in PIDConL or cause premature step timeouts in SFCs. Always synchronize continuously.