Eliminating Plant-Wide Time Synchronization Drift in SIMATIC PCS 7: Central NTP Masters and Terminal Bus Rules

In SIMATIC PCS 7, time drift where workstations and controllers diverge by several seconds within minutes of manual clock setting occurs because PC real-time clocks (RTC) naturally drift without continuous deterministic synchronization. Plant-wide chronological alarm sequencing requires deploying a central Time Master. In PCS 7, manual clock adjustment is strictly prohibited; systems must synchronize via a dedicated hardware NTP master (such as SICLOCK TC 400 or GPS server). In WinCC, configure Time Synchronization to designate OS Servers as Terminal Bus Time Masters and OS Clients as Slaves. In STEP 7, configure S7-400 CPUs to receive NTP synchronization via CP 443-1.

The Critical Role of Time Synchronization in High-Availability Process Control

In continuous process automation, determining root cause analysis during plant trips requires precise chronological alarm sequencing. If an electrical protection trip occurs in an S7-400 controller, followed 50 milliseconds later by a compressor trip in a second controller, the SCADA alarm journal must reflect the exact millisecond sequence. If controllers and OS Servers drift apart by even 2 seconds, consequential trips appear to precede root causes in alarm archives, corrupting diagnostic investigations.

Hardware Real-Time Clock (RTC) Drift vs Deterministic Network Synchronization

Computer motherboards and PLC hardware clocks rely on inexpensive quartz crystal oscillators:

Thermal fluctuations and voltage jitter cause standard PC RTCs to drift by 1 to 5 seconds per day.
Manually adjusting local clocks via Windows Date & Time is completely futile: clocks immediately begin diverging again within minutes.
Ad-hoc, unsynchronized clocks cause WinCC Redundancy synchronization failures, split-screen timestamp conflicts, and Process Historian archive rejection.

Central Time Master Hierarchy: GPS / SICLOCK -> OS Server -> OS Clients & AS

Siemens PCS 7 mandates a hierarchical time distribution topology:

External Time Master (Tier 1): A dedicated hardware GPS/GLONASS receiver or industrial master clock (such as SICLOCK TC 400) connected to the Terminal Bus and Plant Bus. It broadcasts Network Time Protocol (NTP) telegrams.
OS Servers (Tier 2): Redundant OS Servers act as Time Slaves to the SICLOCK on their Plant Bus adapter, and act as Time Masters on the Terminal Bus.
OS Clients & Engineering Stations (Tier 3): Configured strictly as Time Slaves on the Terminal Bus, synchronizing to the active OS Server.
S7-400 Controllers: Synchronize via CP 443-1 using NTP protocol or SIMATIC mode.

Configuring WinCC Time Synchronization on Terminal Bus

In WinCC Explorer on each station, open the Time Synchronization editor:

On OS Servers:
Check 'Synchronization via Terminal Bus (Slave)' if an external SICLOCK exists, OR check 'Make this computer time master'. Designate OS Server 1 as Master 1 and OS Server 2 as Master 2 (Redundant Partner).
On OS Clients:
Check 'Synchronization via Terminal Bus', select 'Slave', and check 'Receive time from time master'. Enter the Terminal Bus network card.

Configuring S7-400 CPU and CP 443-1 NTP Time Synchronization

To synchronize S7-400 controllers:

In HW Config, open the properties of the Industrial Ethernet processor (CP 443-1).
Navigate to the Time-of-Day Synchronization tab.
Check 'Enable NTP time-of-day synchronization'.
Enter the IP addresses of the primary and secondary NTP time masters (e.g. SICLOCK TC 400 or OS Servers).
Check 'Forward time of day to CPU'.
In the S7-400 CPU properties -> Diagnostics/Clock, set Synchronization mode in AS to As slave on the MPI/DP or internal bus.

Comparative Architecture Matrix: SIMATIC Time Synchronization Protocols

The following table compares synchronization mechanisms across PCS 7 communication tiers:

Synchronization MethodProtocol UsedTypical AccuracyNetwork LayerTechnological ScopeNTP Protocol (Ethernet)RFC 1305 / RFC 59051 ms to 10 msTerminal Bus & Plant BusStandard for modern PCS 7 OS and S7-400 CP 443-1SIMATIC Mode (Industrial Ethernet)Siemens Layer-2 Multicast< 1 msPlant Bus (CP 1623)High-accuracy synchronization for SOE (Sequence of Events)PROFIBUS DP Clock ModeDP-V1 Clock Telegram1 ms to 5 msFieldbus / ET 200MSynchronizes distributed I/O racks for 1 ms SOE stampingWindows Time Service (W32Time)Windows Domain Sync1 to 5 SecondsCorporate LANNot Supported for PCS 7 process control networks

Step-by-Step Implementation of Centralized NTP Time Synchronization

Follow this engineering procedure to establish deterministic time synchronization:

Deploy NTP Time Master: Connect the industrial NTP master clock (SICLOCK TC 400) to the managed network switch. Set static IP (e.g. 192.168.1.250).
Configure S7-400 CP 443-1: In HW Config, open CP 443-1 -> Time Synchronization. Check Enable NTP, enter 192.168.1.250, set update interval to 10 seconds, check Forward to CPU. Save, compile, and download to AS.
Configure CPU Clock Mode: In CPU Properties -> Diagnostics/Clock, set Synchronization in AS to As Slave.
Configure OS Servers: In WinCC Explorer -> Time Synchronization. Set Terminal Bus adapter to synchronize to NTP Master.
Configure OS Clients: In WinCC Explorer -> Time Synchronization on client projects. Check Synchronization via Terminal Bus, set mode to Slave.
Disable Windows Time Service: Disable the native W32Time service on all OS Servers and Clients to prevent Windows from overriding WinCC synchronization.

Verification

Confirm plant-wide synchronization accuracy through these tests:

AS Clock Readout: In SIMATIC Manager, select CPU -> PLC -> Set Time of Day. Verify the module time matches the NTP master within milliseconds.
WinCC Time Diagnostic: Check C:\Program Files (x86)\Siemens\WinCC\diagnose\WinCC_Sys_01.log. Confirm entries: Time synchronization started: Time master active.
Cross-Station Clock Compare: Open command prompts on ES, OS Server, and OS Client simultaneously. Execute time /t at the exact same second. Confirm identical timestamps.
Alarm Timestamp Audit: Generate a test alarm in CFC. Confirm the alarm timestamp in the WinCC Alarm journal matches the exact AS event occurrence.

Limitations and Operational Precautions

Observe the following time synchronization safeguards:

Never Change Clock Backwards During Production: Setting system clocks backwards while WinCC Runtime is active causes duplicate primary key violations in SQL Server and corrupts historical archive indexes. If a backward adjustment is necessary, stop WinCC Runtime first.
Daylight Saving Time (DST) Handling: Always configure AS controllers in Coordinated Universal Time (UTC). WinCC converts UTC to local display time, completely eliminating daylight saving time transition glitches.
Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Engineer FAQ

Can Windows Time Service (W32Time) be used instead of WinCC Time Synchronization?

No. The native Windows W32Time service lacks the millisecond precision and sequence-of-events coordination required for industrial DCS process control.

What happens if a system clock is set backwards while WinCC is in runtime?

Setting the clock backwards causes primary key collision errors in Microsoft SQL Server and corrupts historical archive segmentation; always stop runtime before large clock changes.