Why the Engineering Station Must Not Be Time Master in SIMATIC PCS 7 - Redundant OS Server Clock Synchronization

In SIMATIC PCS 7, the Engineering Station (ES / EWS) must never be configured as the Time Master because the CP 1623 time broadcast is active only when WinCC Runtime is running. Because an ES is frequently shut down, rebooted, or has its runtime stopped to perform engineering changes, using it as a Time Master causes severe clock drift, timestamp gaps, and out-of-sequence alarm logging across the plant. The manufacturer-recommended architecture is to configure redundant WinCC OS Servers as cooperative Time Masters, with the automation stations and ES operating strictly as Time Slaves.

Time Synchronization Architecture and Master Clock Allocation

In SIMATIC PCS 7, deterministic sequence-of-events timestamping requires an unbroken, high-precision clock reference distributed across the plant bus.

When configuring time synchronization on Industrial Ethernet:
* An active Time Master transmits time telegrams (in SIMATIC Mode or NTP) at regular intervals (every 10 seconds).
* Automation stations (S7-400H / CPU 410-5H) and Operator Stations synchronize their internal clocks to these incoming frames.
* If the master clock experiences interruptions, controllers drift independently, resulting in scrambled alarm lists and invalid historical trend alignments.

Why the Engineering Station Fails as a Reliable Time Master

Engineers commissioning new systems often attempt to designate the Engineering Workstation (EWS / ES) as the Time Master because it sits centrally in the engineering office.

This configuration represents a severe operational flaw:
1. Intermittent Operational Lifecycle: Unlike OS Servers, an ES is not an uninterrupted 24/7 runtime node. It is powered down during plant maintenance, rebooted after software updates, and restarted frequently.
2. WinCC Project Exclusivity: An engineer cannot open and edit an OS Server project in SIMATIC Manager on the ES while a local WinCC Runtime is active. The runtime must be closed to make project changes, immediately halting the time master!
3. Clock Hunting: When the ES reboots, its local clock may have drifted by several seconds. When it broadcasts its new time, controllers experience abrupt clock steps, corrupting active batch recipes in SIMATIC BATCH.

CP 1623 Hardware Behavior and WinCC Runtime Interlocking

Engineers frequently wonder why the CP 1623 (6GK1162-3AA00) appears locked as a "Slave" in HW Config:
* Firmware Interlock: The CP 1623 communications processor does not transmit time frames autonomously from a static hardware setting.
* WinCC Runtime Control: The WinCC Time Synchronization application dynamically commands the CP 1623 driver to switch into "Master" mode only after WinCC Runtime has fully initialized and verified network connectivity.
* If WinCC Runtime is not running on the ES, the CP 1623 remains permanently locked in passive slave mode.

Recommended Cooperative OS Server Master Topology

The following table details the Siemens-certified time synchronization architecture:

Station NodeHardware CardTime Synchronization RoleSource / Access PointOS Server A (Primary)CP 1623 (6GK1162-3AA00)Time Master (Active)WinCC Time Synchronization via Plant BusOS Server B (Standby)CP 1623 (6GK1162-3AA00)Time Master (Standby)Automatically assumes mastership if Server A failsS7-400 / CPU 410 ASCP 443-1 / Onboard PNTime SlaveConfigured as "Slave in AS", LAN to stationOS ClientsStandard NIC / SOFTNETTime SlaveSynchronized from OS Server via Terminal BusEngineering Station (ES)Standard NIC / CP 1623Time SlaveSynchronized via DCF77 tool or Terminal Bus

Verification

To confirm that the cooperative OS Server time synchronization is functioning properly:
1. Open WinCC Explorer on OS Server A and Server B; verify that both have Time Synchronization via system bus enabled.
2. In SIMATIC Manager, check the S7-400 CPU time (PLC -> Set Time of Day); confirm the CPU clock matches the OS Server within 10 milliseconds.
3. Deliberately shut down OS Server A (or deactivate WinCC Runtime); verify that OS Server B immediately assumes the Time Master role without clock discontinuity.
4. Verify on the Engineering Station that the local clock synchronizes from the Terminal Bus without drifting.
5. In WinCC Alarm Logging, verify that incoming alarms display accurate UTC timestamps with synchronized chronological order.

Limitations and Operational Precautions

Never configure an Engineering Station to transmit time frames onto the plant bus when redundant OS Servers are already configured as Time Masters. Introducing a third competing time master causes packet collisions on the industrial Ethernet switches, erratic clock jumps, and immediate synchronization failure alarms in the controller diagnostic buffer.

Engineer FAQ

How does the Engineering Station synchronize its own clock if it is a slave?

The ES synchronizes its clock from the OS Server across the Terminal Bus using the Siemens DCF77 service tool (available on the PCS 7 installation media) or via Windows NTP time service.

What happens if both OS Server A and Server B are set as Time Master?

WinCC Redundancy coordinates the two servers. Only the currently active server broadcasts time frames. The partner server monitors the bus and seamlessly takes over time transmission if the primary fails.

Why did the CP 1623 show as Slave in HW Config even after I wanted it as Master?

The mastership of the CP 1623 is set dynamically at runtime by the WinCC Time Synchronization editor; HW Config sets only baseline protocol parameters.

Can an external GPS clock be used instead of OS Servers?

Yes. Industrial GPS master clocks (such as the Siemens SICLOCK TC 400) provide high-precision atomic clock references and are recommended for plants requiring microsecond SOE precision.