C135 PCS 7 (PCS7)
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Diagnosing Active Master Bumping and Cyclic Time Synchronization Alarms in SIMATIC PCS 7

Diagnosing Active Master Bumping and Cyclic Time Synchronization Alarms in SIMATIC PCS 7

In SIMATIC PCS 7 (such as V7.0 and V7.1), recurring system alarms appearing every minute stating that the OS has become the 'Active Master'—followed by immediate bumping back to Standby Master or Slave—are caused by time synchronization master arbitration conflicts. When evaluating internal tags @TimeSyncDevice1State and @TimeSyncDevice2State, engineers frequently mistake the tag status (e.g., status 0x80 = Good, 0x4C = Unconfigured second access point) for the actual text value. The root cause is typically multiple competing time masters on the plant bus: for example, a WinCC OS Server configured as Active Master while an external SICLOCK central master clock, corporate Active Directory Domain Controller, or S7-400 CP 443-1 is simultaneously broadcasting time frames, causing the OS to repeatedly seize and lose master status.

Table of Contents

Architecture of Time Synchronization in SIMATIC PCS 7

Deterministic sequence-of-events reporting and millisecond alarm resolution in a distributed DCS require that all Automation Stations (AS), OS Servers, OS Clients, and Process Historians share an identical time base. In SIMATIC PCS 7, time synchronization is administered across two independent subnets:

  1. Terminal Bus: Synchronizes OS Clients, OS Servers, and Engineering Stations using standard Windows Network Time Protocol (NTP) or SIMATIC PDC.
  2. Plant Bus: Synchronizes S7-400 / CPU 410 controllers and OS Server communication processors (CP 1623 / CP 1613) using the ISO Industrial Ethernet / SIMATIC time-of-day protocol.

Understanding Internal TimeSync Tags and Status Codes (80 vs 4C)

WinCC tracks time synchronization state using internal system tags:

  1. Tag Names: @TimeSyncDevice1State (Access Point 1 / CP 1623) and @TimeSyncDevice2State (Access Point 2).
  2. The Status vs Value Confusion: In WinCC V7.0, hovering over a tag in Tag Management displays its Tag Status, not its text value. Status 16#80 indicates quality code 'Good' (valid data exists). Status 16#4C indicates that Access Point 2 is unconfigured in the Time Synchronization editor.
  3. Viewing the Actual String Value: To inspect the real text string (e.g., Active Master, Standby Master, Slave, Error), place an I/O field connected to @TimeSyncDevice1State formatted as a String on a diagnostic screen, or read it using Siemens OPC Scout.

Root Cause Analysis: Why 'Master Bumping' Occurs Every Minute

The symptom where an OS Server broadcasts 'OS is Active Master' every 60 seconds, followed by loss of mastership, is known as Time Master Bumping:

  1. Multiple Competing Masters: The WinCC Time Synchronization editor is configured as 'Active Master'. However, another device on the Plant Bus—such as a GPS central clock (SICLOCK TC 100/400) or an S7-400 CPU—is also broadcasting time-of-day synchronization frames.
  2. Arbitration Knockout: The OS Server asserts mastership, sending a broadcast frame and logging an alarm. Seconds later, it detects a higher-priority or external sync telegram from the competing master. By protocol rule, it demotes itself to Slave.
  3. Recurrent Cycle: Seeing no further frames from its perspective, the OS Server attempts to reclaim mastership 60 seconds later, creating an endless alarm flood.

Step-by-Step Procedure to Establish a Single Time Master Hierarchy

To eliminate time bumping and achieve a stable synchronization hierarchy:

  1. Designate a Single Master Source:
    - Scenario A (External GPS/SICLOCK): The SICLOCK unit is the sole Active Master on the Plant Bus. In WinCC Time Synchronization editor, configure the OS Server as a Slave (Access Point 1 = Slave).
    - Scenario B (OS Server Master): If no external clock exists, designate the Primary OS Server (SERV_A) as Master and the redundant partner (SERV_B) as Standby Master. In HW Config of all S7-400 CPUs, set the CP 443-1 / CPU time synchronization mode to In SIMATIC mode as Slave.
  2. Eliminate Domain Controller Time Clashes: Ensure the Windows Time Service (w32time) on the OS Server does not conflict with SIMATIC Time Sync. Point all PC stations to a single centralized NTP source.
  3. Save and Restart: In WinCC Explorer, open Time Synchronization, click OK to save, and restart WinCC Runtime.

Comparison of PCS 7 Time Synchronization Topologies

The following table compares different time synchronization architecture options in SIMATIC PCS 7:

Synchronization StrategyMaster DevicePlant Bus ProtocolTerminal Bus ProtocolMillisecond AccuracyExternal GPS / SICLOCK (Best Practice)SICLOCK TC400 HardwareSIMATIC Time Telegrams (10 ms)NTP / SIMATIC PDCHighest (< 1 ms resolution)Redundant OS Server Pair as MastersPrimary OS Server (SERV_A)SIMATIC Time BroadcastNTPHigh (< 10 ms resolution)S7-400 Controller as MasterCentral S7-400H CPUSIMATIC Master BroadcastForwarded via OSModerate (Depends on CPU load)Uncoordinated Multi-MasterConflicting OS + S7-400Clashing TelegramsBroken SynchronizationUnstable (Bumping every min)

Verification

Verify time synchronization stability:
1. In WinCC Runtime, monitor the diagnostic screen displaying @TimeSyncDevice1State; confirm that the string value stays locked at Active Master (or Slave) without fluctuating.
2. Open the WinCC Alarm Logging list; verify that the recurring 1-minute 'Active Master' event message disappears completely.
3. In SIMATIC Manager, check the diagnostic buffer of the S7-400 CPUs; verify that system events report 'Time synchronization received from external master' with zero clock jump warnings.
4. Verify that sequence-of-events alarms generated across different controllers display identical synchronized millisecond timestamps.

Limitations and Operational Precautions

Never configure both partners of a redundant OS Server pair as 'Active Master'; one must be configured as 'Master' and the other as 'Standby Master'. If the primary server fails, the standby automatically promotes itself to active master without telegram collision. When using CP 1623 network cards, ensure that time synchronization is enabled inside the Siemens 'Communication Settings' console under the CP 1623 device properties.

Engineer FAQ

What does tag status 80 mean in WinCC?

Status 16#80 is the hex quality code for 'Good'. It indicates that communication to the tag is healthy; it is NOT the text value of the tag.

Why does @TimeSyncDevice2State show status 4C?

Status 16#4C means the second access point is unconfigured or inactive. If your server only uses one network card (CP 1623) for time synchronization, status 4C on device 2 is completely normal and expected.

How do I view the actual text inside @TimeSyncDevice1State in WinCC V7.0?

Create an I/O Field in Graphics Designer, set the data type to 'String / Text', and connect it to @TimeSyncDevice1State. Alternatively, query the tag using Siemens OPC Scout.

Technical References

Related Articles

Related Tags

  • Time Synchronization
  • TimeSync
  • SICLOCK
  • CP 1623
  • WinCC OS Server
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