Almost every electronic device equipped with a CPU contains an internal clock used for timestamping operational data and events.
In industrial automation systems, these timestamps are extremely important because they help engineers analyze alarms, process trends, sequence of events (SOE), and system faults.
In a standalone controller, the internal clock may operate independently without causing any issues. However, in modern industrial plants where Distributed Control Systems (DCS), Programmable Logic Controllers (PLC), Safety Instrumented Systems (SIS), historians, SCADA platforms, and industrial networks exchange data continuously, synchronized system time becomes increasingly important.
Time synchronization ensures that all devices in the plant reference the same clock source, allowing operators and engineers to accurately understand what happened, when it happened, and in what sequence.
DCS and PLC: Different Time Synchronization Characteristics
DCS Systems Depend on Network-Wide Unified Time
A DCS is fundamentally a network-centric control system.
Controllers, operator stations, engineering stations, historians, and servers are interconnected through system networks to perform integrated monitoring and process control.
Because the entire DCS relies on coordinated network operation, most DCS vendors implement a unified system clock architecture. One station within the system typically serves as the master clock, and all other nodes synchronize with it.
Although synchronization methods vary between manufacturers, the principle remains the same:
- The entire DCS operates under a unified internal time base
- Internal synchronization is normally sufficient
- External time servers are not always required
This architecture allows alarm records, historical trends, and sequence events to maintain consistent timestamps across the entire control system.
PLC Systems Are More Independent
Unlike DCS systems, PLC systems are controller-centric rather than network-centric.
A PLC can independently execute control logic even without network communication. Multiple PLCs may later be connected to supervisory systems, SCADA platforms, or HMIs for monitoring and data exchange, but their core control functions usually do not depend on clock synchronization.
However, when multiple PLCs, monitoring stations, or information systems exchange data, unified timestamps become beneficial for:
- Alarm analysis
- Data logging
- Fault investigation
- Historical event reconstruction
Therefore, many PLC systems also implement clock synchronization, although the synchronization methods vary depending on hardware and software architecture.
Why Petrochemical Plants Require Time Synchronization
Modern petrochemical plants typically contain:
- DCS systems
- SIS safety PLCs
- Conventional PLCs
- Fire and gas systems
- Historians
- SCADA systems
- Plant information management systems
All these systems continuously generate large amounts of operational and event data.
During normal production, small time differences between systems may not cause obvious problems. However, during trips, shutdowns, or emergency events, inconsistent timestamps can create serious confusion.
For example:
- Did the SIS trip first?
- Did the process alarm occur before the shutdown?
- Was the valve command delayed?
- Which event actually initiated the sequence?
Without synchronized clocks, engineers may struggle to determine the real sequence of events.
In one refinery shutdown investigation, engineers discovered that the DCS and SIS clocks differed by nearly one minute. Although both systems functioned correctly, the inconsistent timestamps made fault analysis extremely difficult and delayed troubleshooting efforts.
This is why many industrial plants synchronize the clocks of DCS and SIS systems even though their control functionality does not strictly depend on external time sources.
Common Industrial Time Synchronization Methods
Internal System Clock Synchronization
The simplest synchronization method is internal synchronization within the control system itself.
For example:
- A DCS may distribute time signals to all its network stations
- A PLC supervisory node may synchronize connected controllers
- One controller may periodically update the clocks of other devices
This approach is common in smaller systems.
External Time Servers
Large industrial plants often use dedicated time synchronization servers.
These servers may obtain accurate time references from:
- GPS satellites
- BeiDou satellite systems
- NTP servers
- Precision Time Protocol (PTP) sources
The time server then distributes synchronization signals to DCS, SIS, PLC, and other plant systems.
Modern Industrial Time Synchronization Protocols
NTP (Network Time Protocol)
NTP is the most widely used industrial time synchronization protocol.
Typical accuracy:
- Millisecond level
Applications:
- DCS systems
- Historians
- SCADA platforms
- General industrial Ethernet networks
NTP is suitable for most conventional process automation applications.
SNTP (Simple Network Time Protocol)
SNTP is a simplified version of NTP commonly used in PLCs and embedded devices.
Advantages:
- Lightweight
- Easy to implement
- Low network overhead
However, its synchronization precision is lower than full NTP implementations.
PTP (Precision Time Protocol – IEEE 1588)
PTP provides extremely high synchronization accuracy.
Typical accuracy:
- Microsecond level
Applications:
- Power systems
- High-speed automation
- Motion control
- Substation automation
- High-speed event recording
PTP is increasingly used in modern digital industrial systems.
GPS and BeiDou Time Synchronization
Large petrochemical complexes and critical infrastructure facilities often use satellite-based time references.
Advantages:
- High precision
- Plant-wide synchronization
- Independent reference source
In China, many industrial projects prefer BeiDou-based synchronization systems combined with Beijing standard time references.
Important Engineering Considerations
Time Synchronization Signals Should Not Be Updated Too Frequently
Clock synchronization is typically performed periodically rather than continuously.
In many plants:
- Synchronization may occur daily
- Weekly
- Or monthly during stable operating conditions
Frequent synchronization adjustments are unnecessary because modern industrial controllers already maintain highly stable internal clocks.
External Time Sources Can Introduce Risks
External clock sources should be carefully managed.
Abnormal synchronization signals may cause:
- Incorrect timestamps
- Historical data confusion
- Event sequence errors
- Communication instability
In severe situations, major time deviations may even affect system operation or data integrity.
For this reason, engineers should:
- Monitor synchronization status
- Validate time source reliability
- Isolate abnormal external signals
- Avoid uncontrolled external clock inputs
Best Practice for DCS and SIS Synchronization
In many industrial projects, the preferred architecture is:
- A dedicated industrial time server acts as the master reference
- DCS and SIS systems receive synchronization independently
- Each system maintains its own internal clock structure
- Synchronization signals are periodically verified and monitored
Importantly, SIS systems should generally not act as the time master for DCS systems.
Conclusion
In modern industrial automation systems, time synchronization is far more than a simple clock adjustment function.
It is a critical foundation for:
- Accurate alarm analysis
- Sequence-of-event recording
- Historical data consistency
- Fault investigation
- Plant-wide operational coordination
Although DCS, PLC, and SIS systems can usually operate independently using their own internal clocks, synchronized system time greatly improves troubleshooting efficiency and operational transparency.
As industrial networks become increasingly interconnected, reliable and secure time synchronization will continue to play an essential role in process automation and industrial digitalization.