Time synchronisation: NTP, SNTP and PTP
Why controllers, SCADA and historians need a common time, NTP/SNTP vs PTP (IEEE 1588) accuracy, time sources, time zones/UTC and practical configuration rules.
Alarm lists, sequence-of-events analysis, historian trends and audit trails are only meaningful if all devices share one time base. A PLC that drifts by two minutes makes a trip analysis impossible.
Protocols
| NTP / SNTP | PTP (IEEE 1588) | |
|---|---|---|
| Transport | UDP port 123 | Ethernet or UDP (ports 319/320) |
| Typical accuracy on a LAN | 1–10 ms (SNTP: tens of ms) | sub-microsecond with hardware timestamping |
| Infrastructure | Any switches | Boundary/transparent clocks in switches for best results |
| Typical use | PLCs, HMIs, SCADA, servers, historians | Substations (IEC 61850), motion, high-speed SoE, power systems |
SNTP is a simplified client implementation found in many PLCs and panels — sufficient for alarm time stamps.
Architecture
- One or two site time servers (GPS/GNSS receivers or a domain controller synchronised to a reliable source).
- Controllers and SCADA servers synchronise to the site servers, not directly to the internet.
- Use UTC internally; convert to local time only for display. Daylight saving changes then do not create gaps or duplicates in history.
- Define who timestamps an event: the I/O module (best), the controller, or the SCADA server (worst — includes communication delay).
Checklist
- NTP server addresses and poll interval configured on every PLC/HMI; firewall allows UDP 123 between zones.
- Time zone and DST settings consistent; controller clocks in UTC where the platform supports it.
- Monitor offset: alarm on time sync loss (most PLCs expose a status bit or diagnostic event).
Connections
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Sources
Updated 2026-10-04