DNP3 Time Synchronization
Key Takeaway
How DNP3 time synchronization works between master and outstation — LAN and non-LAN procedures, accuracy, GPS alternatives, and why timestamps matter.
Quick Answer
DNP3 includes built-in time synchronization where the master sets the outstation's clock. IEEE 1815 defines two procedures: non-LAN (DELAY_MEASURE / function code 0x17, then a compensated Write Time — for serial and WAN links) and LAN (RECORD_CURRENT_TIME / 0x18, then Write Time with Group 50 Variation 3 — for Ethernet LAN). Do not use the LAN procedure over serial or high-latency WAN links. Accurate timestamps are critical for event sequencing, production accounting, and regulatory compliance.
Why Time Matters
DNP3 events carry timestamps from the outstation's clock. Inaccurate time causes event sequence ambiguity (did the alarm trigger before or after the shutdown?), production accounting errors, and regulatory compliance issues in custody transfer measurement. Outstations commonly assert the NEED_TIME IIN bit when their clock needs refresh.
Non-LAN Time Sync
Use for serial, radio, and IP WAN paths where propagation delay is material:
- Master sends a Delay Measurement request (DELAY_MEASURE, 0x17)
- Outstation responds immediately
- Master calculates round-trip delay
- Master sends Write Time compensated for half the measured delay
- Outstation sets its clock
LAN Time Sync
Use for Ethernet LAN only. The master sends RECORD_CURRENT_TIME (0x18); the outstation records when the last octet arrives. The master then Write Times with Group 50 Variation 3 using the time it recorded when the last octet left its Ethernet hardware. The outstation adjusts for its own receipt-to-clock-set interval. Delay measurement is not used on LAN.
GPS Alternative
For custody transfer and high-accuracy applications, GPS time receivers provide on the order of ±1 microsecond accuracy independent of master communication. Many RTUs (ABB Totalflow, SCADAPack) support GPS receivers for direct time synchronization.
SNTP/NTP
Ethernet-connected outstations can also synchronize via NTP/SNTP servers as an alternative or complement to DNP3 time sync.
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Frequently Asked Questions
Two IEEE 1815 procedures: non-LAN uses DELAY_MEASURE then a Write Time compensated for half the round-trip delay (serial/WAN); LAN uses RECORD_CURRENT_TIME then Write Time (Group 50 Var 3) without delay measurement (Ethernet LAN only).
Typically tens of milliseconds over serial/radio links (±10–100 ms is a common planning range). For higher accuracy, GPS time receivers provide about ±1 microsecond independent of the master.
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