Can the Siemens S7‑400H Synchronize Its System Clock with an NTP Server over Ethernet

For engineers managing redundant high-availability plants, time synchronization is not a luxury—it is a safety prerequisite. The Siemens S7‑400H is a fault-tolerant PLC widely used in power, oil & gas, and water treatment. A common question is whether this system can align its internal clock with an external NTP server via standard Ethernet. The short answer is yes, but with clear architectural constraints. This post explains exactly how to achieve it, what limitations exist, and how Tongtu helps global teams implement this integration reliably.

NTP Support in the S7‑400H: The Technical Reality

The Siemens S7‑400H does not have a built‑in NTP client in its firmware (unlike S7‑1500). However, synchronization is possible through two proven pathways:

Critical note: The CPU’s internal clock (hardware RTC) drifts ~2‑5 seconds per day. Periodic NTP correction is mandatory for timestamped events (e.g., fault records, SOE).

Step‑by‑Step Configuration for CP‑Based NTP (Preferred)

For most Tongtu‑supported projects, we recommend the CP 443‑1 Advanced (6GK7443‑1EX30‑0XE0) with firmware V3.2 or higher. Procedure:

1. Assign an IP to the CP in HW Config (same subnet as NTP server).

2. Open CP properties → “Time synchronization” tab → enable “Activate NTP client”.

3. Enter up to 4 NTP server IPs (primary + secondary).

4. Set update interval (default 60 s; adjust based on network load).

5. Download hardware configuration and reboot the CP.

The CP then periodically polls the NTP server and adjusts the CPU system time automatically—no extra code required.

When No CP Is Available: SNTP via User Program

If you use an older communication processor or only the onboard PN interface (CPU 41x‑H), you must implement a lightweight SNTP request in STEP 7 (V5.6) or TIA Portal:

• Use AG_SEND/AG_RECV (or T‑blocks) to send a UDP packet to NTP server port 123.

• Parse the 64‑bit timestamp in the response (seconds since 1900).

• Adjust for timezone offset and write to the system clock using SFC0 “SET_CLK” .

This approach requires careful programming but works reliably. Tongtu offers pre‑tested function blocks for this scenario, reducing engineering risk.

Performance & Redundancy Considerations

In an S7‑400H redundant pair, both CPUs must have identical system times to ensure consistent failover logs. The CP‑based NTP syncs both units simultaneously (because they share the same CP if configured in the rack). For software‑based SNTP, you must trigger the request on both CPUs separately—and handle the sync‑up time gap during master‑standby role changes.

3 Essential “Siemens S7‑400H” FAQ – Common Questions from the Field

Q1: Can the Siemens S7‑400H synchronize time via NTP if the NTP server is on a different subnet (routed network)?
A1: Yes, provided your CP or CPU’s gateway (router) is correctly set in the hardware configuration. The NTP protocol works over routed IP networks. However, you must ensure UDP port 123 is not blocked by firewalls. For high‑availability, place at least two NTP servers on separate subnets and configure both in the CP’s server list. If using software‑based SNTP, your code must handle the routing via the configured default gateway in the hardware settings. Tongtu recommends using the CP‑based method for routed environments, as it automatically retries on timeout and logs connection failures in the diagnostic buffer.

Q2: Does an NTP sync affect the S7‑400H’s cyclic interrupt (OB 10–17) timing or watchdog monitoring?
A2: No. The NTP correction is applied smoothly to the system clock (RTC) without altering the CPU’s internal 1‑ms tick counter used for cycle monitoring and time‑driven OBs. The CPU adjusts the clock gradually (by slowing or accelerating the RTC) rather than performing a hard jump, unless the time difference exceeds a built‑in threshold (typically > 1 minute). In that case, a hard correction occurs, which may cause a brief discontinuity in timestamped logs but never triggers a CPU stop or watchdog fault. To avoid large jumps, set the NTP poll interval to ≤ 60 seconds and ensure your NTP server is stable.

Q3: How can I verify that the NTP synchronization is actually working on my running Siemens S7‑400H?
A3: Use three independent methods:
(1) Read the CP’s diagnostic buffer via STEP 7 (look for event IDs indicating “NTP server responded” or “time updated”).
(2) In the CPU’s online diagnostics, monitor the “Time” tab—the “Last sync source” will show “NTP” if active (for CP‑based sync).
(3) For software‑based SNTP, implement a status bit in your data block (e.g., “NTP_OK”) and log the offset value each cycle. Additionally, you can compare the CPU clock against a known reference using SFC1 “READ_CLK” and output the result to a test variable. Tongtu’s standard commissioning checklist includes a mandatory NTP validation step with a screenshot saved to the project documentation.

Why This Matters for Compliance & Auditability

Regulatory frameworks (e.g., NERC CIP, IEC 62443) demand traceable event logs with synchronized timestamps across all control devices. Without NTP, a plant’s historian, HMI, and PLC clocks drift apart—making root‑cause analysis of trips nearly impossible.

Common Pitfalls to Avoid

• Firewall blocking UDP 123 – Always test with a simple PC‑based NTP client first.

• Incorrect timezone settings – The CP sends UTC to the CPU; you must configure the local offset in HW Config.

• Leap‑second handling – Standard NTP ignores leap seconds; for sub‑ms precision, consider PTP (IEEE 1588), which the S7‑400H does not support natively.

Summary Table: NTP Options for Siemens S7‑400H

Final Verdict

The Siemens S7‑400H can indeed synchronize its system clock with an NTP server over Ethernet—most robustly via a CP 443‑1 Advanced, and alternatively through custom SNTP code. Both methods are field‑proven, provided you respect the redundancy logic and network routing.

Contact Us

Time synchronization is just one piece of a resilient control architecture. If you are planning a new Siemens S7‑400H project or upgrading an existing line, Tongtu delivers ready‑to‑use function libraries, remote diagnostics, and on‑site validation services tailored to your plant’s network topology. Reach out to our engineering team for a free configuration review—we respond within 4 business hours. Contact Tongtu today and ensure every millisecond counts.