Siemens S7 PLC NTP Time Synchronization: Configuration Guide

Overview

Time synchronization between Siemens SIMATIC S7 controllers and an external NTP (Network Time Protocol) server is a routine requirement in process automation, batch recording, event logging, MES data exchange, and WinCC HMI/SCADA timestamping. Whether the PLC is the NTP client and pulls time from a Meinberg, Windows, or Stratum-1 server, or the PLC acts as a master for downstream HMIs, the configuration path differs between the S7-300/400 generation (STEP 7 V5.x / HW Config) and the S7-1200/1500 generation (TIA Portal V15-V20). This reference covers both, the daylight-saving (DST) caveats engineers hit in the field, and the WinCC/WinCC Unified synchronization models.

NTP (RFC 5905) operates over UDP port 123 and supports Stratum levels 1-15. Industrial deployments typically use a local Stratum-1 or Stratum-2 server (Meinberg lantime, GPS-disciplined, or a Windows Server / Linux ntpd instance) to keep the PLC within tens of milliseconds of UTC. The S7 controller stores time as UTC in its system clock; local time, timezone offset, and DST are applied by application code or by the HMI.

Prerequisites

Before configuring NTP, verify the following:

• CPU / CP support: Built-in PROFINET interfaces on S7-1500 CPUs (article numbers 6ES7 5xx-xxx00-0AB0 and newer with firmware V2.0+) and S7-1200 CPUs (firmware V4.0+) support NTP client mode natively. For S7-300/400, an Ethernet CP is mandatory: CP 343-1 (6GK7 343-1EX30, 6GK7 343-1CX10, 6GK7 343-1GX30 and later), CP 343-1 Advanced (6GK7 343-1GX31), and CP 443-1 (6GK7 443-1EX20, 6GK7 443-1EX41, 6GK7 443-1GX30, and Advanced variants). Older CP 343-1 IT (6GK7 343-1GX20) does not support NTP client.
• Firmware: TIA Portal V15.1 or later recommended for unified handling of S7-1200/1500 NTP. STEP 7 V5.5 + SP4+ for legacy S7-300/400.
• Network reachability: UDP/123 must be permitted from the PLC IP to the NTP server. Validate with a Windows w32tm /monitor /computers:<plc_ip> or Linux ntpq -p <plc_ip> from a service laptop.
• NTP server reachable: Field-proven sources include Meinberg Lantime NTP time servers (industrial Stratum-1 with GPS), Windows Server w32time role, and Linux chrony/ntpd. Avoid public internet NTP (pool.ntp.org) in air-gapped plants — use a local appliance.
• Time zone awareness: Document plant timezone (e.g., Europe/Berlin = UTC+1 / UTC+2 with DST) and the cutover rule (last Sunday of March / October in EU; second Sunday of March / first Sunday of November in US).

S7 System Time Data Types and SFCs

Siemens controllers use several time data types per IEC 61131-3:

Core system blocks for time manipulation:

Configuring NTP on S7-1500 in TIA Portal (V15-V20)

1. Open the TIA Portal project and select the S7-1500 CPU in the project tree.
2. Navigate to Properties > General > Time of day.
3. Set the Time zone for the plant (e.g., (UTC+01:00) Amsterdam, Berlin, Bern, Rome, Stockholm, Vienna with daylight-saving rule).
4. Open Properties > PROFINET interface [X1] > Time synchronization.
5. Enable "Synchronize with NTP server".
6. Add up to four NTP servers in priority order. Each entry requires: IP address, port (default 123), and polling interval (default 10 s; range 1-86400 s). Multiple servers provide redundancy — the CPU selects by stratum and reachability.
7. Optionally enable "Forward time of day to station" so the S7-1500 becomes an NTP source for downstream devices. When enabled, the CPU acts as an NTP server on UDP/123 and serves time derived from its own clock (which is the NTP-corrected value). This is the recommended method when the S7-1500 must be the master for ET 200SP IMs, SCALANCE switches, or SINAMICS drives.
8. Compile and download. The configuration writes to the active PROFINET interface parameters; no user program is required.

Per the WinCC Unified Time Synchronization documentation, at least one NTP server must be specified when the option is enabled, and the same Inspector window path is used for HMI panels in the Unified Comfort line.

Verifying NTP on S7-1500

1. From TIA Portal online: Online & diagnostics > Time of day shows the CPU's current system time, status, and last synchronization source.
2. From a service laptop, run w32tm /stripchart /computer:<plc_ip> /dataonly /samples:5 (Windows) or ntpdate -q <plc_ip> (Linux) to query the offset.
3. Read RD_SYS_T in a watch table and compare against a reference wristwatch or GPS-disciplined source. Acceptable offset for MES/batch is typically < 1 s; for OPC UA pub/sub with microsecond timestamps use PROFINET IRT synchronization (separate mechanism, not NTP).

Configuring NTP on S7-300/400 via STEP 7 V5 (HW Config)

1. Open SIMATIC Manager and the project; launch HW Config.
2. Double-click the Ethernet CP (e.g., CP 343-1, 6GK7 343-1EX30).
3. Open the Time-of-Day Synchronization tab.
4. Select NTP Mode > Activate NTP time-of-day synchronization.
5. Check "Forward time of day to station" if downstream S7-300 stations or HMIs should receive the corrected time from this CP.
6. Enter the NTP server IP address (e.g., 192.168.10.50 for a Meinberg Lantime). Multiple NTP servers can be specified comma-separated on capable CP firmware.
7. Set the synchronization interval (default 10 s; minimum 1 s).
8. Download the HW Config to the CPU/CP. The CP becomes the NTP client on UDP/123 and updates the CPU clock at the configured interval.

Configuring NTP on S7-1200 in TIA Portal

1. Open CPU properties > General > Time of day and set the timezone.
2. Open PROFINET interface > Time synchronization (S7-1200 firmware V4.0+; G2 CPUs 6ES7 2xx-1AG40-0XB0 and later support NTP).
3. Enable NTP and add the server. Polling interval default 10 s.
4. Compile and download.

S7-1200 acts as NTP client only — there is no "forward time of day" server mode on 1200. To synchronize a downstream device, use a separate S7-1500 or external server.

PLC as NTP Master for HMIs and Drives

When the S7-1500 should serve time to WinCC Unified panels, ET 200SP stations, or SINAMICS S120 drives, two models exist:

Daylight Saving Time (DST) Handling

Legacy S7-300/400 PLCs do not automatically apply DST rules. NTP delivers UTC, and the S7 system clock is UTC. If the HMI displays "local time" with DST applied, you must either:

• Configure the timezone (including DST rule) in TIA Portal on the S7-1500/1200 under Time of day > Time zone. The CPU will then expose the local time via the system clock register for HMI display.
• In S7-300/400, write application logic that compares the current DATE_AND_TIME against the DST cutover dates (e.g., last Sunday of March and last Sunday of October for the EU) and adds/subtracts one hour. A typical block:
  

  // Pseudocode for EU DST transition
  
  IF (Month = March) AND (Day = LastSunday) AND (Hour >= 1 AND Hour < 2) THEN
  
    // Spring forward: 02:00 becomes 03:00 UTC+1 → UTC+2
  
  END_IF;
  
  IF (Month = October) AND (Day = LastSunday) AND (Hour >= 2 AND Hour < 3) THEN
  
    // Fall back: 03:00 becomes 02:00 UTC+2 → UTC+1
  
  END_IF;

For S7-1500 the recommended approach is to enable the time zone with the correct DST rule in the CPU properties — Siemens encodes the IANA tz database transitions for the selected region. No code required.

Recommended NTP Server Setup

For industrial deployments, deploy a local Meinberg Lantime Stratum-1 NTP server (e.g., Lantime M600, M1000, M3000) with GPS or DCF77 reference. The Meinberg firmware supports the industrial NTP/PTP/SNTPv4 daemon with full RFC 5905 implementation. Configure the S7 PLC, HMIs, SCALANCE XC/XR switches, and SINAMICS drives as NTP clients pointing to this single local server; never use a public internet NTP source in OT networks.

For cost-sensitive sites, a Linux server running chrony or ntpd synchronized to a USB GPS receiver (e.g., u-blox) provides comparable Stratum-1 quality. On Windows, enable the W32Time service in NTP mode (w32tm /config /manualpeerlist:<gps_host> /syncfromflags:manual /reliable:YES /update).

Alternative: PC-Set Time via S7 Link (Non-NTP)

When the CP does not support NTP client mode (e.g., older CP 343-1 IT or CP 343-1 Lean without NTP), a Windows PC can be configured as the NTP server and the time can be pushed to the PLC using the IP-S7-LINK or a custom S7 connection that calls SET_CLK (SFC 1) on the PLC. The PC must run an NTP server (Meinberg NTP daemon for Windows, or chrony in WSL) and a service that periodically reads the OS time and writes it to the PLC's DATE_AND_TIME tag via S7 PUT/GET or OP-communication. This is a workaround, not preferred — use only when a CP upgrade is not feasible.

Troubleshooting Matrix

Verification Procedure

1. Online diagnostic: In TIA Portal, go to Online & diagnostics > Time of day. The "Status of time synchronization" must show the NTP server IP and "synchronized" or "synchronized, server not reachable temporarily".
2. Offset measurement: From a service laptop, run w32tm /stripchart /computer:<plc_ip> /samples:30 for 5 minutes. The offset should converge to under 10 ms for a local Meinberg server. Stratum should be 1 or 2.
3. DST transition test: Force a time on the NTP server that crosses the DST boundary and verify the PLC's local-time display (e.g., in a tag visible on the HMI) jumps forward or back by 1 h at the correct wall-clock moment.
4. Power-cycle test: Restart the CPU and confirm the time re-syncs within one polling interval after boot. On S7-1500 the buffered RTC retains time across power loss within capacitor hold-up (~6 weeks typical), but re-syncing from NTP must be observed.
5. Failover test: If two NTP servers are configured, disconnect the primary and verify the CPU switches to the secondary within a few poll intervals and continues to keep time within tolerance.

Standards and Further Reading

• RFC 5905 — NTPv4 protocol (IETF) — defines the packet format and stratum hierarchy used by the S7 PROFINET NTP client.
• IEC 61131-3 — defines DATE_AND_TIME, DTL, and TOD data types used in S7 system time.
• Siemens Function Manuals: S7-1500 Communication (article ID 109751654), S7-1200 System Manual, CP 343-1 Operating Instructions (6GK7343-1EX30 manual, A5E02682276).