Configuring S7-1200 NTP Time Synchronization with DST in TIA

1. Overview

Reading the real-time clock from a SIMATIC S7-1200 CPU is not as trivial as calling a single instruction. The PLC maintains two distinct notions of time: a system time (UTC, monotonically referenced) and a local time (wall-clock time adjusted for the configured time zone and daylight saving rules). Without an external time reference, the on-board hardware RTC drifts at a rate of typically ±2.5 s/day, which accumulates to several minutes per month. For applications that log events at roughly 100 transitions per day with second-level accuracy, the RTC alone is not adequate.

This reference describes how to:

• Distinguish RD_SYS_T from RD_LOC_T and select the correct block for a given use case.
• Configure NTP synchronization on a CPU 1214C DC/DC/DC (6ES7 214-1AG40-0XB0) under TIA Portal V16.
• Set the local time zone and DST rules in the device configuration.
• Verify synchronization through the diagnostic buffer and online watch tables.
• Resolve the most common NTP/DST faults (firewall blocks, wrong UTC offset, mixed-protocol networks).

2. Prerequisites

3. S7-1200 Time Architecture

The S7-1200 keeps three coordinated concepts of time that engineers routinely confuse:

The PLC evaluates system time whenever RD_SYS_T is executed. Local time is evaluated only at the moment RD_LOC_T (SFC 19) runs; it does not change the underlying system clock.

4. Time and Date Data Types

All S7-1200 time instructions return a DTL (Date_Time_Long) structure, which is the only data type that fully captures year-to-nanosecond resolution. Auxiliary types are used in arithmetic.

Per the TIA Portal S7-1200 manual, TIME data is stored as a signed double integer interpreted as milliseconds, and the editor display uses d, h, m, s, ms, ns tokens. Conversion to DTL uses DT_TO_DTL; arithmetic uses T_ADD for adding durations or T_SUB for differences.

Reference: S7-1200 Programmable Controller - System Manual (chapter on basic PLC concepts, data types)

5. RD_SYS_T vs RD_LOC_T - Block-by-Block Comparison

In SCL, declare a tag tEventTime : DTL; at the rising edge of the input, then call RD_LOC_T(OUT => tEventTime);. The block reads the configuration that was downloaded as part of the hardware setup (Properties > Time of day > Local time zone > DST rules), so the program code never has to be modified for DST transitions.

6. Configuring NTP Synchronization in TIA Portal V16

1. Open the project in TIA Portal and select the CPU in the project tree.
2. Open Device Configuration and switch to Properties > General > Time of day.
3. Tick "Synchronize the time of day via NTP server". This is a single checkbox; everything below becomes available.
4. Click Add NTP server and enter:
   • IP address: the IPv4 address of the NTP source (e.g., a Windows domain controller at 10.20.30.5, or a SICLOCK at 10.20.30.10).
   • Port: default 123 (only change if the server is non-standard).
   • Update interval: 10 s is a reasonable starting point. Smaller intervals increase CPU utilization marginally; larger intervals tolerate more network loss.
5. Compile and download the hardware configuration.

The PLC behaves as an NTP client in symmetric mode. On every interval it sends a request, calculates the round-trip delay and offset, and slews the system clock by slewing rather than stepping for offsets under 1 second. Larger offsets trigger a step. Every accepted update is logged into the diagnostic buffer as event ID 0xE0FE0001 ("Time successfully synchronized"); each rejection or timeout is logged as 0xE0FE0002.

Reference: NTP Time Synchronization with S7-1200 / S7-1500 - Siemens FAQ

7. Configuring Time Zone and DST in the PLC

Even with NTP, local-time arithmetic still requires time-zone and DST rules. Without them, RD_LOC_T returns a value that is offset by the wrong number of hours and DST transitions are silently skipped.

1. Device Configuration > Properties > Time of day.
2. Set Local time zone: e.g., (UTC-05:00) Eastern Standard Time (US & Canada).
3. Under Daylight saving time settings, tick Activate daylight saving time.
4. Enter the DST start rule (e.g., Second Sunday of March, 02:00 local standard time).
5. Enter the DST end rule (e.g., First Sunday of November, 02:00 local standard time).
6. Enter the time offset (typically 60 minutes = 1 hour).

Download the hardware configuration to the PLC. The rule set is stored in non-volatile configuration memory and survives power cycle.

8. Application Example: Event Time Stamping

The following SCL snippet produces a 12-byte DTL log each time a discrete input rises. It is engineered for an OB (e.g., OB1 or OB200) that runs cyclically and uses edge detection on an input tag iEvent : BOOL;.

// SCL example for rising-edge time stamping using RD_LOC_T
VAR
    tEventTime : DTL;       // wall-clock timestamp
    bPrevEvent : BOOL;      // edge memory
    arrLog : ARRAY[1..500] OF DTL;
    nIdx    : INT := 1;
END_VAR

IF iEvent AND NOT bPrevEvent THEN
    RD_LOC_T(RET_VAL := _dummy_ret_, OUT => tEventTime);
    arrLog[nIdx] := tEventTime;
    nIdx := nIdx + 1;
    IF nIdx > 500 THEN nIdx := 1; END_IF;
END_IF;
bPrevEvent := iEvent;

The complementary block uses RD_SYS_T when logs from multiple sites are consolidated in UTC:

9. Verification and Diagnostics

After downloading the configuration:

1. Go Online and force the diagnostic buffer view (Project tree > right-click CPU > Online & Diagnostics > Diagnostic buffer).
2. Look for 0xE0FE0001 with a recent timestamp. Absence of this event means NTP is not working.
3. Open a watch table and observe the system time tag. Compare it to an authoritative UTC source (e.g., time.is). Drift between updates should be <1 s on a stable network.
4. Verify DST behavior at the seasonal crossover - the clock should step at exactly 02:00 local on the configured date. TIA Portal does not provide a built-in DST simulator; the test must wait for the real transition or be forced by an NTP server with a fictional time.

Reference: How to Evaluate the Diagnostic Buffer Entries of S7-1200/S7-1500 Regarding NTP Synchronization - Siemens FAQ

10. Troubleshooting Matrix

11. Best Practices

• Two-tier time strategy: keep one RD_LOC_T tag for operator-facing displays and one RD_SYS_T tag for event logs destined for cross-plant aggregation.
• Use NTP, not SNTP, when the network is congested; SNTP is a stripped-down subset and can be fooled by jittery links.
• Quarantine the NTP source from production traffic: place the NTP server in a small VLAN shared with the PLCs and route nothing else across it.
• Document the DST rules in the project documentation: if the PLC ever has its configuration reloaded at a customer's site, the engineer will need to know which rules to enter.
• Track RTC battery health: a depleted battery is the only hardware-level cause of time loss; nothing in the software can protect against it.
• Consider a Siemens SICLOCK TM or SICLOCK TC for plants with many controllers - the SICLOCK can synchronize to GPS and emit NTP from a single source for dozens of PLCs.

FAQ