Configuring S7-1200 Date/Time: DTL, RD_LOC_T, and KTP400 HMI Sync

Field-tested procedure for capturing event timestamps in an S7-1200 data block and synchronizing the CPU real-time clock to a KTP400 Basic Panel via the TIA Portal area pointer. Includes the DTL byte layout, RD_LOC_T and SET_TIMEZONE usage, and a troubleshooting matrix for the most common clock-mismatch faults.

1. Overview

The SIMATIC S7-1200 CPU maintains a battery- or capacitor-backed real-time clock (RTC) that continues running across power cycles. The clock is exposed to user programs as a DTL (Date and Time, Long) value, an eight-field structure that occupies 12 contiguous bytes. Three system instructions form the core date/time API in the S7-1200 instruction set:

• RD_LOC_T (Read local time) — returns the current CPU local time as a DTL.
• WR_LOC_T (Write local time) — writes a DTL value to the CPU clock.
• SET_TIMEZONE (extended instruction) — configures the local time zone and DST rules (firmware V4.2 and later).

For an HMI such as the KTP400 Basic, the panel maintains its own RTC. Without explicit synchronization the panel clock drifts relative to the CPU, producing the symptom reported most often: the HMI displays a date and time that does not match the controller. TIA Portal resolves this through the Area Pointer for Date/Time on the HMI connection, which points to a DTL tag in the PLC that the panel reads on startup and on each user request to refresh the clock. The mechanism is documented in the official S7-1200 System Manual and the FAQ How do you synchronize the time between an S7-1200 and a SIMATIC Panel? on the Siemens support portal.

2. Prerequisites

• STEP 7 Basic / Professional V15.1 or later (V16, V17, or V18 recommended for current firmware).
• S7-1200 CPU, firmware V4.2 or later. DTL and the area pointer require V4.0 minimum; SET_TIMEZONE requires V4.2.
• KTP400 Basic (6AV2 123-2DB03-0AX0) or KTP400 Comfort firmware V12.00.xx or later.
• HMI connection configured in TIA Portal over PROFINET with a valid IP address on the panel.
• CPU battery (where applicable) installed and healthy; otherwise the clock resets to 01.01.2011 00:00:00 after a power off long enough to discharge the retention capacitor.
• Project consistency check passes (right-click PLC > Compile > Software (rebuild all blocks)) before downloading.

3. DTL Data Type Structure

DTL is a structured data type with eight fields. When declared in a DB the total length is 12 bytes:

Size(DTL) = 2 (YEAR, UINT) + 1 (MONTH) + 1 (DAY) + 1 (WEEKDAY) + 1 (HOUR) + 1 (MINUTE) + 1 (SECOND) + 4 (NANOSECOND, UDINT) = 12 bytes

4. Step 1 — Create a Global DB for RTC and Event Timestamps

In the project tree, expand Program blocks > Add new block > Data block. Name it DB_RTC and uncheck Instance data block. Inside the DB declare the following variables:

Decide deliberately between Optimized block access and standard access. Both layouts support DTL; however, the area pointer on KTP Basic panels historically resolves only by absolute byte address. If you plan to use the area pointer mechanism and you may recompile the project in older TIA Portal versions, choose a non-optimized DB with manually addressed symbols. TIA Portal V14 SP1 or later resolves symbolic names in the area pointer, so optimized access is acceptable in modern projects. If in doubt, keep "Optimized block access" disabled for the clock DB.

5. Step 2 — Implement RD_LOC_T to Read the Local Time

Open OB1 (or any cyclic OB) and insert the Date and time-of-day > RD_LOC_T instruction. The OUT parameter must point to the LiveClock DTL tag you just created.

SCL example (preferred for readability):

// Cyclic OB — update LiveClock every scan
"DB_RTC".LiveClock := RD_LOC_T();

Ladder equivalent:

1. Drag RD_LOC_T from the instructions catalog (Extended Instructions > Date and time-of-day).
2. Click the OUT operand placeholder and type "DB_RTC".LiveClock.
3. Compile the program and download to the CPU.

To verify the tag is being written, open an online watch table and add "DB_RTC".LiveClock. The YEAR, MONTH, DAY, HOUR, MINUTE, and SECOND fields should change each second. If the values are frozen, check that OB1 is actually being executed (CPU is in RUN, not STOP) and that the tag is fully qualified with the DB name. RD_LOC_T is non-blocking; you do not need to evaluate a status word unless you want to detect a CPU-in-STOP condition.

6. Step 3 — Latch an Event Timestamp with Edge Detection

To capture the time of an event rather than a continuously running value, add a small edge-detect block that latches LiveClock into EventStamp on a rising edge of an event trigger (e.g., a start pushbutton, an alarm condition, or a recipe-loaded bit).

// SCL — rising-edge latch in OB1
IF "EventTrig" AND NOT "DB_RTC".EventTrigOld THEN
    "DB_RTC".EventStamp := "DB_RTC".LiveClock;
    "DB_RTC".EventID    := "SourceTag";
END_IF;
"DB_RTC".EventTrigOld := "EventTrig";

This pattern is preferable to a direct MOVE of LiveClock to EventStamp, which would over-write the captured value on every scan. The single DTL assignment in SCL compiles to a block copy of 12 bytes; the compiler can typically map it to a single CPU instruction, so the cycle-time impact is negligible. For an array of events, use an SCL FOR loop with a sliding buffer:

// Shift a 16-entry event log on every new event
IF "EventTrig" AND NOT "DB_RTC".EventTrigOld THEN
    FOR #i := 15 TO 1 BY -1 DO
        "DB_RTC".EventLog[#i] := "DB_RTC".EventLog[#i - 1];
    END_FOR;
    "DB_RTC".EventLog[0] := "DB_RTC".LiveClock;
END_IF;
"DB_RTC".EventTrigOld := "EventTrig";

7. Step 4 — Configure the KTP400 HMI Area Pointer

The area pointer is the mechanism that tells the HMI where the PLC's DTL clock value lives. Without it the panel reads its own internal RTC only and the time will drift. The Siemens FAQ 39182145 covers this configuration in detail.

1. In the project tree, expand HMI > Connections and double-click the S7-1200 connection.
2. Select the Area pointer tab.
3. Activate Date/Time by checking the checkbox.
4. In the PLC tag field, click the selector and pick DB_RTC > LiveClock.
5. Click OK and recompile the HMI.
6. Download the HMI project to the panel and restart the runtime.

8. Step 5 — Bind HMI Tags for Date and Time Display

The HMI's internal time/date display on the system screen, and the time/date field on any user screen, is fed from the area pointer. To show the PLC time on a custom screen:

1. Create an HMI tag named PLC_Clock, type Date/Time.
2. Set PLC connection to the S7-1200 connection and PLC tag to DB_RTC.LiveClock.
3. Drag an Output field onto the screen, switch its display mode to Date/Time, and bind it to PLC_Clock.
4. Format the display in the Properties > Appearance > Format dialog (for example yyyy-MM-dd HH:mm:ss).

The system clock at the bottom-right of the HMI runtime is driven from the same area pointer. If the screen shows the correct PLC time after step 4, the system clock will follow suit on the next refresh cycle. To force a manual write from the HMI to the PLC (for example, to push an operator-entered time back to the controller), add a button with the SetPLCDateTime system function on the HMI side; it will write through the same area pointer.

9. Time Zone and Daylight Saving Configuration

By default the S7-1200 reports local time without DST adjustment. SET_TIMEZONE lets you declare the offset from UTC and the DST rules. The instruction is found under Extended Instructions > Date and time-of-day > SET_TIMEZONE.

// Configure UTC+1 with European DST rules, called once on startup
#Status := SET_TIMEZONE(
    TimeZone            := WSTRING#'CET-1CEST,M3.5.0,M10.5.0/3',
    Dst                 := TRUE,
    TimeZoneHourOffset  := 1
);
IF #Status <> 0 THEN
    // Log error to diagnostic buffer or alarm
END_IF;

The string CET-1CEST,M3.5.0,M10.5.0/3 follows POSIX TZ syntax: standard time CET is UTC-1 (i.e., 1 hour ahead of UTC), DST is CEST, DST starts on the last Sunday of March (month 3, week 5, day 0) and ends on the last Sunday of October, switching at 03:00 local time. Adjust the string to match your local convention (e.g., EST5EDT,M3.2.0,M11.1.0/2 for US Eastern Time, or <-03>3<-02>,M10.1.0/0,M2.3.0/0 for Argentina, which does not observe DST). Place the call in OB100 (warm restart) so it runs every time the CPU returns to RUN.

10. Diagnostics and Verification

Use the following checks to confirm the clock chain is healthy after commissioning:

1. Online > Diagnostics > Time: the CPU's online diagnostic page shows the current local time and time zone. Compare it to a reference (laptop NTP-synced time). A difference greater than 2 seconds indicates a missed refresh in the HMI area pointer or a stalled OB1.
2. HMI diagnostic buffer: connect with WinCC to the panel and read Diagnostics > Buffer. Entries containing Area pointer confirm whether the panel found the DTL tag on startup. Persistent "Area pointer invalid" entries mean the wrong tag was selected or the DB is not reachable from the HMI connection.
3. Watch table cross-check: with the HMI online, the DB_RTC.LiveClock value on the HMI tag monitor should match the PLC online view within 1 second.
4. Power-cycle test: shut the CPU down for 5 minutes and power back up. The clock should resume within ±10 s of the original time. If it resets to 01.01.2011, the battery is dead or absent.
5. PROFINET diagnostics: from the PLC side, look at Online > Online & Diagnostics > PROFINET interface > Port statistics. Excessive CRC errors on the HMI port will prevent the area pointer handshake from completing.

11. Troubleshooting Matrix

12. Frequently Asked Questions