Logging Heater On/Off Events with Siemens MP370 HMI and S7 PLC

Problem Overview

Recording every transition of a heater contactor on an extruder supervised by a Siemens MP370 12-key HMI is a recurring field requirement: operators want to know when the heater was turned on, when it was turned off, and how many such events occurred between two product batches. A naive approach using an S7 run-time meter does not produce a time-stamped event list, and a naive alarm configuration emits duplicate alarms because the alarm bit is level-triggered rather than edge-triggered.

The proper technique combines three standard SIMATIC building blocks:

1. SFC 1 READ_CLK to read the PLC's wall-clock (DATE_AND_TIME) at the exact instant a state transition is detected.
2. Edge detection (R_TRIG / P_TRIG, or the legacy FP bit logic) so that a single on-transition produces a single log entry instead of being retriggered every OB1 scan.
3. A FIFO buffer stored in a data block as an array of DATE_AND_TIME tags, shifted each time the buffer overflows.

The MP370 panel (Multi Panel family, xP170/xP270/MP370 series) is engineered for ProTool and WinCC flexible 2004 through 2008 SP5; it is not supported by TIA Portal. All coding in this article targets STEP 7 V5.5 / V5.6 with S7-300 or S7-400 firmware, which is the native pairing for the MP370.

Architecture and Prerequisites

Minimum hardware/software stack for a working implementation:

The MP370 supports the SIMATIC S7 MPI/DP protocol driver, which exposes DB tags directly. Each DATE_AND_TIME (8 bytes BCD) is read by the panel as a raw 8-byte field; the panel's own date/time display fields convert the bytes via the standard S7 decoding.

Reading the System Clock with SFC 1 (READ_CLK)

SFC 1 READ_CLK reads the CPU's internal real-time clock and returns it as a DATE_AND_TIME value. The function has no input parameter; the output is written to a temporary variable of type DATE_AND_TIME.

Reference: SFC 1 / SFC 0 — READ_CLK / SET_CLK — SIMATIC S7-300/400 Standard and System Functions manual (entry ID 109751706).

The DATE_AND_TIME data type is a fixed 8-byte (64-bit) BCD structure:

This format is what the MP370 expects when a tag of type Date/Time is configured to point at a DB byte offset that is byte-aligned. Always keep the offset to an even byte; mis-aligned offsets produce W#16#80Bx decode errors on the panel.

Edge Detection: Why Duplicate Alarms Occur

The original poster reports that two alarms are generated when the heater turns on and two when it turns off. This is the classic symptom of level-triggered alarm logic. Each alarm has two states (inactive / active) and is re-evaluated every cycle. As long as the trigger bit is "1", the alarm remains "active"; when the trigger bit returns to "0", the alarm raises a "cleared" event. The panel then displays:

• Heater ON alarm — appears (rising edge of trigger)
• Heater ON alarm — cleared (falling edge, almost immediate if trigger is short)

If both "on" and "off" alarms share the same trigger (the heater bit), the HMI also fires the off-alarm when the bit drops, producing a "two on, two off" pattern. The fix is to:

1. Evaluate the heater bit in edge-triggered mode so that a transition is detected exactly once.
2. Use separate on-edge and off-edge bits that latch for one OB1 scan.

The two standard edge detectors are:

Reference for edge FBs: R_TRIG / F_TRIG — SIMATIC S7-300/400 Standard Library Bit Logic Functions (entry ID 12154079).

FIFO Buffer Implementation in S7

A FIFO (first-in / first-out) is required because each new event must push the oldest entry out. The simplest, code-efficient implementation for S7-300/400 is an array-shift FIFO: an ARRAY[1..N] of DATE_AND_TIME, an integer counter, and a FOR loop that shifts all entries one slot down when the buffer is full.

Step 1 — Data Block

DATA_BLOCK DB_HeaterLog
TITLE = 'Heater On/Off Event Log'
  STRUCT
    evt : ARRAY[1..100] OF DATE_AND_TIME;  // 800 bytes
    cnt : INT;                              // number of valid entries
    heater_evt_on  : BOOL;                  // one-shot rising edge flag
    heater_evt_off : BOOL;                  // one-shot falling edge flag
  END_STRUCT
END_DATA_BLOCK

With 100 entries the DB is roughly 802 bytes. S7-300 CPUs handle this comfortably in the work area; for S7-314 with 96 KB of work memory, keep the array ≤ 200 entries.

Step 2 — SCL Logic in OB1

FUNCTION_BLOCK FB_HeaterLogger
VAR
    Heater       : BOOL;     // I-input: heater contactor feedback
    LastHeater   : BOOL;     // static: previous scan state
    EdgeOn       : BOOL;     // rising-edge pulse
    EdgeOff      : BOOL;     // falling-edge pulse
    CDT_now      : DATE_AND_TIME;
    i            : INT;
    R_TRIG_On    : R_TRIG;
    F_TRIG_Off   : F_TRIG;
END_VAR

BEGIN
    // ---- edge detection ----
    R_TRIG_On(CLK := Heater, Q => EdgeOn);
    F_TRIG_Off(CLK := Heater, Q => EdgeOff);

    // ---- timestamp and shift ----
    IF EdgeOn OR EdgeOff THEN
        SFC1(CDT := CDT_now);

        // overflow handling: shift older entries down
        IF "DB_HeaterLog".cnt >= 100 THEN
            FOR i := 1 TO 99 DO
                "DB_HeaterLog".evt[i] := "DB_HeaterLog".evt[i+1];
            END_FOR;
            "DB_HeaterLog".cnt := 99;
        END_IF;

        "DB_HeaterLog".cnt := "DB_HeaterLog".cnt + 1;
        "DB_HeaterLog".evt["DB_HeaterLog".cnt] := CDT_now;
        "DB_HeaterLog".heater_evt_on  := EdgeOn;
        "DB_HeaterLog".heater_evt_off := EdgeOff;
    END_IF;
END_FUNCTION_BLOCK

Step 3 — STL Equivalent (no SCL required)

// Heater = I 0.0   (input bit)
// DB20   = DB_HeaterLog
// EdgeMem = M 10.0 (static memory for edge bit)

NETWORK 1  // Rising edge of Heater
  A   "Heater"
  FP   "EdgeMem_Heater"
  =    "DB_HeaterLog".heater_evt_on

NETWORK 2  // Falling edge of Heater
  AN   "Heater"
  FN   "EdgeMem_Heater"
  =    "DB_HeaterLog".heater_evt_off

NETWORK 3  // Read clock and store
  A    "DB_HeaterLog".heater_evt_on
  O    "DB_HeaterLog".heater_evt_off
  JCN  END3
  CALL SFC1
        CDT   := #CDT_now
        RET_VAL := #ret

  // shift if full
  L     "DB_HeaterLog".cnt
  L     100
  >=I
  JC    SHIFT
  JU    STORE

SHIFT:
  L     1
T_LOOP: T   #i
  L     #i
  L     1
  -I
  SLD   3
  LAR1
  L     DBW [AR1,P#0.0]
  T     #temp_word
  L     #temp_word
  // ... (omit: shift loop body — see SCL version)
  L     #i
  L     99
  <I
  JC    T_LOOP
  L     99
  T     "DB_HeaterLog".cnt

STORE:
  L     "DB_HeaterLog".cnt
  +    1
  T     "DB_HeaterLog".cnt
  // copy CDT_now into evt[cnt]
  LAR1 P##CDT_now
  L     DBB [AR1,P#0.0]
  T     DBB [AR1,P#0.0]   // (illustrative; proper AR2 usage required)

END3:  NOP 0

The SCL version is strongly preferred for clarity. The STL excerpt is shown to confirm that the algorithm ports to STEP 7 installations without an SCL license.

MP370 HMI Configuration in WinCC flexible

1. Connection: In the WinCC flexible project tree, open Communication → Connections. Add a SIMATIC S7 MPI/DP connection. Set MPI address 2 for the panel, 2 for the CPU, and bus profile 1.5 Mbps.
2. Tags: Create area pointers and process tags:
   • Tag HeaterState — BOOL — DB20.DBX0.0 (in practice, point at the live heater bit, not the edge bit).
   • Tag HeaterOnPulse — BOOL — DB_HeaterLog.heater_evt_on
   • Tag HeaterOffPulse — BOOL — DB_HeaterLog.heater_evt_off
   • Tag EvtCnt — INT — DB_HeaterLog.cnt
   • Tag EvtArray[0..99] — DATE_AND_TIME — DB_HeaterLog.evt[1..100]
3. Alarm configuration: In Alarms → Discrete Alarms, create two alarms:

   Alarm Text	Trigger Tag	Trigger Edge
   "Heater ON — logged"	HeaterOnPulse	Rising
   "Heater OFF — logged"	HeaterOffPulse	Rising

   Setting trigger edge to rising ensures exactly one event per OB1 pulse, regardless of how long the heater remains on.
4. Alarm view: Place an Alarm View object on a screen, configured to display the alarm buffer with date/time stamp. The MP370 inserts the panel's own timestamp; for an authoritative PLC-side timestamp, configure an additional Output field with the EvtArray[EvtCnt-1] tag formatted as Date/Time.
5. Export: Connect the alarm buffer to the Alarm Logging persistent storage on the MP370 (CF card slot at the rear of the unit). Set the buffer size to ≥ 512 entries and the retention to ring buffer.

Sample SCL Function Block (drop-in)

The complete drop-in FB that can be inserted into an SCL source file in STEP 7 V5.5+:

FUNCTION_BLOCK FB_500 "HeaterEventLogger"
VERSION : 1.0
VAR_INPUT
    HeaterInput : BOOL;     // TRUE = heater contactor closed
END_VAR
VAR_OUTPUT
    NewEvent : BOOL;        // TRUE for one OB1 scan after a log entry
END_VAR
VAR
    sLastHeater : BOOL;
    sRTrig : R_TRIG;
    sCDT  : DATE_AND_TIME;
    sFbk  : INT;
    sI    : INT;
END_VAR
BEGIN
    sRTrig(CLK := HeaterInput, Q => NewEvent);
    IF NewEvent THEN
        SFC1(CDT := sCDT, RET_VAL := sFbk);
        IF sFbk <> 0 THEN
            // clock read failure; abort
            RETURN;
        END_IF;
        IF "DB_HeaterLog".cnt >= 100 THEN
            FOR sI := 1 TO 99 DO
                "DB_HeaterLog".evt[sI] := "DB_HeaterLog".evt[sI+1];
            END_FOR;
            "DB_HeaterLog".cnt := 99;
        END_IF;
        "DB_HeaterLog".cnt := "DB_HeaterLog".cnt + 1;
        "DB_HeaterLog".evt["DB_HeaterLog".cnt] := sCDT;
    END_IF;
    sLastHeater := HeaterInput;
END_FUNCTION_BLOCK

Call in OB1:

CALL FB 500 , DB 500
     HeaterInput := "DB_Heater".contactor_closed
     NewEvent    := "DB_HeaterLog".evt_pulse

Troubleshooting Matrix

Verification and Commissioning

1. Offline simulation: In STEP 7, run PLCSIM with the project. Force the heater bit TRUE/FALSE repeatedly and watch DB_HeaterLog.cnt increment by exactly 1 per transition. Inspect evt[i] in DB view; the date must be the PLCSIM clock date.
2. Online panel test: On the MP370, navigate to the alarm view screen. Trigger the heater manually from the panel's own I/O area. Confirm a single "Heater ON — logged" entry and a single "Heater OFF — logged" entry per physical transition.
3. Time-stamp sanity: Record a transition; the panel's timestamp and the PLC's DATE_AND_TIME should agree to within ±1 second (PROFIBUS acquisition latency). If the panel time diverges by minutes, the panel's internal clock has never been synchronized; enable Time Synchronization under Device Settings → Date/Time.
4. Overflow test: Force DB_HeaterLog.cnt to 100 in STEP 7 and trigger one more event. Confirm cnt remains 100 and the array has shifted down by one slot, with the new timestamp at index 100.
5. Power-cycle test: Power off the CPU for 5 minutes and back on. Verify the buffer contents are retained (the DB is retentive by default if configured Non-retentive: no for cnt and evt[]).

Edge Cases and Field-Proven Caveats

• Chattering contact: A faulty contactor can produce multiple on/off transitions within a single OB1 scan. Add a debounce: only log if the new state has been stable for ≥ 200 ms. Implement with a TON timer whose ET bit gates the edge detector.
• Clock drift on power-up: S7-300 CPUs without a battery lose the time after a power cycle of more than ~30 days (capacitor backup). Always install a fresh battery (e.g. 6ES7971-1AA00-0AA0 for CPU 31x).
• Time zone / DST: DATE_AND_TIME has no time-zone field. If the plant operates across DST boundaries, store the raw PLC time and convert to local time on the HMI.
• Lossy semantics of "off" event: If the PLC loses power while the heater is on, the off-event is missed. To capture this, add an OB100 startup routine that detects the contactor feedback at startup and logs an "off event" if it is unexpectedly TRUE.
• Multi-heater extruders: For machines with 4–8 heater zones, allocate one FB_500 instance per zone and a 2-D array evt[1..8][1..100]; the index to the Alarm View on the MP370 is then driven by a zone-selection key on the panel.

Alternative Implementations

• Pointer-based FIFO using the indirect addressing of the STEP 7 ANY-pointer is faster for very large buffers but harder to maintain; not recommended unless you have already mastered ANY-pointers.
• S7-1500 with DTL: replace DATE_AND_TIME with DTL and SFC1 with the IEC standard RD_SYS_T; the FB is otherwise identical. See SIMATIC S7-1500 Time Functions (entry ID 109751706).
• Direct logging to the panel CF card: configure Logging tags in WinCC flexible with the EvtArray tags set to Logging on change; the panel produces a CSV file on the CF card, no PLC-side array required. This consumes no PLC work memory but ties log retrieval to the panel.