Flow Totalization on SIMATIC S7-300/400 and S7-1200/1500 PLCs

Problem Statement: From Instantaneous Flow to Integrated Total

A flow transmitter with a 4-20 mA output delivers a signal proportional to the instantaneous volumetric or mass flow rate (for example, m³/h or kg/h). The operator, however, usually wants the cumulative quantity that has passed through the line since a defined start: shift total, batch total, daily total, or lifetime total. The conversion from a rate to a quantity is an integration:

V_total(t) = V_total(t₀) + ∫_t0^t q(τ) dτ

On a discrete-time controller such as the SIMATIC S7-300, S7-400, S7-1200, or S7-1500, the integral is approximated by a Riemann sum. Implementing it correctly requires three engineering decisions:

1. How the analog raw value (INT 0..27648) is scaled into engineering units. The classic STEP 7 block is FC105; in TIA Portal the equivalent is the pair NORM_X and SCALE_X.
2. How the cycle time of the integrating task is obtained, and whether the integrator uses a fixed nominal Δt or a measured timestamp difference.
3. How the integrator state is retained across power cycles and how overflow / rollover is handled.

Siemens documents this exact application in its Industry Online Support FAQ "How can you accumulate physical flow measurements (e.g. mass flow rate, flow velocity) to an overall value?" under entry ID 24000437. The technique below is the field-proven implementation of that reference.

Flow Measurement Signals: Analog Rate vs. Pulse Quantity

Before writing any code, classify the transmitter output. The integration math differs substantially between the two main cases.

Signal type	Output characteristic	Engineering unit	Integration approach
Analog (4-20 mA / 0-10 V)	Proportional to instantaneous rate	m³/h, L/min, kg/h	Numeric accumulation: q(t) × Δt
Pulse / frequency	One pulse per fixed volume	pulses / unit volume	Event counting via K-factor
HART / Modbus register	Two registers: rate and device-side total	mixed	Read the device's pre-integrated total

The pulse case is mechanically simpler because the transmitter itself performs the integration and the PLC only counts edges; this is also the most accurate method for custody transfer. The analog case, which is the focus of this article, requires the controller to compute the sum. Keyence's flow knowledge base defines the underlying terms: integrated flow is the cumulative value of flow used from the start to finish of measurement, while instantaneous flow is the value at a specific moment (see Keyence Flow Knowledge - Integrated Flow).

Prerequisites

• SIMATIC S7-300 (CPU 31x), S7-400, S7-1200 (CPU 12xx), or S7-1500 (CPU 15xx) with an analog input module (SM 331, SM 431, SM 1231, SM 1531) configured for 4-wire 4-20 mA. See the S7-1200 system manual 109741593 and the S7-1500 system manual 59191792.
• STEP 7 V5.x (classic) for S7-300/400, or TIA Portal V15..V18 for S7-1200/1500.
• Transmitter datasheet with: flow unit, full-scale (20 mA) value, lower-range value (4 mA), and output type.
• Defined scan time for the integration task. Use a cyclic interrupt OB (OB30 on S7-1500, OB35 on S7-300/400) with a configurable period instead of OB1, to make Δt deterministic.
• Retentive DB or retentive variable to survive CPU STOP/RUN transitions and power loss.

Scaling the Analog Input: FC105 vs. NORM_X / SCALE_X

The first non-trivial step is converting the raw AI value (0..27648 for the Siemens default unipolar range) into engineering units. The scaled value is the instantaneous rate q that feeds the integrator.

Classic STEP 7: FC105 "SCALE"

FC105 is a library function in the STEP 7 standard library under TI-S7 Converting Blocks. Calling convention:

CALL  FC  105
     IN     := MW20          // raw INT 0..27648
     HI_LIM := 1.000000e+02  // 100.0 m3/h at 20 mA
     LO_LIM := 0.000000e+00  // 0.0 m3/h at 4 mA
     BIPOLAR:= FALSE
     RET_VAL:= MW22          // scaling error code
     OUT    := MD30          // REAL flow in m3/h

TIA Portal: NORM_X and SCALE_X

In S7-1200 and S7-1500, FC105 is replaced by the two-instruction pair NORM_X (normalize to 0.0..1.0) and SCALE_X (scale to engineering range). SCALE_X accepts REAL limits and produces a REAL output directly, which eliminates the need for a separate conversion step.

// NORM_X: 0..27648 -> 0.0..1.0
NORM_X(ENABLE := TRUE,
       VALUE  := "AI_raw",
       MIN    := 0,
       MAX    := 27648,
       RET_VAL=> "norm_val");

// SCALE_X: 0.0..1.0 -> 0.0..100.0 m3/h
SCALE_X(ENABLE := TRUE,
        VALUE  := "norm_val",
        MIN    := 0.0,
        MAX    := 100.0,
        RET_VAL=> "flow_m3h");

Integration Mathematics: Discrete-Time Implementation

The continuous integral V = ∫ q dt is approximated in the PLC by:

V(k) = V(k-1) + q(k) × Δt

where Δt is the time between two samples. Three Δt handling strategies exist:

Strategy	Source of Δt	Accuracy	Recommended use
Fixed nominal	OB30/OB35 configured period (e.g., 100 ms)	±5% under heavy CPU load	Lightly loaded, non-custody applications
Timestamp difference (SFC1 / RD_SYS_T)	Two system clock reads	±0.1%	Recommended for all production systems
Counter-based Δt	Count OB passes; integrate every N	±0.5%	When system clock unavailable

Unit Conversion

If q is in m³/h and Δt is in seconds, the increment must include a time-unit conversion:

ΔV [m³] = q [m³/h] × Δt [s] / 3600 [s/h]

The constant 1/3600 must be present. Forgetting it is the single most common totalizer bug and produces a totalizer that runs 3600× too fast (or, if the inverse error is made, 3600× too slow).

Implementation: Classic S7-300/400 in STEP 7

Create a function block FB100 "Flow_Totalizer" with an instance DB (DB100) holding the integrator state in its STAT section. The integrator must be retentive so it survives a warm restart.

FB100 Interface

Section	Name	Type	Comment
INPUT	i_Flow_m3h	REAL	Instantaneous flow from FC105
INPUT	i_Dt_s	REAL	Measured Δt in seconds
INPUT	i_Reset	BOOL	Reset integrator to zero
INPUT	i_Hold	BOOL	Freeze accumulator
OUTPUT	q_Total_m3	REAL	Cumulative volume in m³
OUTPUT	q_Status	WORD	16#0001 = overflow flag
STAT	s_Total	REAL	Retentive accumulator
STAT	s_OverflowCount	DINT	Retentive 109 m³ rollover counter

In the instance DB properties, mark s_Total and s_OverflowCount as retentive (Non-Retain off). Configure the CPU's retentive area to include DB100.

STL Body

FUNCTION_BLOCK FB100
VAR
   s_Total          : REAL;   // retentive
   s_OverflowCount  : DINT;   // retentive
END_VAR
BEGIN
   IF i_Reset THEN
       s_Total := 0.0;
       s_OverflowCount := 0;
   ELSIF i_Hold THEN
       // freeze
   ELSE
       // increment with unit conversion (m3/h -> m3)
       s_Total := s_Total + i_Flow_m3h * i_Dt_s / 3600.0;
   END_IF;

   // rollover at 1.0e9 m3 (well below REAL precision floor)
   IF s_Total > 1.0E+09 THEN
       s_Total := s_Total - 1.0E+09;
       s_OverflowCount := s_OverflowCount + 1;
   END_IF;

   q_Total_m3 := s_Total;
   IF s_OverflowCount > 0 THEN
       q_Status := W#16#0001;
   ELSE
       q_Status := W#16#0000;
   END_IF;
END_FUNCTION_BLOCK

OB1 Wiring (Single-Call Variant)

CALL  FB100, DB100
     i_Flow_m3h := MD30      // output of FC105
     i_Dt_s     := MD40      // measured cycle time
     i_Reset    := M10.0     // from HMI reset button
     i_Hold     := M10.1     // from HMI hold button
     q_Total_m3 := MD50
     q_Status   := MW52

For better accuracy, place the FB call inside OB35 so Δt is the OB35 period (e.g., 100 ms) and read the actual cycle time once per second via SFC1 "READ_CLK".

Implementation: S7-1200 and S7-1500 in TIA Portal

The TIA Portal version uses an FB written in SCL (Structured Control Language) for cleaner math. The logic is identical, but data-type handling is stricter and the RETAIN qualifier replaces the older "non-volatile DB" technique.

FUNCTION_BLOCK "Flow_Totalizer"
VAR_INPUT
    i_Flow_m3h : REAL;          // from SCALE_X
    i_Dt_s     : REAL;          // measured delta time
    i_Reset    : BOOL;          // rising edge resets
    i_Hold     : BOOL;
END_VAR
VAR_OUTPUT
    q_Total_m3 : REAL;
    q_Overflow : BOOL;
END_VAR
VAR RETAIN
    s_Total    : REAL;          // retained across power cycle
    s_Rollover : DINT;
END_VAR

BEGIN
    IF i_Reset THEN
        #s_Total    := 0.0;
        #s_Rollover := 0;
    ELSIF NOT #i_Hold THEN
        #s_Total := #s_Total + #i_Flow_m3h * #i_Dt_s / 3600.0;
    END_IF;

    IF #s_Total > 1.0e9 THEN
        #s_Total    := #s_Total - 1.0e9;
        #s_Rollover := #s_Rollover + 1;
        #q_Overflow := TRUE;
    ELSE
        #q_Overflow := FALSE;
    END_IF;

    #q_Total_m3 := #s_Total;
END_FUNCTION_BLOCK

Place the FB call inside the cyclic interrupt OB30 (typical period 100 ms). On S7-1500, OB30 is preferred for fast periodic tasks; OB1 remains free for slower I/O scans. Note the use of the RETAIN keyword in the VAR block: this is the TIA Portal mechanism for retentive storage and replaces the STEP 7 classic non-volatile DB property.

Delta-Time Measurement Patterns

Method 1 - Fixed OB Period (Simplest)

Configure OB30 on S7-1500 or OB35 on S7-300/400 to a known period. The integrator uses this constant. The downside is that the period is only nominal; actual jitter on a loaded CPU can be ±10%.

// inside the cyclic OB
#i_Dt_s := 0.1;   // 100 ms period from OB30 configuration

Method 2 - Timestamp Difference (Most Accurate)

Read the system clock each pass and compute the actual Δt. Siemens blocks for this:

• S7-300/400 classic: SFC1 "READ_CLK" returning a DATE_AND_TIME structure.
• S7-1200/1500: RD_SYS_T (returns DTL) and DTL subtraction.

// S7-1500 timestamped integrator
VAR
    s_t_last : DTL;
    s_init   : BOOL;
    temp_t   : DTL;
END_VAR

IF NOT s_init THEN
    RD_SYS_T(RET_VAL := s_t_last);
    s_init := TRUE;
ELSE
    RD_SYS_T(RET_VAL := temp_t);
    #i_Dt_s := DINT_TO_REAL(
                 TIME_TO_DINT(temp_t - s_t_last) / 1000);
    s_t_last := temp_t;
END_IF;

Method 3 - Counter-Based Δt (Mid Accuracy)

Increment a DINT every OB cycle and integrate the total every time the counter reaches N. Use this if the OB period is too short for a timestamp read but you need a known quantity per integration step.

Pulse-Based Totalization

Many flowmeters expose a pulse output (one pulse = K cubic meters or liters). Counting pulses is robust against analog drift and is the preferred method for custody transfer. Siemens provides fast counters on most CPUs.

S7-1200 with HSC (High-Speed Counter)

1. In device configuration, configure a high-speed counter on input I0.0 with counting mode "Count once".
2. Assign a process image address, e.g., ID1000.
3. Use the CTRL_HSC instruction to control the counter; the current count value (CV) is in ID1000.
4. Compute the total: total = CV × K-factor (for example, 0.001 m³ per pulse).

// in cyclic OB
"total_m3" := "total_m3" + INT_TO_REAL(ID1000 - "last_count") * 0.001;
"last_count" := ID1000;

S7-1500 with TM Count / TM Pulse

For higher pulse rates (>1 kHz) or for two-channel metering, use a TM Count 2x module with "Count and Measure" mode. The TM module exposes totalizer registers directly, eliminating the need for PLC-side integration math.

Edge Cases and Field-Proven Caveats

1. Power Loss and Restart

If the integrator is not retentive, a STOP/RUN transition resets the total. Mark the storage tags as retentive in the DB, or use the S7-1500 RETAIN qualifier. For backup, mirror the integrator value to a non-volatile medium: use a recipe DB on S7-300/400, or use a DataLog written to a memory card on S7-1500.

2. REAL Precision Degradation

REAL (IEEE-754 single precision) has approximately 7 significant decimal digits. If the running total reaches 10,000,000 m³, the increment resolution degrades below 1 mL. Switch to LREAL (double precision) on S7-1500 if high precision is required at high total counts, or wrap the totalizer at a chosen rollover threshold.

3. Wrong Unit Conversion

The most common bug is mismatched time units (minutes in q, seconds in Δt) leading to totals off by a factor of 60, or 3600 if hours are involved. Always display both q and Δt in the HMI to verify consistency.

4. Scan Time Drift

If OB1 cycle time is used without compensation, the totalizer will drift under heavy communication load. Move the integrator call into a cyclic interrupt OB whose period is hardware-fixed.

5. Negative Flow (Bidirectional Meters)

Some meters report bidirectional flow (for example, reverse flushing or backflow). Subtract the absolute reverse flow from the forward total, or report two separate registers (forward total, reverse total). Do not simply use SIGN(q).

6. Out-of-Range Detection

If the AI shows >27648 or <0 (overrange), FC105 returns a status code and SCALE_X returns an error. Suppress the integrator increment when the quality is bad to avoid injecting garbage.

// suppress integration when AI quality is not "good"
IF "AI_flow".%Quality = 16#80 THEN
    // integrate
END_IF;

7. Integrator Mode State Machine

The integrator has three valid states: Running, Hold, and Reset. The Reset action should be edge-triggered (rising edge only) to avoid repeated clears from a stuck input. The Hold action is level-triggered, so the operator can keep the total frozen for as long as required.

// edge-triggered reset
i_Reset_RTrig(CLK := i_Reset);
IF i_Reset_RTrig.Q THEN
    s_Total := 0.0;
END_IF;

Commissioning Procedure

1. Force the AI to 4 mA (0% flow) and verify FC105 / SCALE_X outputs 0.0.
2. Force the AI to 12 mA (50% flow) and verify the output equals HI_LIM / 2.
3. Force the AI to 20 mA (100% flow) and verify full-scale output.
4. Set i_Hold = TRUE on the FB. Verify the integrator is frozen.
5. Set i_Reset = TRUE for one OB cycle, then unset. Verify the integrator clears to 0.0.
6. Release the hold. Compare the integrator accumulation against a reference: weigh tank, calibrated volumetric vessel, or a calibrated pulse-counter reference.
7. Run for 24 h at typical operating point. The drift between the PLC totalizer and the reference should be within the transmitter's accuracy specification (typically ±0.5% to ±1.0%).

Document the following so future modifications do not silently break the integrator:

• FB tag name, instance DB number, retention setting.
• OB used (OB1, OB30, OB35) and its configured period.
• Method of Δt acquisition (fixed, timestamp, counter).
• Unit conversion constants (for example, the 1/3600 divisor).
• Rollover threshold and overflow behavior.
• Reset procedure and authorization level.

Troubleshooting Matrix

Symptom	Likely cause	Diagnostic	Fix
Totalizer drifts much faster than expected	Wrong time unit (q in m³/h but Δt in seconds without /3600)	Watch q, Δt, and increment in VAT; verify HMI units	Add or correct the /3600 divisor
Totalizer reads zero regardless of flow	Wrong MD/MW wiring; i_Reset stuck TRUE; i_Hold stuck TRUE	Monitor i_Reset and i_Hold in VAT	Correct wiring; verify HMI button logic
Totalizer resets on power cycle	Storage tags not marked retentive	Open DB properties / FB VAR RETAIN	Enable retentivity; verify CPU retentive range
Totalizer accuracy drifts over time	OB1 cycle time varies under load	Use RD_SYS_T to log min/max Δt	Move call into OB30/OB35 with fixed period
Negative total when flow goes negative	Bidirectional meter; integrator not absolute	Verify q sign during flow reversal	Split into forward and reverse totals
Total freezes at large value	REAL precision floor; LREAL not used	Inspect q_Total_m3 precision	Switch to LREAL on S7-1500 or use rollover
Large step changes in total	AI noise spikes (raw >27648 then back)	Inspect AI raw value in VAT	Add input smoothing or status qualification
Integrator runs in OB1 instead of OB30	FB placed in main scan; Δt is wrong	Check OB call stack in online view	Move the FB call to the cyclic OB
Totalizer shows correct value but HMI shows wrong	HMI scaling or unit mismatch on the tag	Compare HMI tag and PLC tag in online	Correct HMI scaling field or use raw REAL