Calculating Flow Rate (L/h) from Pulse Input on S7-1200 HSC

1. Problem Definition

A S7-1214C receives a pulse train from a process flow meter and must derive an engineering-units flow rate (L/h) for display and downstream control. The relevant signal chain is:

• Pulse source: Paddle-wheel / turbine flow meter, sinking or sourcing output, 24 V DC.
• Meter constant: K = 1000 pulses per liter (nominal).
• Maximum specified flow: Q_max = 80 L/h.
• Observed pulse rate: f = 25 Hz (period 40 ms) at the operating point.

The expected pulse frequency at the rated flow is

f_rated = Q_max · K = 80 L/h · 1000 pulses/L = 80 000 pulses/h = 22.22 Hz

A measured 25 Hz therefore sits roughly 12.5 % above the meter's nominal Q_max. Treat the first task on the bench as verifying that the meter is not being over-driven and that the K-factor is constant over the flow range; a paddle-wheel meter in turbulent flow can drift several percent and produce this kind of mismatch.

2. Prerequisites

2.1 Hardware

• SIMATIC S7-1214C DC/DC/DC (firmware V4.2 or later recommended for full HSC functionality; V4.5+ if you intend to use the symbolic HSC IDB fields directly).
• 24 V DC pulse output flow meter, K = 1000 pulses/L.
• Wiring: meter output → DI of the CPU; for sourcing outputs use a 24 V input; for sinking outputs add a pull-up or use an interposing relay or signal conditioner.

2.2 Software

• STEP 7 Basic / Professional in TIA Portal (V15.1 or later; V17/V18 recommended for the latest HSC application example).
• S7-1200 System Manual, edition 09/2022 or later.

2.3 Official references

• SIMATIC S7-1200 Programmable Controller System Manual (109766074)
• S7-1200: Application Examples for High-Speed Counters (109751224)
• S7-1200 Motion Control - HSC overview (109751049)

3. HSC Architecture on the CPU 1214C

The CPU 1214C exposes six independent HSC channels (HSC1-HSC6) implemented in hardware, independent of the OB1 scan. Each HSC can be configured for one of three counting modes:

Default input pin assignment on the CPU 1214C (DC/DC/DC) for an unused channel is reassignable in the device configuration. The factory defaults that matter for this application are:

4. Engineering Units: From Pulses to Liters per Hour

Two equivalent conversions are useful. Define a sampling window W (in seconds). The number of pulses accumulated in the window is ΔN. The instantaneous volume throughput is then

V_dot [L/s] = ΔN / (K · W)

and the displayed engineering value is

Q [L/h] = V_dot · 3600 = (ΔN · 3600) / (K · W)

For K = 1000 pulses/L and W = 1 s, this simplifies to

Q [L/h] = 3.6 · ΔN [pulses in 1 s]

At the rated 22.22 Hz (Q = 80 L/h), ΔN = 22 pulses per second; at the observed 25 Hz, ΔN = 25 pulses per second, giving 90 L/h. Either the meter constant is slightly off, or the meter is being run above its specified Q_max - in either case, the FB below exposes the raw pulse count and the scaled L/h value so the mismatch is visible at commissioning.

4.1 Window length trade-off

• W = 100 ms: responsive but quantised - at 80 L/h only 2.2 pulses accumulate, so the displayed value jitters between 72 and 108 L/h (jumps of 36 L/h).
• W = 1 s: 22 pulses at 80 L/h, LSB ≈ 3.6 L/h, jitter 0 to 7.2 L/h - acceptable for display.
• W = 10 s: 222 pulses, LSB ≈ 0.36 L/h - excellent stability but 10 s display lag.

Pick W = 1 s for HMI display. If the value is also used in a closed-loop control loop, run a second faster loop at 100 ms with a low-pass filter.

5. TIA Portal Device Configuration

1. Open the project and the CPU 1214C device view.
2. Select Properties → Digital inputs and assign HSC1 to the physical input that the meter is wired to (e.g. I0.0). Set the input filter for that channel to 0.1 ms.
3. Select Properties → High-Speed Counters (HSC). Enable HSC1, choose Count mode, single-phase, internal direction control (count up).
4. Set initial count value, initial reference, and any preset / upper limits you need. For this application, no preset is required; just count up.
5. Compile and download the hardware configuration.

After download, the HSC exposes an Instance DB (IDB) of type HSC_Count (or CTRL_HSC_EXT in older projects). The fields you will read from the SCL FB are:

• IDB_HSC1.CurrentCount - DInt, live count value.
• IDB_HSC1.NewDirection - Bool, direction status.

6. SCL Implementation: FlowRate FB

Create a function block FB_FlowRatePulses in a new SCL source. The FB latches the HSC count on every cycle of a 1 s timer, computes the delta, and scales to L/h.

FUNCTION_BLOCK "FB_FlowRatePulses"
TITLE = 'Pulse-train flow rate'

VAR_INPUT
    iRawCount   : DInt;     // IDB_HSC1.CurrentCount snapshot
    iPulsePerL  : Real := 1000.0;   // K [pulses/L]
    iWindow_s   : Real := 1.0;      // sampling window W [s]
    iEnable     : Bool;
END_VAR

VAR_OUTPUT
    oFlow_Lph   : Real;     // engineering-units flow rate [L/h]
    oDeltaN     : DInt;     // pulses accumulated in last window
    oAlarmHi    : Bool;     // 1 = above configured high limit
    oAlarmLo    : Bool;     // 1 = below configured low limit
END_VAR

VAR
    sCountPrev  : DInt;     // last latched count
    sFirstRun   : Bool := TRUE;
    sIEC_Timer  : IEC_TIMER; // TP or TON, 1 s pulse
    sWindow_Trig: Bool;
END_VAR

VAR CONSTANT
    cHighLimit_Lph : Real := 80.0;
    cLowLimit_Lph  : Real := 0.0;
END_VAR

BEGIN
    // Edge-triggered 1 s window
    sIEC_Timer(IN := iEnable, PT := T#1s);
    sWindow_Trig := sIEC_Timer.Q;

    IF NOT iEnable THEN
        sFirstRun := TRUE;
        oFlow_Lph := 0.0;
        oDeltaN   := 0;
        oAlarmHi  := FALSE;
        oAlarmLo  := FALSE;
        RETURN;
    END_IF;

    IF sFirstRun THEN
        sCountPrev := iRawCount;
        sFirstRun := FALSE;
        oFlow_Lph := 0.0;
        oDeltaN   := 0;
    END_IF;

    IF sWindow_Trig THEN
        // signed delta; flow meter is single-direction here
        oDeltaN := iRawCount - sCountPrev;
        sCountPrev := iRawCount;

        // guard divide-by-zero
        IF iWindow_s > 0.0 AND iPulsePerL > 0.0 THEN
            oFlow_Lph := (DINT_TO_REAL(oDeltaN) * 3600.0)
                         / (iPulsePerL * iWindow_s);
        ELSE
            oFlow_Lph := 0.0;
        END_IF;

        oAlarmHi := oFlow_Lph > cHighLimit_Lph;
        oAlarmLo := oFlow_Lph < cLowLimit_Lph;
    END_IF;
END_FUNCTION_BLOCK

Call the FB from OB1, supplying the HSC's CurrentCount snapshot. A clean snapshot is simply reading "HSC_1".CurrentCount into a local DInt; the HSC hardware register is updated in the background by the high-speed counter logic, so there is no race condition with OB1.

// OB1 - calling the FB
"Inst_FlowRate"(
    iRawCount  := "HSC_1".CurrentCount,
    iPulsePerL := 1000.0,
    iWindow_s  := 1.0,
    iEnable    := TRUE
);

// Push the engineering value to the HMI tag
"HMI_Flow_Lph" := "Inst_FlowRate".oFlow_Lph;
"HMI_AlarmHi"  := "Inst_FlowRate".oAlarmHi;

7. LAD/FBD Alternative (Pure Ladder)

If the project prohibits SCL (some legacy code bases), the same flow can be built in LAD/FBD without any HSC. The trade-off is that the count is taken on the OB1 scan edge rather than in hardware, so you must verify that no pulses are lost between scans. With a 6 ms OB1 and a 40 ms pulse period the worst case is one lost pulse in 7, which is tolerable for display.

1. Wire the meter output to a standard DI (e.g. I0.0). Configure the input filter to 0.1 ms.
2. Use a standard up-counter CTU with PV set above the maximum expected count over the window (e.g. 200 for 1 s at 25 Hz).
3. Add a 1 s on-delay timer (TON) whose output resets the counter via the CTU R input.
4. Capture the counter value on the timer's falling edge into a static word using a MOVE block triggered by the timer's Q output.
5. Apply the scaling Q = 3.6 · ΔN as a math block or a CALCULATE instruction.

The math step uses the CALCULATE instruction in TIA Portal with the expression

OUT = (IN1 * 3.6) / 1000.0   // IN1 = delta pulses in 1 s

For a 500 ms window replace 3.6 with 7.2; for 100 ms use 36. The trade-off is a noisier display at small W, since the integer delta quantises to 1 pulse.

8. Sampling Window and Filtering

For HMI display, a single-pole exponential filter on top of the 1 s sample is usually enough to calm the visual jitter:

Q_filt(t) = α · Q_raw(t) + (1 - α) · Q_filt(t - 1)

with α in [0, 1]. A 1 s sample window with α = 0.4 gives a display time constant of about 1.5 s, which is comfortable for operators. If the output is used as a PV in a PID loop, drop the filter and use the raw 1 s value; the loop's integral action provides the smoothing.

9. Edge Cases, Overflow, and Alarms

9.1 Negative delta from HSC wraparound

The HSC on the S7-1200 has a 32-bit count, so at 25 Hz it would take more than 4.8 years to roll over. If you ever change to a faster meter or remove the rollover, the difference oDeltaN must be treated as DInt, not as a Word, so the sign is correct.

9.2 Direction change

If the meter can read bi-directional flow, switch the HSC to two-phase mode and use the direction bit to gate the calculation. The SCL FB above assumes single-direction; for bi-directional, scale a positive delta by +1 and a negative delta by -1, or run two accumulators keyed off IDB_HSC1.NewDirection.

9.3 Stalled flow / wire break

A broken wire presents as zero pulses. Add a stall alarm: a TON with PT = 30 s that triggers when the HSC has not changed for 30 s, while the enable is true. This is often more useful than a low-flow alarm because it distinguishes "valve closed, no flow" from "flow at zero".

9.4 High-flow alarm

Set cHighLimit_Lph = 80 L/h. At 25 Hz the calculated 90 L/h will fire this alarm, which is the desired behaviour - it tells the operator the meter is over-driven and the K-factor should be re-verified or a smaller meter installed.

9.5 Totaliser

For totalised volume, do not average - add the unscaled count and divide by K once per minute. Store the total in a retentive LReal tag (e.g. Tot_L) and reset only on a manual command or a daily schedule.

10. Commissioning and Verification

1. Disconnect the process and feed a calibration pulse generator into the meter input. Set the generator to 22.22 Hz (1 pulse every 45 ms) and verify that the displayed Q reads 80.0 L/h ± 1 L/h within 2 s.
2. Repeat at 11.11 Hz (50 %) and 5.56 Hz (25 %) to confirm linearity.
3. Set the generator to 0 Hz and check that the stall alarm fires after 30 s.
4. Restore the process. Compare the integrated volume over 10 min against a manual draw into a calibration vessel. Aim for ± 1 %.
5. Watch the HSC IDB online - CurrentCount should advance monotonically.

10.1 Online watch table

Add the following tags to a watch table during commissioning:

"HSC_1".CurrentCount         // raw hardware count
"Inst_FlowRate".oDeltaN      // pulses in last window
"Inst_FlowRate".oFlow_Lph    // scaled flow rate
"Inst_FlowRate".oAlarmHi     // high alarm
"Inst_FlowRate".oAlarmLo     // low alarm

11. Troubleshooting Matrix

12. Variants and Migration

The same flow scales cleanly to other S7-1200 CPUs (CPU 1211C, 1212C, 1215C, 1217C) and to the S7-1500. On the S7-1500 the HSC is replaced by a TM Count or by a high-speed counter on the technology module, and the period measurement is built in. The scaling formula in §4 is identical, so the SCL FB body is portable verbatim. For a 1217C (DC/DC/DC) the HSC ceiling is 1 MHz and the same code is the right starting point; only the device configuration differs.

If the application is being ported from a MicroLogix or a third-party PLC, the only item that changes is the input filter default; the SCL math and the windowed scaling are the same.