Siemens FB100 Totalizer in STEP 7 5.5: Drift, LREAL, Pulse Method

1. Overview

Siemens FB100 "Totalizer" from the standard library is one of the most commonly requested blocks for S7-300/S7-400 programs written in STEP 7 V5.5 (and earlier). It is invoked from OB1 or, more typically, from a cyclic interrupt such as OB35 to integrate an instantaneous flow rate (m³/h, L/min, kg/h) into a running total (m³, L, kg). Field experience shows that this block, when fed from an analog flow input, drifts by 10-15% within 30-40 days of continuous operation. This article explains why the drift occurs, why no amount of analog-based "Totalizer" FB tuning will eliminate it, and how to implement either an LREAL 64-bit version or, preferably, a pulse-input totalizer that is exact by construction.

2. FB100 "Totalizer" - Block Mechanics

The FB100 Totalizer is delivered with the STEP 7 V5.5 standard library Standard Library > TI-S7 Converting Blocks (catalog name varies by service pack) and with the older stdlibs CD. The block multiplies the instantaneous input by the OB scan time and adds the result to a running accumulator.

Internally the integration is essentially:

CV := CV + (AI_IN * TI)
// where TI is converted to hours

Both AI_IN and CV are stored as REAL (32-bit IEEE-754 single precision), giving approximately 7 significant decimal digits of mantissa. The flow range of any real-world process (e.g. 0.05 to 250 m³/h integrated over weeks) easily exceeds that precision window.

3. Why Analog Totalization Drifts - The Floating-Point Root Cause

Field reports match published Siemens support material: integrating a small REAL (flow rate) into a large REAL (totalizer) repeatedly creates rounding loss because the relative magnitude of the addend shrinks as the totalizer grows. The classic example:

After about a month, the 32-bit mantissa cannot represent the 0.0014 m³ increment against the 480 m³ running total. Each scan either drops the increment entirely or rounds it to zero, producing the reported 10-15% loss versus the meter's mechanical register. This is not a bug in FB100; it is a property of IEEE-754 single precision documented in the Siemens KB article on floating-point arithmetic accuracy: Entry ID 14844391 - Accuracy of floating-point arithmetic in S7-300/S7-400.

4. LREAL (64-Bit Real) Library for S7-300/S7-400

STEP 7 V5.5 supports the LREAL (64-bit IEEE-754 double precision, ~15-17 significant digits) data type on S7-300 CPUs with firmware ≥ V2.x and on all S7-400 CPUs. The Siemens-contributed library of LREAL arithmetic functions (add, subtract, multiply, divide, compare, conversion to/from REAL) is published on the Siemens Support forum entry "Using Double Float (64-bit Real) in S7-300/400 Library with Functions" and is the canonical extension for any totalizer that must remain analog-driven. Key functions:

An LREAL-based totalizer in OB35 reduces drift from ~1 part in 10⁴ to ~1 part in 10¹⁵ at the same data rate - effectively drift-free for any practical plant life. However, the analog noise remains; use LREAL when you have no pulse output, not because it is "more accurate" than a counter.

5. Pulse-Based Totalization - The Only Exact Method

Every modern flow meter (electromagnetic, vortex, Coriolis, turbine, ultrasonic) offers a scaled digital pulse output - typically one pulse per litre, per 0.1 m³, per kg, or per gallon. The meter's own register is driven by the same pulse train, so the PLC totalizer will agree with the meter to within one pulse, by definition. The required S7-300 hardware is one digital input module (SM321 DI32 or DI16 × DC24V); no analog scaling required.

5.1 Pulse Counter Hardware Configuration

On S7-300, use the FM350-1 / FM350-2 counter modules or, more commonly, the integrated CPU 31xC fast counters:

• CPU 312C / 313C / 313C-2 PtP / 314C-2 PtP: 4 built-in counters, 10 kHz max.
• CPU 31x-2 PN/DP (V3.x and later): none integrated; use FM350-1.

For S7-400 use FM450-1 (8 channels, 100 kHz) or the IM178-4 high-speed module. Always terminate the pulse line through the meter's recommended pull-up/down resistor (typically 1-2.2 kΩ to +24 V for an open-collector output).

5.2 Pulse to Engineering Units

Define the meter's pulse weight at the HMI/parameter level, not hard-coded in the FB. A typical configuration block (DB100) for OB1 initialization:

DATA_BLOCK DB100
STRUCT
  PulseWeight : REAL := 0.001;  // 1 pulse = 0.001 m³ (1 L)
  Total_m3    : LREAL;          // accumulated volume
  Pulses_raw  : DINT;           // raw counter from FM350 / DI
  Reset_cmd   : BOOL;
END_STRUCT
END_DATA_BLOCK

5.3 The Integration Network (OB35, 100 ms)

// Network 1: Read counter
     L     "DI_Counter".Pulses_raw          // MD100 - actual counter
     T     DB100.DBD   8                    // mirror to DB100.Pulses_raw

// Network 2: Compute delta pulses this scan
     L     DB100.DBD   8                    // pulses_now
     L     DB100.DBD  12                    // pulses_last
     -I                                    // delta = now - last
     T     #delta_pulses                    // INT scratch
     L     DB100.DBD   8
     T     DB100.DBD  12                    // update last

// Network 3: LREAL accumulation
     L     #delta_pulses
     DTR                                    // DINT -> REAL
     L     DB100.PulseWeight                // REAL
     *R
     DTR                                    // already REAL; repeat-safe
     L     DB100.Total_m3.LB0                // load low word of LREAL
     // (call FC101 LREAL_ADD here from the 64-bit library)
     CALL FC 101
       IN1   := DB100.Total_m3
       IN2   := #inc_LREAL
       OUT   := DB100.Total_m3

For a DINT-only implementation (preferred for absolute minimum drift), store the total in m³ × 1000:

// Per scan: Total_dL := Total_dL + delta_pulses * (PulseWeight * 1000)
// PulseWeight = 0.001 m³ -> PulseWeight*1000 = 1 dL/pulse
// => Total_dL := Total_dL + delta_pulses   // pure DINT addition, no FP

This DINT counter is exact for ~24 days at 1 L/pulse and 1 Hz flow; for longer campaigns, add a high-word rollover using two DINTs (low / high 32 bits = 64-bit total in dL).

6. OB35 Cycle Time and Sampling Theory

OB35 default cycle is 100 ms on both S7-300 and S7-400 and is configurable in HW Config → CPU Properties → Cyclic Interrupts from 1 ms to 60 000 ms. Trade-offs:

For pulse counting in OB35, prefer the absolute delta-pulses approach (Section 5.3) over simple "count since last scan", because OB35 can be delayed by higher-priority OBs (OB82, OB121, OB122). Always read the counter from the FM350 hardware image rather than from a software flag.

7. Step-by-Step LAD Implementation (STEP 7 V5.5)

Prerequisites

• STEP 7 V5.5 + SP4 or later
• S7-300 CPU 31x-2 PN/DP or S7-400 CPU 41x
• FM350-1 counter module (or DI module + pulse meter with 24 V OC output)
• Siemens LREAL library (download from Siemens Support, copy FC101-FC112 into your S7 program)

Step 1 - Hardware Configuration

1. Open HW Config, insert FM350-1 in the slot next to the CPU.
2. Set channel 0 to continuous counting, 32-bit, gate always open.
3. Set the output value address, e.g. PQW 304 (DO load value) and PIW 304 (DI actual value).

Step 2 - Create the Instance DB

DATA_BLOCK DB100 "TotData"  // Shared DB
  PulseWeight  : REAL  := 0.001;   // m³ / pulse
  Total_m3     : LREAL;            // LREAL accumulator
  Total_dL     : DINT;             // integer accumulator (backup)
  Pulses_now   : DINT;             // last raw count
  Pulses_last  : DINT;
END_DATA_BLOCK

Step 3 - OB35 Network 1: Read raw counter

|    L     PIW 304                // low word
|    L     PIW 306                // high word
|    T     MD    100              // store as DINT in flags

Step 4 - OB35 Network 2: Compute delta

|    L     MD    100              // pulses_now
|    L     DB100.DBD  12          // pulses_last
|    -I
|    T     MW    104              // delta this scan
|    L     MD    100
|    T     DB100.DBD  12          // update pulses_last

Step 5 - OB35 Network 3: DINT accumulator (primary)

|    L     MW    104              // delta (pulse count)
|    L     DB100.DBD   0          // not used - pulse weight is unitless
|    // Assuming pulse weight = 1 L/pulse, delta is direct in L
|    L     DB100.DBD  16          // Total_dL
|    +I
|    T     DB100.DBD  16          // Total_dL := Total_dL + delta

Step 6 - OB35 Network 4: LREAL accumulator (for HMI display)

|    CALL FC 101
|      IN1 := DB100.Total_m3
|      IN2 := #inc_LREAL            // delta * PulseWeight, as LREAL
|      OUT := DB100.Total_m3

Step 7 - WinCC / HMI Tag

Expose DB100.Total_dL and DB100.Total_m3 to WinCC as integer and floating-point tags. Use scaling on the integer tag: 0.001 m³/dL.

8. Verification Procedure

1. Force DB100.Reset_cmd := TRUE for one OB35 scan; verify both totals return to 0.
2. Inject a known pulse burst from a signal generator (e.g. 1000 pulses at 100 Hz, weight = 0.001 m³) and confirm both Total_dL = 1000 and Total_m3 = 1.000.
3. Run 24 h with the meter on a closed loop; compare PLC total to the meter's mechanical register - they must agree to within ±1 pulse.
4. For analog/LREAL validation, compare against the pulse totalizer and graph the residual over 72 h. Residual must be flat or slowly diverging at <0.01% per week, not the 10-15% per month reported for FB100.

9. Method Comparison

10. Troubleshooting Matrix

11. Specifications & Limits Reference

12. Field-Commissioning Checklist

• □ Confirm the meter has a pulse output and the weight (e.g. 1 pulse / L) is documented.
• □ Wire the pulse output through a shielded twisted pair; terminate with the recommended pull-up.
• □ Configure the DI / FM350-1 channel in HW Config and download hardware.
• □ Insert DB100 (TotData) and verify all tag addresses against the symbol table.
• □ Insert FC101-FC112 (LREAL library) into the S7 program blocks.
• □ Replace any call to FB100 with the new pulse-counter network.
• □ Force a reset, generate a 1000-pulse test burst, verify totals.
• □ Run for 24 h against the meter register; record residual.
• □ Document the pulse weight and OB35 period in the loop drawing and HMI tag list.
• □ Mark DB100 as retentive and verify after power-cycle that the total is preserved.

13. Frequently Asked Questions