Programming Siemens S7 Analog Inputs: FC105/FC106 with 4-20mA Level Control

Siemens SIMATIC S7-300 and S7-200 PLCs read 4-20 mA process signals from level, pressure, flow, and temperature transmitters as 16-bit integer counts in the process-image input word (PIW). The IEC 61131-3 standard library TI-S7 Converting Blocks supplied with STEP 7 V5.x exposes FC105 (SCALE) and FC106 (UNSCALE) to linearize that integer to engineering units (e.g., 0.0-100.0 % tank level) and back again for analog outputs. This technical reference walks through FC100, FC101, FC105, FC106 operation, raw-value mapping for 4-20 mA on SM331 (S7-300) and EM231 (S7-200), hardware configuration in STEP 7 Classic, and a complete LAD and FBD example that starts and stops a pump on high and low level setpoints with hysteresis.

Overview: When to Use FC100, FC101, FC105, and FC106

Siemens ships a fixed set of standard converting FBs/FCs in the STEP 7 Standard Library that bridge raw I/O counts and engineering units. The two most commonly called blocks are FC105 (read side) and FC106 (write side). FC100, FC101, FC102, and others handle less common or older conversion tasks.

Prerequisites

• STEP 7 V5.5 SPx (Classic) or TIA Portal V13 SP1+ with S7-300 / S7-400 HSP support package installed (TIA V15.1 or later is recommended for current CPUs).
• S7-300 SM331 analog input module (typical catalog numbers: 6ES7331-7KF02-0AB0 for 8 AI 13-bit, 6ES7331-1KF02-0AB0 for 8 AI 12-bit, 6ES7331-7PF11-0AB0 for 8 AI 16-bit with HART).
• For S7-200: STEP 7 Micro/WIN V4.0 SPx with EM231 (6ES7231-0HC22-0XA0, 4 AI) or EM235 (6ES7235-0KD22-0XA0, 4 AI + 1 AO).
• A 4-20 mA two-wire or four-wire level transmitter (e.g., Siemens SITRANS Probe LU, Prosonic FMU, Endress+Hauser FMX21, or generic 24 V loop-powered unit) wired to AI channel 0.
• 24 VDC supply for the analog loop. SM331 does NOT supply loop power for 4-wire transmitters; for 2-wire transmitters set the SM331 measuring range to 2DMU or use the integrated 24 V on the front connector only if the SM331 variant supports it.
• Shielded twisted-pair cable for the analog run, grounded at the cabinet end only.

SM331 Module Selection Matrix

Raw Value Mapping: 4-20 mA on S7-300 SM331

SM331 modules configured for measuring range "4DMU" (4-wire current, 4-20 mA) deliver a 16-bit signed integer in the standard SIMATIC range. The same integer range applies to 0-20 mA and 0-10 V. Bipolar ranges (-10 to +10 V, -20 to +20 mA) use -27648 to +27648.

The FC105 block automatically clips a -32768 input (wire break) to LO_LIM and returns RET_VAL = W#16#0000. The actual "wire break" condition must be read from the S7 diagnostic buffer (SFC51 / RD_REC) or from the SF LED on the SM331.

FC105 SCALE — Parameters and Math

FC105 is located in the STEP 7 Standard Library at Standard Library > TI-S7 Converting Blocks > FC105. Calling it in OB1 reads PIWxxx and returns a REAL in your configured engineering range.

Math implemented by FC105:

OUT = LO_LIM + (IN / 27648.0) * (HI_LIM - LO_LIM)   // unipolar (4-20 mA, 0-10 V)
OUT = LO_LIM + (IN / 27648.0) * (HI_LIM - LO_LIM)   // bipolar (-10 to +10 V)

For 4-20 mA with HI_LIM = 100.0 and LO_LIM = 0.0, a raw value of 13824 (12 mA, half range) yields OUT = 50.0. The actual process current at any scaled OUT is recovered by Process_mA = 4.0 + (OUT / 100.0) * 16.0.

FC106 UNSCALE — Parameters and Math

FC106 is the mirror block used for analog outputs. The PLC writes PQWxxx to the SM332 module, which converts 0-27648 to 0-20 mA or 4-20 mA at the output.

OUT = (IN - LO_LIM) / (HI_LIM - LO_LIM) * 27648.0   // unipolar

Hardware Configuration in STEP 7 (Classic)

1. Open the S7-300 station in SIMATIC Manager and double-click Hardware to launch HW Config.
2. Insert your CPU (e.g., 6ES7315-2EH14-0AB0, CPU 315-2 PN/DP) in slot 2 and the SM331 (e.g., 6ES7331-7KF02-0AB0) in slot 4.
3. Open SM331 properties. In the Addresses tab, confirm the input start address (slot 4, AI area → default 288 for the first 16 bytes, so channel 0 = PIW288, channel 1 = PIW290, ...).
4. In the Inputs tab, set channels 0-7 to 4DMU (4-wire current, 4-20 mA). The setting is per-group: groups of 2 channels share a measuring range.
5. Set Integration time to 20 ms (50 Hz mains rejection, used in Europe) or 16.67 ms (60 Hz rejection, used in North America). Default 60 ms works for most tank-level apps but adds update delay.
6. Enable Diagnostics Interrupt if you need OB82 to catch wire break and overrange events.
7. Save and compile (Station → Save and Compile). Download to the PLC.

LAD Example: Read 4-20 mA Level Transmitter and Start/Stop Pump

Hardware assumed: SM331 at slot 4 (start address 288). Pump output on Q0.0. High setpoint 80 % (start pump), low setpoint 20 % (stop pump). Manual Run-permit at I0.0, manual start at I0.1, manual stop at I0.2, thermal fault at I0.3.

Network 1 — Read and scale AI0 (4-20 mA level transmitter, 0-100 %):

      PIW288
--|EN|  |--[ FC105 SCALE ]--|EN|       MD100    MW110
                          IN:    PIW288
                          HI_LIM: 1.0e+02
                          LO_LIM: 0.0e+00
                          BIPOLAR: FALSE
                          RET_VAL: MW110
                          OUT     -> MD100   (Level % REAL)

Network 2 — Set (start) pump on high level with Run permit and no fault:

   MD100        80.0          I0.0       I0.3
==|----[ GE ]----80.0----+---[/]---+---[/]---+----(S)---- Q0.0
                          |  (I0.0   |  (I0.3
                          |   Run    |   Thermal
                          |   permit)|   OK) 
                          |          |

Network 3 — Reset (stop) pump on low level:

   MD100        20.0
==|----[ LE ]----20.0-----------------(R)---- Q0.0

Network 4 — Manual start (I0.1) and stop (I0.2) overrides:

   I0.1                              Q0.0     I0.3
--[ ]--+--(S)---- Q0.0     (I0.2 stop)--[/]--+--(R)---- Q0.0

Network 5 — Optional wire-break alarm from FC105 RET_VAL:

   MW110        W#16#0007       M10.0
--[ == ]-----W#16#0007-----(S)---- M10.0   (Level_Tx_WireBreak_latch)

FBD Example: Same Application in Function Block Diagram

Network 1 — Scale input:

PIW288 ---| FC105 SCALE |---> MD100
            HI_LIM  = 100.0
            LO_LIM  =   0.0
            BIPOLAR = 0
            RET_VAL = MW110

Network 2 — Start pump on high level with Run permit and no fault:

MD100 --[ GE ]-- 80.0 --+-- I0.0 --[ AND ]-- I0.3 --[ AND ]--| S |--- Q0.0
                          (Run permit)        (Thermal OK)

Network 3 — Stop pump on low level:

MD100 --[ LE ]-- 20.0 ---------------------| R |--- Q0.0

Network 4 — Manual start / stop and fault reset:

I0.1 --[ OR ]-- I0.2 --[ NOT ]--| S |--- Q0.0   (Manual start)
I0.2 ------------------------------| R |--- Q0.0   (Manual stop)

Complete Pump Control FB (Hysteresis + Fault Latching)

For a robust field implementation, package the level scaling, hysteresis, and fault latching in a single Function Block (FB100 "PUMP_CTRL") with instance DB100. This is the pattern that survives a long service life in a process plant.

FUNCTION_BLOCK FB100
VAR_INPUT
    PIW_Level       : INT;     // PIW from SM331 (e.g., PIW288)
    Sp_High         : REAL;    // Start setpoint (e.g., 80.0 %)
    Sp_Low          : REAL;    // Stop setpoint (e.g., 20.0 %)
    RunPermit       : BOOL;    // I0.0
    ThermalOK       : BOOL;    // I0.3
    ManualStart     : BOOL;    // I0.1
    ManualStop      : BOOL;    // I0.2
END_VAR
VAR_OUTPUT
    PumpRun         : BOOL;    // Q0.0
    LevelPct        : REAL;    // 0.0 to 100.0 %
    LevelmA         : REAL;    // 4.0 to 20.0 mA
    WireBreak       : BOOL;    // DI from RET_VAL or PIW = -32768
END_VAR
VAR
    FC105_RV        : WORD;    // RET_VAL from FC105
    LevelPrev       : REAL;    // Edge-detection helper
END_VAR

BEGIN
    // 1. Scale raw level
    FC105(IN       := PIW_Level,
          HI_LIM   := 100.0,
          LO_LIM   := 0.0,
          BIPOLAR  := FALSE,
          RET_VAL  := FC105_RV,
          OUT      := LevelPct);

    // 2. Convert to mA for HMI display
    LevelmA := 4.0 + (LevelPct / 100.0) * 16.0;

    // 3. Wire-break detection (PIW = -32768 when open circuit)
    IF PIW_Level = -32768 THEN
        WireBreak := TRUE;
    ELSE
        WireBreak := FALSE;
    END_IF;

    // 4. Hysteresis pump control
    IF (LevelPct >= Sp_High) AND RunPermit AND ThermalOK AND NOT WireBreak THEN
        PumpRun := TRUE;
    END_IF;
    IF (LevelPct <= Sp_Low) OR NOT RunPermit OR NOT ThermalOK OR WireBreak THEN
        PumpRun := FALSE;
    END_IF;

    // 5. Manual override
    IF ManualStart AND RunPermit AND ThermalOK THEN
        PumpRun := TRUE;
    END_IF;
    IF ManualStop THEN
        PumpRun := FALSE;
    END_IF;
END_FUNCTION_BLOCK

Call FB100 from OB1 with instance DB100. Drive Q0.0 from DB100.DBX0.0 directly. This pattern gives you a 60 % deadband (80 - 20) and prevents pump chattering at the threshold.

S7-200 Differences (EM231 / EM235)

The S7-200 family does NOT have FC105/FC106; the user must scale the raw integer manually because the EM231/EM235 raw count range differs from S7-300:

EM231 4-20 mA scaling ladder (STEP 7 Micro/WIN V4.0):

Network 1 — Subtract 6400 (4 mA offset)
   AIW0          VW100         VW102
--[ SUB_I ]--  VW100,  6400,  VW102    ; VW102 = raw - 6400

Network 2 — Convert to REAL
   VW102         VD104         AC0
--[ MOV_R ]--  VD104,   AC0           ; AC0 = (raw-6400) as REAL

Network 3 — Scale to 0.0-100.0 (multiply by 100.0/25600.0 = 0.00390625)
   AC0           0.00390625    AC0
--[ MUL_R ]--  AC0,   0.00390625,  AC0

Network 4 — Store result
   AC0           VD108
--[ MOV_R ]--  AC0,   VD108           ; VD108 = Level %

For S7-200 SMART (the modern replacement for S7-200, 6ES7288-...), the AI raw range follows S7-300 (0-27648) and the SCALE / NORM_X instructions are available in the Instruction tree. Use the same FC105-equivalent path as S7-1200.

S7-1200 / S7-1500 Alternative (TIA Portal)

For S7-1200/1500, FC105/FC106 are replaced by the box-style NORM_X and SCALE_X instructions in the Basic Instructions > Converter operations palette. NORM_X performs the integer-to-fraction (0.0-1.0) step; SCALE_X performs the fraction-to-engineering-range step. The pair is functionally identical to FC105, with the BIPOLAR parameter replaced by selecting the appropriate data type (INT for 0-27648 unipolar, INT for -27648 to +27648 bipolar). For complete analog input specifications on S7-1200 SM 1231, SM 1232, and SM 1234 modules, see the SIMATIC S7-1200 Programmable Controller System Manual — Analog Inputs (CPU 1212C / SM 1231 / SM 1234).

Verification & Commissioning

1. Monitor PIW: Open Monitor/Modify in STEP 7. Apply a known current from a calibrator (Fluke 754, Beamex MC6, or similar). Confirm PIW reads 0 at 4.000 mA and 27648 at 20.000 mA (±1 LSB).
2. Monitor scaled REAL: With calibrator at 12.000 mA, MD100 (FC105 OUT) should display 50.0 ±0.1 %.
3. Wire break test: Disconnect the +ve loop wire at the transmitter. SM331 should drive PIW to -32768 (8000H), FC105 should clip OUT to LO_LIM (0.0), and the S7 diagnostic buffer should record "Channel 0: Wire break" with SF LED on the SM331. If wired to a non-Siemens HMI, the alarm word M10.0 (Network 5) latches.
4. Setpoint sweep: Ramp the calibrator from 4 to 20 mA in 1 mA steps. Q0.0 (PumpRun) should set at 80 % (PIW = 22118) and reset at 20 % (PIW = 5529). The 60 % deadband should be observed.
5. Fault test: Open the thermal contact (I0.3 = 0). Pump must stop within one OB1 cycle (typically 10-100 ms) regardless of level.
6. HMI cross-check: Compare the HMI displayed % with the level transmitter's local display; typical agreement ±0.5 % of span.

Troubleshooting Matrix

Field-Commissioning Notes

• Set SM331 hardware integration time to 20 ms (50 Hz European mains) or 16.67 ms (60 Hz North America) for power-line noise rejection. In STEP 7 HW Config: SM331 → Inputs → Channel → Integration time. The default 60 ms is conservative but adds 40 ms update delay.
• For redundant level measurement (two transmitters in the same tank), build the scaling in FC105 on each PIW first, then use a HIGH_SELECT (FC37) or AVERAGE (FC36) function to combine. This preserves the raw value for diagnostics on each transmitter.
• Compare PIW as a signed INT. In LAD, comparing PIW directly to 27648 with == will raise a type clash. Always convert via FC105 to REAL, or use ITD (Integer-to-Double Integer, 32-bit) followed by DTR (Double Integer-to-REAL) before comparison.
• Loop-powered 2-wire transmitters need a separate 24 V supply or use the SM331-7NF10 / SM331-7PF11 "2-wire" (2DMU) configuration. 4-wire transmitters (separate 24 V + signal pair) work with the "4DMU" mode used in this article.
• Always wire the cable shield to the cabinet ground bar at one end only (the cabinet end). Ground loops from both-end grounding inject 50/60 Hz noise that pushes the reading 50-200 counts.
• For 4-20 mA signals on long runs (>100 m), increase the cable gauge from 0.75 mm² to 1.5 mm² to keep loop resistance below the transmitter's compliance voltage budget (typically 250 Ω at 24 V for a 2-wire unit = 12 V drop budget).
• On S7-200, the EM231 has DIP switches SW1-SW6 that MUST match the input range. For 4-20 mA on channel 0, set SW1=ON, SW2=OFF, SW3=ON, SW4=OFF, SW5=ON, SW6=OFF (per the 6ES7231-0HC22-0XA0 datasheet). Wrong DIP setting yields a reading that is always 0 or always 32767.
• For S7-300 firmware V3.x and later, the OB82 (diagnostic interrupt) is automatically inserted by STEP 7 if you enable SM331 diagnostic interrupts in HW Config. Use OB82 to latch a wire-break bit and force the pump off, independent of FC105's clipping.