Configuring 10 S7-300 Analog Inputs with a Generic Pressure FB

Overview: 10-Channel Analog Pressure Acquisition on SIMATIC S7-300

A SIMATIC S7-300 station can be expanded to 10 analog inputs (AI) and 10 analog outputs (AO) using a combination of SM 331 analog input modules and SM 332 analog output modules configured in HWConfig. The CPU 315-2DP supports up to 32 analog channels in its standard address space, and STEP 7 automatically assigns peripheral input/output (PII/PIQ) addresses outside the process image (PI/PIQ) — typically starting at byte 256 for SM modules on rack 0.

For 10 pressure transmitters with 4–20 mA or 0–10 V outputs, the cleanest approach is to use a generic function block (FB) that wraps the standard Siemens converter FC105 "SCALE" and is called ten times from OB1 with different instance DBs. Each call uses its own peripheral input word (PIW) and its own engineering scale. This article documents the hardware configuration, the address map, the FB code, the wiring rules, and the verification procedure.

Prerequisites

• STEP 7 V5.5 (or compatible) with Simatic Manager and HWConfig installed.
• CPU 315-2DP (6ES7 315-2AG10-0AB0 or later) with sufficient free slots in the central rack.
• SM 331 analog input module(s) supporting the required signal type. For 4–20 mA: SM 331-7KF02 (8AI, 12-bit) or SM 331-1KF02 (8AI, 13-bit). For 0–10 V: same modules with the appropriate range card.
• SM 332 analog output module(s) for any companion outputs (8AO: 6ES7 332-5HF00).
• 10 pressure transmitters (4–20 mA, 2-wire or 4-wire) with documented full-scale and zero values.
• 24 V DC sensor supply (PS 307 5A or separate DC supply) and shielded twisted-pair signal cable.
• TI-S7 Converting Blocks library (installed by default with STEP 7 V5.5; contains FC105 and FC106).

S7-300 Analog Module Selection for 10 Channels

For 10 AI and 10 AO, the cleanest hardware combination is two SM 331s and two SM 332s in the central rack. The peripheral address map generated by HWConfig for this combination is:

The split at 256/288 reflects the S7-300 convention: SM 331 input modules start the analog PII at byte 256, and SM 332 output modules start the analog PIQ at byte 288. The byte numbers shown above are produced by a CPU 315-2DP in slot 2 with no other analog modules on rack 0; if additional analog modules are already configured, the addresses will shift accordingly.

For 0–10 V pressure transducers, set the range card on the SM 331 side to the "B" position (voltage, 0–10 V). For 4–20 mA transmitters, use the "C" or "D" position depending on the exact SM 331 variant — see the range-card legend printed on the module housing and the S7-300 Module Data manual.

HWConfig: Building the Rack and Allocating Peripheral Addresses

Open Simatic Manager, double-click the S7-300 station, and launch HWConfig. Drag the modules from the hardware catalog into slots in the following order:

1. Slot 1: PS 307 power supply.
2. Slot 2: CPU 315-2DP.
3. Slot 3: leave empty or place a digital module (digital I/O does not consume the analog PII/PIQ area).
4. Slot 4: SM 331 (8AI) — addresses PIW 256..271 are assigned automatically.
5. Slot 5: SM 331 (2AI) — addresses PIW 272..275 are assigned automatically.
6. Slot 6: SM 332 (8AO) — addresses PQW 288..303 are assigned automatically.
7. Slot 7: SM 332 (2AO) — addresses PQW 304..307 are assigned automatically.

Save and compile (Station → Save and Compile). HWConfig downloads the configuration to the CPU on the next programming device online connection; the CPU will not enter RUN until the actual module types match.

The ten 16-bit peripheral input words are at byte addresses 256, 258, 260, 262, 264, 266, 268, 270, 272, 274. STEP 7 numbers analog channels from 0 to 7 inside an 8AI module and 0 to 1 inside a 2AI module; the byte offset is module base + (channel × 2). Do not use the bit-level I and Q operand areas for these words — they only exist in the PII/PIQ peripheral area.

FC105 SCALE — Inputs, Outputs, and Bipolar Handling

FC105 ("SCALE") is part of the TI-S7 Converting Blocks library. It converts a 16-bit peripheral input word into a REAL engineering value using the formula:

OUT = ((IN - IN_LO) / (IN_HI - IN_LO)) × (HI_LIM - LO_LIM) + LO_LIM

where:

• IN is the raw peripheral word (0..27648 unipolar, -27648..+27648 bipolar).
• IN_LO / IN_HI are fixed by the module and the BIPOLAR pin: 0/27648 (unipolar) or -27648/+27648 (bipolar).
• LO_LIM / HI_LIM are the engineering low and high values supplied by the user as REAL.
• OUT is the scaled REAL engineering value.
• RET_VAL is W#16#0000 on success; W#16#0007 indicates an IN value out of range; W#16#0008 indicates a LO_LIM ≥ HI_LIM configuration error.

For 4–20 mA pressure transmitters, the correct FC105 parameters are:

CALL "SCALE"   // FC105
    IN      := PIW256          // raw 0..27648
    HI_LIM  := 3.500002E+02    // 350.0 bar at 20 mA
    LO_LIM  := 0.0             // 0.0 bar at 4 mA
    BIPOLAR := "M0.1"          // FALSE — unipolar
    RET_VAL := MW100
    OUT     := MD200

For ±10 V bipolar transducers the same block is used with BIPOLAR := "M0.0" (TRUE) and the LO_LIM / HI_LIM are set to negative/positive full scale, e.g. -10.0 / +10.0 V, or to the engineering equivalent of ±1000 mbar for a vacuum-pressure sensor.

Designing the Generic Pressure Function Block (FB100)

The user requirement is a single FB called ten times, one per gauge, with different peripheral addresses and engineering ranges. STEP 7 supports this directly: create an FB with IN/OUT/STAT/TEMP parameters, call it from OB1 (or a cyclic OB such as OB35) ten times with separate instance DBs, and the FB holds the per-channel scaling parameters in its instance DB.

Insert a new FB in the S7 program: Insert → S7 Block → Function Block, name it FB100, family "PRESSURE". Open the declaration table and define the following interface:

The body of FB100 contains a single FC105 call. The BIPOLAR pin is wired to a conditional that selects s_bZero or s_bOne based on the i_bBipolar input:

// FB100 - PRESSURE_SCALE (generic pressure scaling FB)
//
// One instance per gauge. The calling block (typically OB1 or
// a cyclic OB such as OB35) supplies the peripheral input word
// and the engineering limits. This block internally calls the
// standard Siemens FC105 "SCALE" converter to produce the
// engineering value in REAL.

      A     #i_bBipolar
      JCN   uni
// --- bipolar branch (e.g. ±10 V) ---
      L     #s_bOne                  // TRUE
      T     #s_bBipolarWire          // internal helper flag
      JU    call
uni:  L     #s_bZero                 // FALSE
      T     #s_bBipolarWire
call: CALL "SCALE"                   // FC105
          IN     := #i_RawValue
          HI_LIM := #i_HiLim
          LO_LIM := #i_LoLim
          BIPOLAR:= #s_bBipolarWire
          RET_VAL:= #o_wError
          OUT    := #o_EngValue

The LAD/FBD view of the same code uses a comparator and a MOVE / assignment to switch the BIPOLAR input. From the caller's perspective, no M-flag is required and the wiring of BIPOLAR is handled inside the FB.

Create the instance DBs: in the S7 program, insert ten Data Blocks — DB101..DB110 — each as Instance DB associated with FB100. Alternatively, build a multi-instance DB that owns all ten FB100 instances from a single parent FB. STEP 7 will pre-allocate the static variables (s_bZero, s_bOne) once per instance.

OB1 Cyclic Call: Instantiating the FB Ten Times

In OB1, call FB100 ten times. Use a separate instance DB for each call so the per-channel limits and current scaled value are isolated:

// ===========================================
// Pressure gauge scaling — 10 channels
// ===========================================
//
// Address assignments from HWConfig:
//   Gauge 1  -> PIW 256    Gauge 2  -> PIW 258
//   Gauge 3  -> PIW 260    Gauge 4  -> PIW 262
//   Gauge 5  -> PIW 264    Gauge 6  -> PIW 266
//   Gauge 7  -> PIW 268    Gauge 8  -> PIW 270
//   Gauge 9  -> PIW 272    Gauge 10 -> PIW 274
//
// All gauges are 0–350 bar, 4–20 mA (unipolar).

      CALL FB100, DB101                 // Gauge 1
        i_RawValue  := PIW256
        i_LoLim     := 0.0
        i_HiLim     := 350.0
        i_bBipolar  := FALSE
        o_EngValue  := "PRESS".Gauge1
        o_wError    := "PRESS".Err1

      CALL FB100, DB102                 // Gauge 2
        i_RawValue  := PIW258
        i_LoLim     := 0.0
        i_HiLim     := 350.0
        i_bBipolar  := FALSE
        o_EngValue  := "PRESS".Gauge2
        o_wError    := "PRESS".Err2

      // ... repeat for DB103..DB110 / PIW260..PIW274 ...

The symbol table for the data block "PRESS" (e.g. DB120 — a shared global DB holding the ten REAL results) is:

Pressure Gauge Scaling — Worked Examples

For a 0–350 bar gauge with a 4–20 mA output connected to an SM 331 channel configured for 4–20 mA:

P_bar = (PIW - 0) / (27648 - 0) × (350 - 0) + 0

Worked points:

• 4.000 mA → raw 0 → 0.000 bar
• 8.000 mA → raw 5529.6 → 70.00 bar (20 % of FS)
• 12.000 mA → raw 11059.2 → 140.00 bar (40 % of FS)
• 16.000 mA → raw 16588.8 → 210.00 bar (60 % of FS)
• 20.000 mA → raw 27648 → 350.00 bar (100 % of FS)

For a -1..+10 bar compound gauge (vacuum to 10 bar, ±10 V bipolar), the formula is:

P_bar = (PIW - (-27648)) / (27648 - (-27648)) × (10 - (-1)) + (-1)

Note that the bipolar input range uses the full -27648..+27648 span, so the scaling is symmetric. For asymmetric ranges such as -0.5..+10.0 bar, set LO_LIM = -0.5 and HI_LIM = 10.0; FC105 will linearly scale the same input range to the asymmetric output.

FC106 UNSCALE for Companion Analog Outputs

When the application also needs ten 0–10 V control outputs (e.g. proportional valve commands derived from the pressures), use FC106 "UNSCALE" — the inverse of FC105. The address map for outputs is PQW 288..307 in our hardware example.

CALL "UNSCALE"   // FC106
    IN      := "PRESS".CmdValve1   // REAL, e.g. 0.0..100.0 (%)
    HI_LIM  := 100.0
    LO_LIM  := 0.0
    BIPOLAR := "M0.1"              // FALSE for 0..10 V
    RET_VAL := "PRESS".AO_Err1
    OUT     := PQW288

The OUT pin writes to the peripheral output word and the SM 332 latches the value at the next analog output update. Use the same FB-wrapping technique to make a generic FB200 that contains the FC106 call and is called ten times with the appropriate PQW and engineering range.

Wiring, Shielding, and Range-Module Setup

1. Range card on SM 331. The SM 331 AI8 (6ES7 331-7KF02) has a removable range card on the left side of the housing. Set the card to position "C" for 4–20 mA or "B" for 0–10 V. The range card is read at power-up; if you change it, cycle power to the module.
2. Wiring topology. For 4-wire transmitters, use a separate 24 V supply to the transmitter and return the mA signal on its own pair. For 2-wire transmitters, use the SM 331 channel's M+ / M- terminals and the 24 V supplied by the module (U+ / U- terminal pair, current-limited to ~25 mA per channel).
3. Shielding. Ground the cable shield at the cabinet entry using a PE rail or shield bar. Do not ground at both ends unless the building uses a single-point ground; ground loops will inject 50/60 Hz into the mA loop.
4. Channel grounding. For 4–20 mA, connect Comp (compensation / common) of the channel to M- on the SM 331 terminal block. For 0–10 V with single-ended inputs, leave Comp open or connect to ground as documented.
5. Unused channels. Short the unused channels' M+ to M- with a jumper and set the range card position to "B" (voltage) — open unused current inputs are a common source of diagnostic interrupts and channel fault LEDs.
6. Cable type. Use LiYCY or similar shielded twisted-pair, ≤ 50 m for 4–20 mA to keep loop resistance below the transmitter's compliance voltage limit. For 0–10 V, keep ≤ 10 m to limit capacitive loading.

Commissioning, Verification, and Online Monitoring

1. Compile and download. In Simatic Manager, select the S7 program blocks and click PLC → Download. The CPU should go RUN after a STOP-to-RUN transition if the hardware matches HWConfig.
2. Online → Monitor/Modify. Open OB1 in the online view. Confirm each PIW256..PIW274 shows a value in the expected range (e.g. 0..27648 for 4–20 mA). A reading of -32768 (W#16#8000) usually means wire break on a 4–20 mA channel.
3. Force-test the FC105 call. Set PIW256 to a known value using Variable Table → Monitor/Modify. The corresponding DB120.DBD0 (Gauge1) should be 0.0 at raw=0 and 350.0 at raw=27648. If the value is stuck at 0 or shows a wildly large number, check the FC105 RET_VAL in DB120.DBW40 — non-zero indicates LO_LIM ≥ HI_LIM or an overflow.
4. Calibration sweep. Apply 4 mA, 8 mA, 12 mA, 16 mA, 20 mA from a precision calibrator on each channel and record the engineering value. The deviation at any point should be within the SM 331's accuracy spec (typically ±0.3 % of full scale for the 6ES7 331-7KF02 at 25 °C, ±0.6 % over 0..60 °C).
5. Cross-check FB instance DBs. Open DB101..DB110 online and verify the instance data: i_RawValue (last read), o_EngValue (last scaled), o_wError. If the instance DB and the OB1 call use mismatched names, the FB will be reinitialised each cycle and the static helper bits will lose their state.
6. Diagnostic buffer. On any unexpected STOP, check the CPU diagnostic buffer (PLC → Diagnostic Buffer) for "OB not loaded", "Module failure", or "I/O access error" entries that point to the offending PIW address.

Fault Codes, Diagnostics, and Common Pitfalls

FAQ