S7-1200 PT100 RTD Module: Wiring, TIA Portal & 6-Zone Oven Setup

Overview: Why Direct RTD Connection Beats External Converters

A common misconception when integrating PT100 resistance temperature detectors (RTDs) with a Siemens S7-1200 PLC is that an external PT100-to-voltage or PT100-to-current converter is mandatory. The S7-1200 family has dedicated SM 1231 RTD signal modules and a compact SB 1231 RTD signal board that accept 2-wire, 3-wire, and (on selected variants) 4-wire RTDs directly. This eliminates the cost of six DIN-rail transmitters, removes a layer of failure, eliminates additional 24 V load, and improves noise immunity because the signal never leaves the shielded sensor cable as a low-level analog voltage.

This reference covers a 6-zone oven application: six 3-wire PT100 probes, four ramp/soak setpoints per profile, ±1 °C control tolerance, and a CPU 1214C or CPU 1215C host. The same architecture scales to any S7-1200 RTD application from 1 to 16 channels.

PT100 RTD Sensor Fundamentals

A PT100 is a platinum RTD with a nominal resistance of 100.00 Ω at 0 °C. The IEC 60751 nominal coefficient is α = 0.00385 Ω/Ω/°C (the European/IEC curve, also called PT100 standard). A second curve, α = 0.003916 (American curve, PT100 legacy US), is occasionally encountered on older probes; the SM 1231 lets you select either via the sensor type parameter.

3-Wire vs 2-Wire vs 4-Wire

• 2-wire: Lead resistance adds directly to the measured Ω. At 0.385 Ω/°C, a 1 Ω lead = 2.6 °C error. Acceptable only for short pigtails.
• 3-wire (recommended): Two leads carry the excitation current; the third lead senses the element terminal. The SM 1231 cancels lead resistance by measuring both excitation legs. This is the default for industrial oven installations.
• 4-wire (Kelvin): Two excitation + two sense leads, eliminates lead error completely. Supported on selected SM 1231 RTD variants; use it for lab-grade or sub-±0.5 °C work.

S7-1200 RTD Module Family

The S7-1200 supports RTD inputs through two product lines:

All four modules accept PT100, PT200, PT500, PT1000, Ni100, Ni120, Ni200, Ni500, Ni1000, Cu10 (selected types) and plain resistance. The 5PD32 / 5QF32 series are the current production variants and are the right pick for new builds.

For the official Siemens product page and current firmware, refer to the S7-1200 Programmable Controller System Manual and the SM 1231 RTD module manual entry.

Maximum Module Count per CPU

Module Selection for 6 Oven Zones

For six PT100 probes, the cleanest hardware plan is:

1. Option A (recommended): 1 × SM 1231 RTD 8AI (6ES7231-5QF32-0XB0). 8 channels, 2 spares, no SB needed.
2. Option B: 2 × SM 1231 RTD 4AI (6ES7231-5PD32-0XB0). 8 channels total. Useful if channels must be on different physical modules for serviceability.
3. Option C: 1 × SM 1231 RTD 4AI + 1 × SB 1231 RTD. Only 5 channels — insufficient for 6 zones.

3-Wire PT100 Wiring Procedure

The SM 1231 RTD uses a removable terminal block. The 3-wire PT100 connects to the two excitation terminals (I+ and I-) and one sense terminal (M-). The fourth wire, if present, is tied to M+ for 4-wire mode (not used here).

Terminal layout (SM 1231 RTD, channel 0 shown)

Channel 0  |  Channel 1
  1  I0+       5  I1+
  2  I0-       6  I1-
  3  M0+       7  M1+
  4  M0-       8  M1-

Power:    L+  /  M  (24 V DC supply to module)

3-wire PT100 hookup

1. Strip the probe cable shield back to within 20 mm of the gland; expose the three conductors and drain wire.
2. Tie the drain wire to the cabinet ground bar via a short pigtail — do not land it on the RTD module terminal.
3. Land red (excitation return) on I0+.
4. Land red (excitation source) on I0-. The two reds are interchangeable as long as one is on I+ and the matching pair-side on I-.
5. Land white (sense lead) on M0-.
6. Repeat for channels 1…5. Channels 6 and 7 stay open (or jumper M+ to M- to defeat the open-wire diagnostic; see Diagnostics below).
7. Apply 24 V DC to L+ / M on the module power terminals.

TIA Portal Hardware Configuration

Open the project in TIA Portal (V16 or later recommended for the 5PD32/5QF32 modules). In the project tree: Devices & Configuration → [CPU 1214C] → Device view.

1. From the hardware catalog, expand SM 1231 → AI → RTD.
2. Drag the SM 1231, AI 8x16 bit RTD (order number 6ES7231-5QF32-0XB0) into slot 1 of the CPU. The order is determined by the physical slot in the rack; slot 1 sits immediately to the left of the CPU.
3. If using the SB 1231 instead, drag the SB 1231, AI 1x16 bit RTD (6ES7231-5PA30-0XB0) into the signal board slot of the CPU. The SB mounts on the lower front of the CPU; only one SB per CPU.
4. Double-click the module to open the Properties inspector. Set the channel-by-channel configuration described in the next section.
5. Compile the device configuration (Hardware (rebuild all)) and download to the CPU.

TIA Portal Channel Configuration

Select channels 0…5 (channels 6 and 7 are unused). For each, set the following parameters in the Properties → Inputs tab:

The configured input word (IW) returns temperature in 1/10 °C units by default. An oven at 250 °C reads back as decimal 2500. Confirm this in Properties → IO Tags on the module — Siemens calls it the Process Value Scale.

Temperature Scaling and Smoothing

The RTD module returns a normalized integer in 1/10 °C. For analog scaling math (e.g., when feeding a PID that expects a real number), use the standard TIA Portal NORM_X and SCALE_X instructions, or read the value as INT and divide by 10.0.

// FB "Temp_Raw_To_Float" — SCL
// Input:  iRaw   : INT    — 1/10 °C from SM 1231
// Output: rTempC : REAL   — °C as real

rTempC := INT_TO_REAL(iRaw) / 10.0;

// Optional plausibility check
IF rTempC < -50.0 OR rTempC > 500.0 THEN
    bSensorFault := TRUE;
END_IF;

For moving-average smoothing to fight electrical noise on long probe leads:

// FB "Temp_MA_Filter" — SCL
// 8-sample moving average, runs at the OB1 cycle time

IF iCnt >= 8 THEN
    iCnt := 0;
END_IF;
arSamples[iCnt] := rTempC;
iCnt := iCnt + 1;

rTempFiltered := (arSamples[0] + arSamples[1] + arSamples[2] + arSamples[3]
                + arSamples[4] + arSamples[5] + arSamples[6] + arSamples[7]) / 8.0;

Ramp/Soak Profile Programming (4 Setpoints, 4 Ramps)

"4 setpoints and 4 ramps" describes a four-segment profile. The standard pattern is Ramp1 → Soak1 → Ramp2 → Soak2 … but the implementation below uses a generic setpoint table, so it can be configured as 4 ramps only, 4 soaks only, or any mix.

Profile data block

DATA_BLOCK "OvenProfile_DB"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
  STRUCT
      rSetpoint : ARRAY[1..4] OF REAL := [100.0, 200.0, 250.0, 25.0];
      rRampRate : ARRAY[1..4] OF REAL := [5.0, 3.0, 2.0, 10.0];  // °C/min
      rDwellTime: ARRAY[1..4] OF REAL := [30.0, 45.0, 60.0, 15.0]; // min
      iActiveSeg: INT  := 0;
      bRunProfile: BOOL := FALSE;
  END_STRUCT;
END_DATA_BLOCK

Profile execution FB

// FB "RampSoak_4Seg" — SCL
// Inputs: rTempC (°C), bStart, bStop, rTarget (setpoint of active seg)
// Outputs: rOutput (target to PID), bProfileDone, bFault
// Internal: timer advances the segment when rTempC reaches rTarget within tolerance

IF bStop THEN
    iActiveSeg := 0;
    bRunProfile := FALSE;
    bProfileDone := FALSE;
END_IF;

IF bStart AND NOT bRunProfile THEN
    iActiveSeg := 1;
    bRunProfile := TRUE;
    bProfileDone := FALSE;
    tSegmentTimer(IN := TRUE, PT := T#0ms);
END_IF;

IF bRunProfile THEN
    // Determine next setpoint & ramp slope
    rTarget := "OvenProfile_DB".rSetpoint[iActiveSeg];
    rRate   := "OvenProfile_DB".rRampRate[iActiveSeg];
    rDwell  := "OvenProfile_DB".rDwellTime[iActiveSeg];

    // Linear ramp: dT/dt = rRate °C/min
    rOutput := rLastOutput
             + SIGN(rTarget - rLastOutput) * rRate * (tTick_ms / 60000.0);

    // Clamp to setpoint
    IF (rTarget - rLastOutput) * (rOutput - rLastOutput) <= 0.0 THEN
        rOutput := rTarget;
    END_IF;

    rLastOutput := rOutput;

    // Dwell timer
    IF ABS(rTempC - rTarget) < 1.0 THEN  // ±1 °C tolerance
        tSegmentTimer(IN := TRUE, PT := REAL_TO_TIME(rDwell * 60000.0));
        IF tSegmentTimer.Q THEN
            iActiveSeg := iActiveSeg + 1;
            tSegmentTimer(IN := FALSE);
            IF iActiveSeg > 4 THEN
                bRunProfile := FALSE;
                bProfileDone := TRUE;
            END_IF;
        END_IF;
    END_IF;
END_IF;

PID_Compact Integration per Oven Zone

For each of the six zones, instantiate PID_Compact from the TIA Portal instructions library. The PID output drives either a solid-state relay (SSR) for electric heaters, a 4–20 mA proportional gas valve, or a PWM output on a digital output module.

1. Add PID_Compact to the program. Create a separate instance DB per zone (e.g., PID_Compact_DB_1 through PID_Compact_DB_6).
2. Wire Setpoint to rOutput from the RampSoak_4Seg FB.
3. Wire Input to rTempC from the Temp_Raw_To_Float FB.
4. Configure Input / Output → Input scaling as 0.0 to 400.0 °C.
5. Configure Output → Output scaling as 0.0 to 100.0 %.
6. Select Output → Activate mode: PWM (digital out, 1 s period) for SSR loads, or analog out (0…10 V via AQ module).
7. Run Commissioning → Pretuning once the oven reaches a stable temperature. Pretuning identifies Kp, Tn, Tv automatically.
8. Run Fine tuning at the operating setpoint to capture the actual process dynamics.

Diagnostics and Troubleshooting

Commissioning Verification

1. Module online test: In TIA Portal, right-click the SM 1231 → Online & Diagnostics → Monitor. All six channels should show a sensible ambient temperature (e.g., 18–25 °C).
2. Short-circuit test: On channel 0, disconnect the probe and short I0+ to I0- to M0- with a short wire. The reading should drop to a defined underflow value (e.g., -200 °C) and trigger the wire-break diagnostic.
3. Calibration check: Use a decade resistance box (or calibrated PT100 simulator) connected to channel 0. Inject 100.00 Ω — the module should read 0.0 °C ± 0.3 °C. Inject 138.51 Ω — should read 100.0 °C ± 0.5 °C.
4. Cold-junction observation: For an unheated probe, log the value over 5 minutes. The spread should be < ± 0.1 °C with no smoothing, or < ± 0.05 °C with Weak smoothing.
5. Profile run: With the oven at ambient, start a profile that ramps to 200 °C at 10 °C/min. Verify the FB advances Ramp1 → Soak1 → Ramp2 correctly and that the PID output modulates the heater.
6. Fault injection: Yank the channel 0 probe while the oven is heating. Confirm the wire-break diagnostic trips and the PID output drops to 0 % (fail-safe).

Sourcing the Module in Canada

For Canada-based builds, Siemens partners include:

• Distributors: WESCO, Guillevin Automation, Wesco / CED, Eaton Electrical, Atlas Apex, EBH Elektronik.
• OEM direct: Siemens Canada Limited — order via the local rep.
• Online: PLCDirect (not affiliated), a UBC affiliated surplus reseller, and eBay for development spares (avoid for production builds).

Lead time for the 6ES7231-5QF32-0XB0 is typically 1–3 weeks from a Canadian distributor. Always confirm the firmware version printed on the module label matches the version your TIA Portal project supports.