Siemens 6ES7134-6JD00-0CA1 RTD Scaling and Wire Break Detection

Siemens 6ES7134-6JD00-0CA1 RTD Scaling and Wire Break Detection Reference

Field reference for the SIMATIC ET 200SP AI 4xRTD/TC HF analog input module. Covers integer-to-engineering-unit scaling, decimal-place resolution, the overrange/overflow boundary at 32511, wire break detection at the excitation current and channel-diagnostic level, and the S7 status-byte evaluation in TIA Portal. Targets commissioning engineers migrating 4-20 mA scaling logic to RTD channels or debugging existing Pt100/Pt1000 installations.

1. Module Identification and Order Data

The 6ES7134-6JD00-0CA1 is the high-feature RTD/thermocouple analog input of the SIMATIC ET 200SP distributed I/O family. It is a 4-channel module occupying one BU (BaseUnit) slot and supports Pt, Ni, and Cu RTDs plus type B, E, J, K, N, R, S, T thermocouples in a single firmware image. Module-level key data from the Siemens product page and manual entry ID 59753600:

The base unit determines the connection system (BU15-P16+A0+2B for spring terminals, BU15-P16+A0+2D for push-in). Always check the BU type when commissioning because the wiring diagram and shield contact method differ.

2. Wiring Modes and Sensor Cabling

Each channel can be configured for 2-wire, 3-wire, or 4-wire connection. The wiring mode is set in TIA Portal under Device view → AI 4xRTD/TC HF → Channel n → Sensor → Connection type and must match the physical sensor harness or the linearization is invalidated.

3. RTD Scaling Fundamentals

An RTD module returns the measured temperature as a signed 16-bit integer in the process image (input word IW). To convert to engineering units (°C, °F, K) the integer must be scaled. The scaling factor is dictated by the channel resolution chosen in HW config, not by the RTD type.

The 6ES7134-6JD00-0CA1 supports the following resolution presets, configurable in TIA Portal under Channel → Measurement → Resolution / Integration time:

General formula:

Temperature_EU = INT_ChannelValue / ScaleDivisor

Where the divisor is the engineering-unit resolution: 10 for one decimal place, 100 for two decimal places, 20 for 0.05 °C resolution, etc. The scaling is purely integer-to-real arithmetic; no additional linearization of the Pt curve is required because the module applies the IEC 60751 polynomial internally before the integer is written to the process image.

3.1 Why divide by 10.0 and not 10

Always force a real (REAL / LREAL) divisor. Dividing two INTs in S7-1500 STL or SCL performs integer division and truncates, so 2534 / 10 = 253 (3.4 lost). Use 2534 / 10.0 or explicit INT_TO_REAL(2534) / 10.0 to obtain 253.4. A common field bug is a SCALE block configured with integer IN/OUT that drops the least-significant digit silently.

4. Resolution and Decimal Place Configuration

Resolution is a per-channel setting. The trade-off is integration time vs. noise: higher resolution requires longer ADC integration, which slows the channel update. Per the manual chapters 4.3 and 4.4 referenced in the Siemens support entry ID 59753600, the typical presets are:

If you change the resolution after deployment, the scaling divisor must be updated in the program at the same time. Failing to do so produces a constant offset (factor of 10 if 0.01 °C is loaded into a divide-by-10 block) that is easy to miss during commissioning.

5. Value Range: Nominal, Overrange, Overflow, Underflow

The 6ES7134-6JD00-0CA1 writes the measured value into a 16-bit signed word (INT, range -32768 to +32767). The Siemens convention reserves special values for fault signalling. These values are returned regardless of the configured resolution - they are the underlying channel-value integers before scaling.

As confirmed in the discussion thread, the overrange boundary is 32511 at the input value. Any value at or above 32512 indicates an overflow condition. Any value at or below -32512 (effectively -32768 as the lower sentinel) indicates an underflow. These sentinels are valid regardless of resolution preset - they exist in the underlying channel-value integer space.

6. Wire Break Detection Mechanism

Wire break is a critical safety and process-integrity diagnostic. On the 6ES7134-6JD00-0CA1 it can be implemented at two levels: hardware (channel-diagnostic) and software (raw-value sentinel).

6.1 Hardware-level detection (excitation current path)

At the ADC level, RTD wire break is detected by the excitation current sources (IDAC) integrated into the delta-sigma ADC and by monitoring the reference voltage (VREF). When a sense or current lead opens, the differential voltage seen by the ADC exceeds the normal PTAT (proportional-to-absolute-temperature) range, and the converter flags an out-of-range condition. The principle is documented in the Texas Instruments application brief RTD Wire-Break Detection Using Precision Delta-Sigma ADCs (SBAA483): the IDAC continues to source current into a broken lead, the sense input saturates at the ADC reference rails, and the digital filter reports a code corresponding to either the upper or lower sentinel (overflow / underflow). The Siemens module's internal ASIC applies an equivalent technique and exposes the result as a per-channel wire-break diagnostic bit.

6.2 Enabling the diagnostic in TIA Portal

To get a hardware-level wire-break interrupt:

1. Open the ET 200SP station in the device view.
2. Select the AI 4xRTD/TC HF module.
3. Open Properties → Diagnostics.
4. Enable Wire break for the relevant channel(s).
5. Optionally enable Diagnostic interrupt at module level so an OB82 is triggered when the bit changes state.

Once enabled, the channel status byte (assigned automatically to the channel-quality / QI byte in the process image) reports the wire-break state. In the S7 program this is read from the value-status (quality information) byte associated with the input word.

6.3 Software sentinel detection (fallback)

For installations where the diagnostic interrupt is not enabled (older CPU firmware, slot-routing constraints, or third-party master), the raw-value sentinel method still works. The condition posted in the source:

#ANLi < -865   // underflow sentinel region (negative-side wire break, lead open)

#ANLi > 28513  // overflow sentinel region (positive-side wire break, lead open)

These thresholds are application-specific and assume a particular nominal range and resolution; they are NOT the standard S7 sentinels of 32512 / -32768. Always derive thresholds from the configured sensor type and resolution - for example, on a Pt100 with 0.1 °C resolution (-200.0 to +850.0 °C, scaled to -2000 to +8500), 28513 ≈ 2851.3 °C is well above the physical sensor limit and can be treated as overflow.

The recommended, unambiguous sentinel check is:

IF #RawAI = 16#7FFF OR #RawAI >= 32512 THEN

  #WireBreak := TRUE;

END_IF;

IF #RawAI = 16#8000 THEN

  #WireBreak := TRUE;

END_IF;

7. Diagnostic Interrupt Structure (OB82)

When a diagnostic event is enabled and fires, the CPU enters OB82. The following local-byte layout is typical for the AI 4xRTD/TC HF module (per the manual chapter 4.2, entry ID 59753600):

The standard Siemens pattern is to read the channel vector and trigger a fault routine per channel. The exact offset of the wire-break flag depends on the GSD file revision and TIA Portal version; always validate against the offline-online compare and the diagnostic buffer after the first intentional fault.

7.1 Reading the value-status (QI) byte directly

For cyclic (non-interrupt) wire-break detection, assign the value-status byte in the channel's I/O address configuration. The byte returned has a bit set when the channel is faulted:

Sample ladder check using the QI byte at address IB n:

A    IB[n]      // non-zero = at least one channel faulted

JCN  NOFLT

S    M 100.0    // aggregate wire-break flag

8. TIA Portal Programming Example (SCL)

Reusable FB for scaling, sentinel check, and wire-break flag. The instance DB holds the raw word, scaled REAL, and three boolean flags. Wire to a call in OB1 with the channel IW address and the QI byte IB address passed as inputs.

FUNCTION_BLOCK "FB_RTD_Scale_WB"
VAR_INPUT
    iRawAI     : INT;   // channel value, e.g. IW 0
    iQIByte     : BYTE;  // value-status byte, e.g. IB 4
    iDivisor    : REAL := 10.0;
END_VAR
VAR_OUTPUT
    oTemperature : REAL;
    oOverflow   : BOOL;
    oUnderflow  : BOOL;
    oWireBreak  : BOOL;
    oValid     : BOOL;
END_VAR
BEGIN
    // --- Wire break from value-status (QI) bit ---
    // QI bit per channel: 0 = OK, 1 = fault
    oWireBreak := (iQIByte <> 16#00);

    // --- Overflow sentinel: 32512..32767 ---
    IF iRawAI >= 32512 THEN
        oOverflow := TRUE;
        oValid   := FALSE;
    ELSIF iRawAI = -32768 THEN
        // --- Underflow sentinel: -32768 ---
        oUnderflow := TRUE;
        oValid    := FALSE;
    ELSE
        oOverflow := FALSE;
        oUnderflow := FALSE;
        oValid    := TRUE;
    END_IF;

    // --- Scale only when value is valid ---
    IF oValid THEN
        oTemperature := INT_TO_REAL(iRawAI) / iDivisor;
    ELSE
        oTemperature := 0.0;
    END_IF;
END_FUNCTION_BLOCK

Usage in OB1:

"iDB_RTD".iRawAI   := "AI_RTD_HF".Channel0.IW;
"iDB_RTD".iQIByte   := "AI_RTD_HF".Channel0.ValueStatus;
"iDB_RTD".iDivisor   := 10.0;       // 0.1 °C resolution
"FB_RTD_Scale_WB"(iDB := "iDB_RTD");

IF "iDB_RTD".oWireBreak OR "iDB_RTD".oOverflow OR "iDB_RTD".oUnderflow THEN
    // trip fault, latch into HMI tag, write to alarm log
END_IF;

9. Comparison: RTD vs 4-20 mA AI Scaling

Many control engineers migrate 4-20 mA scaling logic to RTD channels. The two are similar in mechanics but differ in three important ways.

The two wire-break conditions are NOT the same: in 4-20 mA, a broken wire drops the loop current to zero and the raw value falls below the 4 mA floor (underflow). In RTD, a broken lead typically saturates the ADC high (overflow) because the IDAC sources current into an open circuit and the differential sense voltage pegs to VREF. The fault polarity can therefore be inverted between the two sensor types, and a one-size-fits-all 4 mA floor check will not detect an RTD wire break.

10. Troubleshooting Matrix

11. Field Commissioning Checklist

1. Verify the base unit (BU type) matches the wiring plan; mismatched BU types are a frequent cause of intermittent channel faults.
2. In TIA Portal, set the sensor type, wiring mode, resolution, and diagnostic enables per channel before download.
3. Compile, download, and go online. Confirm the device view shows the module with the expected article number and firmware.
4. Force each channel to a known resistance (precision decade box or Pt100 simulator) at three points: 0 °C, mid-scale, and overrange. Verify the integer matches expected Raw = T °C × 10 (or × 100 for 0.01 °C resolution).
5. Open one RTD lead and confirm the wire-break flag (QI bit or OB82) fires within the configured diagnostic time.
6. Short the sense leads at the field terminals and confirm the underflow sentinel (-32768) is reported.
7. Log all three fault scenarios to the alarm buffer and HMI before sign-off.

12. Common Mistakes

• Scaling 32512 with the engineering-unit formula. The sentinel must be intercepted before scaling.
• Using the 4-20 mA wire-break threshold on an RTD channel. The two conditions drive the ADC in opposite directions.
• Forgetting the trailing ".0" on the divisor. Integer division truncates and silently loses precision.
• Leaving the QI (value-status) byte disabled in HW config, so the wire-break diagnostic is only visible in OB82 but not in cyclic read.
• Configuring 2-wire mode in HW config but wiring a 3-wire probe (or vice versa) - linearization is invalid and the offset is roughly proportional to lead resistance.

FAQ