Reading PT100 RTD on Siemens ET 200SP Analog Input in TIA Portal

Overview

This procedure walks through commissioning a Pt100 resistance temperature detector (RTD) on a Siemens SIMATIC ET 200SP distributed I/O station using the AI 4×RTD/TC 2-/3-/4-wire HF analog input module (order number 6ES7134-6JD00-0CA1) inside TIA Portal. The workflow covers hardware wiring, hardware catalog configuration, channel parameter assignment, peripheral addressing of the raw process value, and the integer-to-temperature scaling required to display engineering units.

The Pt100 is the most common platinum RTD used in industrial temperature measurement. At 0 °C its nominal resistance is 100 Ω, and resistance increases with temperature according to the IEC 60751 characteristic curve. The Siemens analog input module converts this resistance into a digital integer value that the CPU program reads, scales, and uses for control or visualization.

PT100 RTD Fundamentals

A Pt100 RTD is a passive temperature sensor based on a platinum resistance element. The "100" designates 100 Ω nominal resistance at 0 °C. Two internationally standardized nominal curves are used:

Approximate resistance values across the measurement range (IEC 60751):

The Callendar-Van Dusen equation describes the relationship:

For T ≥ 0 °C: R(T) = R₀ · (1 + A·T + B·T²)
For T < 0 °C: R(T) = R₀ · (1 + A·T + B·T² + C·(T-100)·T³)
A = 3.9083 × 10⁻³ °C⁻¹
B = -5.775 × 10⁻⁷ °C⁻²
C = -4.183 × 10⁻¹² °C⁻⁴

Because the resistance is small (≈ 100 Ω) and the wire resistance adds directly to it, the wiring topology (2-wire / 3-wire / 4-wire) directly determines measurement accuracy.

Prerequisites

• SIMATIC ET 200SP CPU or IM (interface module), with the AI 4×RTD/TC module already inserted on a suitable BaseUnit type A0 (light) or A1 (dark).
• Order number 6ES7134-6JD00-0CA1 ET 200SP AI 4×RTD/TC 2-/3-/4-wire HF module.
• Appropriate BaseUnit (BU type A0 or A1, depending on terminal block preference), with shield clamp or shield terminal accessory for sensor cable.
• TIA Portal V15.1 or later installed, with HSP (Hardware Support Package) for the AI 4×RTD/TC HF module loaded.
• Configured ET 200SP station with the CPU in the device tree.
• Pt100 sensor with known wiring topology (2-wire / 3-wire / 4-wire).
• Shielded, twisted-pair sensor cable (recommended for industrial environments).

Module Specifications — 6ES7134-6JD00-0CA1

Source: SIMATIC ET 200SP Analog Input Module AI 4×RTD/TC 2-/3-/4-wire HF (6ES7134-6JD00-0CA1) equipment manual, available on the Siemens Industry Online Support portal at support.industry.siemens.com.

Wiring Topologies — 2-Wire, 3-Wire, and 4-Wire

The terminal assignments on the BaseUnit depend on the wiring topology. The diagram below shows the channel pin assignments for the AI 4×RTD/TC HF module. Channels are numbered CH0 through CH3; each occupies four terminals (Ic+, Ic-, Uco+, Uco-).

2-Wire Connection

The simplest topology: only two conductors connect the Pt100 to the module. Lead resistance is measured directly and adds to the Pt100 resistance, producing a positive temperature error of approximately 0.385 °C per Ω of total lead resistance. Use 2-wire only when the sensor is mounted directly on the terminal block (≤ 1 m of cable) and absolute accuracy is not critical.

3-Wire Connection (Industry Default)

Three conductors reach the sensor; the module measures the resistance of one lead and subtracts it from the measurement. This cancels lead resistance assuming all three leads have the same resistance (use identical conductor cross-section). 3-wire is the most common industrial compromise between cost and accuracy.

4-Wire Connection (Highest Accuracy)

Two force leads carry the excitation current; two sense leads measure the voltage directly at the Pt100 element. Lead resistance in the force loop does not affect the measurement. Use 4-wire for high-accuracy applications or when the sensor is far from the module (>10 m).

TIA Portal Hardware Configuration

1. Open the TIA Portal project containing your ET 200SP CPU.
2. In the project tree, expand the ET 200SP station and locate the AI module slot where the 6ES7134-6JD00-0CA1 is mounted.
3. If the module is missing from the device view, right-click the slot and choose "Change Device" or drag the module from the hardware catalog.
4. Search the catalog under:
   SIMATIC ET 200SP > Analog modules > AI 4×RTD/TC 2-/3-/4-wire HF (6ES7134-6JD00-0CA1)
5. Drag the module onto the empty slot.
6. Compile the hardware configuration: Right-click the ET 200SP station > Compile > Hardware and software (only).
7. Download the configuration to the CPU: select the ET 200SP station > Online > Download to device.

After successful download, the module's status LED should display solid green and the channel status LEDs (one per channel) should be off when no fault is present.

Module Parameter Configuration

Select the AI 4×RTD/TC module in the device view and open the Properties inspector. Configure each channel independently:

The configured temperature range determines how the module interprets the resistance and which integer it outputs. With "Pt100 standard" enabled and unit °C, the module returns engineering-unit-ready scaled integers.

Reading the Process Value — Peripheral Addressing

Once downloaded, the module occupies four input words in the I/O address space, one per channel. The default address assignment depends on the slot. For a typical ET 200SP station with the AI module at slot 4, channel 0 returns to address %IW256, channel 1 to %IW258, channel 2 to %IW260, and channel 3 to %IW262. The exact range is shown in the device view under Properties > I/O addresses.

To force an immediate, up-to-date read from the module — bypassing any optimized or buffered access — read the channel using the peripheral (P) access suffix:

// Read Pt100 channel 0 as a direct peripheral value
// IW256 is the configured input word for channel 0
iRaw   : INT;            // raw integer from input card
iTemp  : INT;            // temperature in tenths of a degree C
rTemp  : REAL;           // temperature in °C

iRaw  := %IW256:P;       // peripheral read — accesses module directly
iTemp := iRaw;           // module already outputs 0.1 °C / digit
rTemp := INT_TO_REAL(iTemp) / 10.0;

Scaling the RTD Value

The AI 4×RTD/TC HF module performs linearization internally and returns an integer that already represents the temperature in the configured unit, scaled by a factor of 10 (2-decimal display) or 100 (3-decimal display) depending on the resolution you selected in the module parameters.

The full integer range maps as follows:

• Underrange: -32768 (8000₁₆) — process value below physical minimum
• Overrange: +32767 (7FFF₁₆) — process value above physical maximum, or channel disabled, or wire break
• Wire break / sensor fault: 7FFF₁₆ (with diagnostic interrupt if enabled)

STEP 7 SCL Example (S7-1200 / S7-1500)

// Function block "FB_Pt100_Read" — reads one Pt100 channel
// and returns engineering-unit temperature + status
FUNCTION_BLOCK "FB_Pt100_Read"
VAR_INPUT
    iwRawAddress  : POINTER TO INT;    // e.g. P#%IW256
END_VAR
VAR_OUTPUT
    rTemperature  : REAL;              // °C
    bFault        : BOOL;              // TRUE if sensor fault or out of range
END_VAR
VAR
    iRaw          : INT;
END_VAR
BEGIN
    iRaw         := iwRawAddress^;
    rTemperature := INT_TO_REAL(iRaw) / 10.0;
    bFault       := (iRaw = 16#7FFF) OR (iRaw = 16#8000);
END_FUNCTION_BLOCK

Ladder Logic Example (LAD)

// Network 1: Read raw and divide
       %IW256:P --------------[ MOVE ]-------------- MW100   // raw INT
       MW100   --------------[ DIV_I ]------------- MW102   // /10 → 0.1 °C step
// (Note: DIV_I returns truncated integer; for engineering units
//  load into REAL first, then DIV_R by 10.0)

Verification & Commissioning

1. Online diagnostics: Open the AI module in the device view, right-click and select "Online & Diagnostics". The "Channel diagnostics" tab shows wire-break, overflow/underflow, and short-circuit events per channel.
2. Watch table: Create a watch table and force the input address in "monitor" mode (not force):
   %IW256 — verify the integer reflects ambient temperature × 10 (e.g., 235 = 23.5 °C).
3. Known-resistance test: Disconnect the sensor and connect a precision decade resistance box (e.g., 100.00 Ω for 0 °C, 138.51 Ω for 100 °C). Verify the module reading against the IEC 60751 table.
4. Short-circuit test: Short terminals 1 and 2 of a channel. The module should raise a wire-break / short-circuit diagnostic and return 7FFF₁₆. Disconnect and confirm diagnostic clears.
5. Value status: Enable the value status (QI) bit. Read %IW256 in the watch table; bit 15 = 0 indicates good quality, bit 15 = 1 indicates a bad value.
6. Integration-time trade-off: If readings fluctuate due to mains noise, switch to 16.67 ms or 100 ms integration time. Expect a slower update interval in exchange for noise rejection.

Troubleshooting Matrix

Edge Cases & Field-Proven Notes

• Self-heating: The excitation current flowing through the Pt100 produces ≈ 0.5 mW dissipation at 100 Ω. For low-thermal-mass sensors in air this can raise the element by 0.1–0.3 K. The HF variant uses a pulsed excitation to minimize self-heating.
• Galvanic isolation: The AI 4×RTD/TC HF module provides channel-to-channel and channel-to-bus galvanic isolation, allowing sensors at different potentials to be connected without ground loops.
• Cold-junction compensation (CJC): Required only when measuring thermocouples, not RTDs. The module contains an internal reference junction temperature sensor used when a TC channel is enabled.
• Mixed RTD and TC channels: Each of the four channels can be configured independently as RTD or TC. Channel 0 can be a Pt100 while channel 1 is a Type K thermocouple.
• Resolution trade-off: Selecting 16-bit resolution doubles conversion time. For PID loops with a 100 ms period, 16-bit + 100 ms integration gives the best stability. For fast monitoring, 15-bit + 20 ms is sufficient.
• Calibration drift: Industrial Pt100 sensors drift on the order of 0.1 K per year at 200 °C. Schedule annual verification with a dry-block calibrator for critical process loops.
• Long cable compensation: For 3-wire sensors over 50 m, the lead resistance mismatch (due to different conductor lengths) introduces a small but measurable error. Use 4-wire topology for cable runs > 50 m.
• Module replacement: When replacing a 6ES7134-6JD00-0CA1, the device name and IP parameters are stored on the BaseUnit's coding element only if the original BU type supports it. Without coding, the module is recognized as new and the CPU may report an IO fault.

Companion Modules and System Integration

For multi-channel temperature acquisition at higher density, the following ET 200SP modules are common companions:

For HMI display, configure a tag in WinCC Unified or TIA Portal HMI pointing at the same peripheral address and set the linear scaling to /10.0 (or /100.0 for high resolution). Most WinCC faceplates include a built-in temperature scaling block.

FAQ