Wiring 3-Wire RTD to Siemens 6ES7134-4NB51-0AB0 ET 200S Module

Module Overview and Identification

The Siemens ET 200S distributed I/O system includes two analog input electronic modules designed for resistance temperature detectors (RTDs): the 6ES7134-4NB51-0AB0 (2AI RTD HF, High Feature) and the 6ES7134-4JB51-0AB0 (2AI RTD ST, Standard). Both modules occupy two slots in the ET 200S terminal block and provide two RTD input channels per module. They support 2-wire, 3-wire, and 4-wire RTD connection methods, configurable per channel in the engineering software.

The HF module is the recommended choice when higher resolution, additional sensor types (Cu10/Cu50/Cu100), or full channel-level diagnostics are required. The ST module is suitable for cost-sensitive Pt/Ni applications without advanced diagnostics.

Prerequisites

Before wiring, confirm the following items are available and compatible:

• ET 200S interface module (IM 151-1, IM 151-3 PN, IM 151-3 PN HF, or IM 151-7 CPU) with current firmware
• Appropriate TM-E terminal block matching the RTD module (e.g., TM-E15C26-01 for screw-type, TM-E15N26-01 for spring-type)
• STEP 7 V5.5 + SPx with HSP or TIA Portal V14 SP1 or later (TIA V16+ recommended for current GSDML/GSD support)
• Current GSD file: SIEM80B7.GSE (for PROFIBUS) or GSDML-Vx.x-Siemens-ET200S-xxxxxx.xml (for PROFINET)
• 3-wire RTD probe (Pt100/Pt1000/Ni1000 etc.) with two same-colored leads and one different-colored lead
• Shielded twisted-pair cable for noise-sensitive installations

Terminal Assignments and Pinout

Both 2AI RTD modules share the same 8-terminal pinout for the two measurement channels. The terminals are physically located on the TM-E terminal block. Reference the official manual at ET 200S 2AI RTD HF manual (PDF) for the schematic.

The module injects a precisely controlled current (typically 1 mA) through the I+/I- pair and measures the voltage developed across the resistor via the M+/M- pair. By comparing the two M-channel measurements, the module compensates for lead-wire resistance in 3-wire and 4-wire configurations.

Three-Wire RTD Connection Method

A 3-wire RTD probe contains a single sensing element with three leads: two leads are connected to the same side of the element (commonly red) and one lead is connected to the opposite side (commonly white). This arrangement allows the RTD input module to cancel the lead-wire resistance by measuring the voltage drop in both excitation leads and subtracting it from the element voltage.

Wiring Procedure for Channel 0

1. Connect the two same-colored leads of the RTD (the two reds, or the two leads that tie to the same side of the element) to terminals 1 (I0+) and 3 (M0+).
2. Connect the single different-colored lead (the one tied to the other side of the sensing element) to terminal 4 (M0-).
3. Leave terminal 2 (I0-) open (unconnected). The module's internal excitation current returns through the M0+ lead path for 3-wire mode.
4. Connect the cable shield to the shield bar of the TM-E terminal block (if present) or to a dedicated shield grounding clamp near the module. Ground the shield at one end only (typically the cabinet end) to avoid ground loops.

SVG Wiring Diagram (Channel 0, 3-Wire RTD)

The white lead routes to M0- (terminal 4), the two red leads route to I0+ (terminal 1) and M0+ (terminal 3). I0- (terminal 2) is intentionally left unconnected.

Converting a Four-Wire RTD to Three-Wire

If a 4-wire RTD is on hand but only 3-wire is supported (or desired due to existing cabling), the conversion is straightforward: short the two leads on one side of the sensing element at the RTD end, or at the terminal block. The 4-wire element has two current-carrying leads and two voltage-sensing leads. By tying the two leads on the same side together, the element effectively becomes 3-wire:

1. Identify the two leads tied to the same side of the Pt/Ni element (refer to the RTD manufacturer's data sheet for the element schematic).
2. Twist and solder or Wago-connect these two leads together. Insulate the joint with heat-shrink tubing.
3. Wire the shorted pair to terminal 1 (I0+); the third (single) lead to terminal 3 (M0+); the fourth (single) lead to terminal 4 (M0-). Terminal 2 (I0-) remains open.
4. Configure the channel in the engineering software for "3-wire" measurement.

Configuration in TIA Portal / STEP 7

The wiring method is selected in the hardware configuration, not by physical jumpers. The choice tells the module how to interpret the I+ and M+ measurements.

STEP 7 V5.5 (HSP-based) Procedure

1. Open HW Config and place the 6ES7134-4NB51-0AB0 or 6ES7134-4JB51-0AB0 from the ET 200S catalog onto the slot.
2. Double-click the module to open the properties dialog. Switch to the "Inputs" tab.
3. Select Channel 0. Under "Measurement type," select RTD (or "Thermal resistor").
4. Under "RTD type," select the probe: Pt100, Pt1000, Ni100, Ni1000, LG-Ni1000, Cu10 (HF only), or Cu50/Cu100 (HF only).
5. Under "Connection," select 3-wire (German: "Dreileiter").
6. Under "Temperature unit," select Degrees Celsius (default) or Fahrenheit/Kelvin as required.
7. Set "Temperature coefficient" to Pt 0.00385 (European standard) or 0.00391 (US old standard) or 0.00392 as required by the probe.
8. Enable "Diagnostics" if wire break / short-circuit alarms are required (HF module only for full diagnostic support).
9. Repeat for Channel 1 if used.
10. Compile and download the hardware configuration to the IM 151 interface module.

TIA Portal V16 / V17 Procedure

1. In the project tree, expand the ET 200S station → device view. Select the 2AI RTD module.
2. In the inspector window, click "Properties" → "Module parameters" → expand the channel tree.
3. For each channel, set: Measurement = RTD; RTD type = e.g. Pt100 standard; Connection = 3-wire connection.
4. Optional: Smoothing (averaging) — set 1, 4, 16, or 64 cycles for noise reduction on slow-changing temperatures.
5. Under "Diagnostics," enable "Wire break" and "Short circuit" interrupts if your HMI/SCADA needs to alarm on probe failure.
6. Save, compile, and download to the ET 200S head module.

Configuration Parameters Reference

Verification and Commissioning

After wiring and configuration, perform the following checks before placing the system in production:

1. Online diagnostic test: In TIA Portal, go online with the ET 200S station. Right-click the RTD module → "Online & diagnostics" → check "Channel 0 / Channel 1" status. All enabled channels should show "OK."
2. Value plausibility check: Read the raw value in the I/O image (e.g., IW 256 for slot 1, channel 0). Compare with a hand-held calibrator injecting a known resistance (e.g., 100.00 Ω → 0.00 °C for Pt100 at 0 °C). Acceptable deviation: typically ±0.5 °C for HF module at 25 °C ambient.
3. Wire break simulation: Disconnect one of the RTD leads at the terminal block. Within 2–5 scan cycles the module should raise a "wire break" diagnostic and substitute 7FFFh (overflow) in the process image. Reconnect and confirm the diagnostic clears.
4. Short circuit simulation: Briefly short M0+ to M0-. The module should raise a "short circuit" diagnostic (HF module only; ST module may not detect).
5. Temperature range verification: Apply a calibrator at the lower and upper range limits (e.g., -50 °C and +200 °C for Pt100) to confirm the linearization is correct.

Diagnostics and Error Handling

The HF module (6ES7134-4NB51-0AB0) provides the following diagnostic events, visible in the diagnostic buffer of the IM 151 / CPU:

Troubleshooting Matrix

Field-Commissioning Best Practices

• Use a 4-wire RTD wherever the lead run exceeds 10 m, even if the application can tolerate the small error of 3-wire. The 4-wire mode eliminates lead-resistance error entirely.
• Always torque TM-E screw terminals to 0.6–0.8 N·m. Loose terminals are the single most common cause of "flapping" wire-break diagnostics on these modules.
• Where both channels are used for the same physical point (redundant sensors), configure channels independently to use 3-wire mode but pull each from its own cable run for maximum diagnostics.
• When migrating from a previous ET 200S analog input card (e.g., 6ES7134-4FB51-0AB0) to the HF or ST module, re-verify the temperature coefficient and wiring mode — older GSD versions may default to "4-wire" silently.
• Document the configuration: terminal assignments, probe serial number, calibration date, and the S7 tag name in a commissioning sheet for the asset record.

Calculating Expected Lead-Resistance Error in 3-Wire Mode

The 3-wire connection does not perfectly cancel lead resistance because both leads are assumed identical. The residual error is:

ΔT (°C) ≈ (R_lead_asymmetry / R_probe) × T_range

For a Pt100 with 10 Ω per lead and 1 Ω asymmetry, the error is roughly (1 / 100) × 850 = 8.5 °C worst case across the full range. In practice, with matched lead lengths, the asymmetry is < 0.1 Ω and the error is < 1 °C — acceptable for most process applications but not for laboratory-grade measurement.

FAQ