Wiring 3-Wire Pt100 RTD to Siemens SM334 6ES7334-0KE00-0AB0

SM334 6ES7 334-0KE00-0AB0 Module Overview

The SM334 6ES7 334-0KE00-0AB0 is a Siemens SIMATIC S7-300 analog I/O module that combines 4 analog inputs (AI) and 2 analog outputs (AO) on a single 20-pin front connector. The "KE" suffix designates the RTD-capable variant, distinguishing it from voltage/current-only SM334 versions. The module delivers 12-bit resolution across its inputs and outputs and supports Pt100 resistance temperature detectors in 2-wire, 3-wire, and 4-wire configurations. Reference the SIMATIC S7-300 SM334 Manual for the full hardware data sheet.

Unlike dedicated SM331 (8 AI) and SM332 (4 AO) modules, the SM334 is a compact hybrid suited to applications where a small number of analog points are needed without occupying two rack slots. The trade-off is reduced resolution (12-bit vs 15/16-bit) and reduced accuracy. For high-precision applications that require 4-wire RTD with extended diagnostics, prefer an SM331 with 6ES7 331-7PF01-0AB0 (8 AI, 16-bit, RTD/TC).

The module ships with no sensor-specific configuration baked in; each channel must be parameterized in the engineering software (STEP 7 or TIA Portal) for the connected sensor type. A common mistake is leaving the default "Voltage" measurement type after inserting the module, which causes the input to interpret the Pt100 resistance as an out-of-range voltage and triggers a 0x7FFF (overflow) reading. This single misconfiguration accounts for the majority of field reports of "the Pt100 reads full scale" — including the case described in this article.

Pt100 RTD Measurement Fundamentals

A Pt100 RTD is a platinum resistance thermometer with a nominal resistance of 100 Ω at 0 °C and a temperature coefficient of approximately 0.385 Ω/°C (3.85 × 10⁻³ /K). The resistance/temperature relationship follows the IEC 60751 Callendar-Van Dusen equation:

For T ≥ 0 °C:
  R(T) = R0 × (1 + A·T + B·T²)

For -200 °C ≤ T < 0 °C:
  R(T) = R0 × (1 + A·T + B·T² + C·(T - 100)·T³)

Where:
  R0  = 100 Ω at 0 °C
  A   = 3.9083 × 10⁻³ °C⁻¹
  B   = -5.775 × 10⁻⁷ °C⁻²
  C   = -4.183 × 10⁻¹² °C⁻⁴

Common reference resistance values for Pt100 (per IEC 60751):

For copper conductors, lead resistance is approximately 0.0175 Ω per metre per conductor for 1.0 mm² cross-section (or about 0.39 Ω per 100 ft for AWG 18). A 10-metre run with two conductors adds ~0.35 Ω to the measured resistance, which at 0.385 Ω/°C represents ~0.9 °C measurement error if left uncompensated. This error grows linearly with cable length and is the primary motivator for using 3-wire or 4-wire RTD configurations instead of 2-wire. For higher gauge conductors, divide by the relative cross-section ratio.

3-Wire Compensation Theory and Lead Resistance

The 3-wire RTD configuration uses two identical conductor runs to one side of the Pt100 element and a single return conductor. The SM334 exploits this by sourcing a constant excitation current (typically 1.0 mA to 1.6 mA) through one M+ terminal, returning the current through M-, and using the second M+ (sense) terminal to measure the voltage directly at the Pt100 element. Because the same excitation current flows through both M+ leads, the voltage drop across the lead resistance is identical in both conductors, allowing the module's internal ADC bridge to subtract this common-mode drop and yield the true Pt100 resistance.

The compensation principle depends on three critical assumptions:

1. Both M+ lead resistances are equal (matched conductors, identical length, gauge, and routing temperature).
2. The excitation current is stable and small (typically 1 mA to minimise self-heating; sensor dissipation must remain under 1 mW to avoid measurement drift).
3. The M- lead is not compensated by the 3-wire method; its resistance adds a small residual error proportional to cable length.

With a 1 mA excitation current, the voltage across the Pt100 at 0 °C is 100 mV. The SM334 uses a dedicated RTD measurement circuit with internal gain to amplify this small Pt100 signal across a range appropriate for 12-bit conversion. Configuring the channel in voltage mode by mistake routes the raw voltage to a gain stage sized for ±10 V, where 100 mV is indistinguishable from noise — producing either a near-zero reading or a full-scale overflow. Always configure the channel explicitly for RTD mode in the engineering software before applying the sensor.

For typical industrial installations with shielded twisted-pair cable up to 50 m, 3-wire Pt100 achieves approximately ±0.5 °C accuracy on the 12-bit SM334. For higher accuracy, switch to a 16-bit SM331 module or use a 4-wire connection.

SM334 Pinout and Terminal Map for RTD Mode

The SM334 uses a single 20-pin front connector (6ES7 390-1AJ30-0AA0). For RTD measurement, only the first 13 pins are used; pins 14 through 20 are reserved for the analog outputs, the analog ground reference (ManA), and the optional 24 V sensor supply. The pinout is reproduced from the official Siemens hardware data sheet:

The three pins per RTD channel (M+, M-, M+ sense) follow the same pattern across all four channels. Channel 0 occupies pins 2-4, Channel 1 pins 5-7, Channel 2 pins 8-10, Channel 3 pins 11-13. The sense terminal (4, 7, 10, 13) is the conductor that the module uses internally to measure the Pt100 voltage after subtracting the lead-resistance drop on the excitation conductor.

3-Wire Pt100 Wiring Procedure

The wiring pattern for a 3-wire Pt100 on Channel 0 (pins 2-4) follows this assignment, illustrated below by an inline topology diagram:

The two "matched pair" wires must be the conductors that connect to the same physical side of the Pt100 element internally. Most Pt100 sensors are manufactured with two wires of one colour (typically red) bonded to one side of the platinum film or wire-wound element, and one wire of a different colour (often white, or a different jacket colour) bonded to the opposite side. Confirm this from the sensor's data sheet before wiring — if the lone wire ends up on the wrong terminal, lead compensation will invert and add error instead of subtracting it.

1. Power down the S7-300 station. Remove the 24 V supply from the CPU/IM and any sensor supply feeding the SM334. Working on analog terminals with the system energised risks both electric shock and module damage.
2. Strip and crimp the Pt100 leads. Use wire ferrules (e.g., Weidmüller PZ 6/5 or Phoenix Contact AI 1.5 -8) sized for the conductor gauge. Recommended conductor: 0.5 to 1.5 mm² (AWG 20-16), shielded twisted-pair cable for runs longer than 3 m.
3. Open the front connector. Slide the SM334 front connector (6ES7 390-1AJ30-0AA0) out of the module. The connector has a removable terminal block that allows wiring without disturbing other channels.
4. Connect the matched pair. Insert the two same-colour Pt100 wires into terminals 2 and 4 (M0+ and M0+ sense). Tighten terminal screws to 0.5 N·m.
5. Connect the lone wire. Insert the lone-colour Pt100 wire into terminal 3 (M0-). Tighten to 0.5 N·m.
6. Connect the cable shield. For shielded cable, terminate the shield to a grounded shield bar (e.g., Weidmüller WFB or Phoenix Contact FB) located within 50 cm of the module. Ground one end only (typically the cabinet end); do not ground both ends to avoid ground-loop currents through the shield.
7. Verify no stray strands. Inspect each terminal for stray copper strands that could bridge adjacent pins.
8. Reinsert the front connector. Push the connector firmly into the SM334 module until the latch clicks. The connector is keyed against reverse insertion.
9. Restore 24 V power. Reapply the system supply and verify the SF (group fault) LED on the SM334 is OFF.

ManA Reference and Module Grounding

The ManA terminal (pin 18) is the analog ground reference for the SM334 module. It is internally connected to the module's M- rail for the input channels and to the AO M rail for the output channels. ManA must be referenced to a clean, low-noise ground point — typically the cabinet ground bar or the S7-300 mounting rail's protective earth (PE) connection.

For RTD measurements specifically, ManA is not part of the Pt100 sensing circuit and does not require a dedicated connection to the sensor cable shield. The shield termination is handled at the cabinet ground bar. Miswiring ManA to one of the M+ sense terminals has been reported in the field and causes an apparent short between the sense input and ground, which the module interprets as a Pt100 with near-zero resistance (underflow, 0x8001) or a wire break (0x8000). Confirm the source poster's concern about Mana is resolved by leaving pin 18 connected only to cabinet ground, separate from the sensor cable conductors.

Recommended grounding scheme for a Pt100 + SM334 installation:

• Shield: terminated to cabinet ground bar at the SM334 end only, using a short pigtail (≤ 50 mm) to minimise inductance.
• ManA (pin 18): connected to the cabinet ground bar with a dedicated 1.5 mm² conductor.
• Sensor housing: if the Pt100 is mounted in a metal thermowell or pipe, ensure the thermowell is bonded to plant ground at one point to avoid ground loops.
• Module PE: the SM334's bottom mounting screw should be tightened to engage the S7-300 rail's PE contact, which provides the high-frequency chassis bond.

If the Pt100 sensor is electrically isolated from ground (typical for sheath-type RTDs in plastic piping), no additional ground reference is needed at the sensor end. The SM334 can operate in "floating" mode, with ManA providing the single-ended reference for the differential input stage.

STEP 7 (Classic) Hardware Configuration

Hardware configuration in STEP 7 V5.x establishes the measurement type for each SM334 channel. The default after module insertion is "Voltage ±10 V" which produces a full-scale reading on any Pt100 input. The channel must be reconfigured for RTD measurement before the sensor will read correctly.

1. Open the SIMATIC Manager and load the S7-300 station project.
2. Double-click Hardware to launch HW Config.
3. Locate the SM334 in the rack view (slot matching the physical module).
4. Double-click the SM334 to open the module properties dialog.
5. Select the Inputs tab.
6. For Channel 0, set:
   • Measurement type: R (resistance) or RTD
   • Measuring range: Pt100 (standard, 385 ppm/K)
   • Wire connection: 3-wire
   • Integration time / Interference frequency: 60 Hz (50 ms) for North America, 50 Hz (60 ms) for Europe/Asia. The integration time filters mains-frequency noise and should match the local supply frequency.
7. Repeat for any additional RTD channels. Unused channels should be set to "Deactivated" to suppress unused-channel diagnostics.
8. Click OK to apply.
9. Save and compile (Station > Save and Compile).
10. Download the hardware configuration to the CPU (PLC > Download to Target Station).

The input word assignments follow the slot-based addressing scheme. For an SM334 in slot 4 of an S7-300, the analog inputs occupy IW 4 (Channel 0), IW 6 (Channel 1), IW 8 (Channel 2), and IW 10 (Channel 3). The analog outputs occupy QW 4 (Channel 0) and QW 6 (Channel 1). Confirm the actual addresses in HW Config under the module's input/output address list.

Diagnostic interrupts can be enabled in the same dialog. If enabled, the module will trigger an OB82 diagnostic interrupt on wire break or overflow, allowing the PLC program to react without polling the input word.

TIA Portal Hardware Configuration

For TIA Portal V15 or later, the procedure is similar but integrated into the Device View:

1. Open the project in TIA Portal.
2. Expand the S7-300 device in the Project Tree.
3. Double-click Device Configuration.
4. Select the SM334 module in the rack view.
5. In the Properties inspector, expand Analog inputs.
6. For Channel 0, configure:
   • Measurement type: RTD thermal resistor
   • Sensor: Pt100 (385 ppm/K, IEC 60751)
   • Connection type: 3-wire
   • Temperature unit: Celsius (°C)
   • Smoothing: None / Weak / Medium / Strong (Weak is typical for Pt100)
   • Interference frequency suppression: 50 Hz or 60 Hz
7. Under I/O addresses, note the input address (e.g., %IW64 for slot 4).
8. Save and compile the project.
9. Download to the CPU (Online > Download to device).

In the user program, the temperature value at the configured input address is a normalised integer scaled to the sensor range. For Pt100 with -200 °C to +850 °C range, the value 0 corresponds to -200 °C and 27648 corresponds to +850 °C. Conversion to engineering units:

// SCL: Convert raw RTD value to temperature in °C
#RawInput := "DB_Temperature".AI_Ch0;   // raw input from SM334
#Temp_C   := (INT_TO_REAL(#RawInput) / 27648.0) * 850.0 - 200.0;

// LADDER equivalent using NORM_X and SCALE_X:
//   NORM_X: EN=1, MIN=0, MAX=27648, VALUE=AI_Ch0, RETVAL=N0
//   SCALE_X: EN=1, MIN=-200.0, MAX=850.0, VALUE=N0, RETVAL=TempC

Always use the scaled value (e.g., NORM_X and SCALE_X blocks in STEP 7) for any further arithmetic, and clamp the output to a safe range to handle diagnostic states gracefully. Place a comparison of the raw value against 0x7FFF and 0x8000 before any scaling to detect fault states and prevent control loops from acting on a saturated reading.

Diagnostic Codes and "Full Scale" Analysis

The SM334 reports diagnostic information through two paths: the input word value itself (passive diagnostics) and optional diagnostic interrupts via OB82 (active diagnostics). The most common input word values and their meanings:

The "output was full scale" symptom reported by the original poster most commonly maps to 0x7FFF, which has four overlapping root causes that must be eliminated in sequence:

1. Channel not configured for RTD. Open HW Config (STEP 7) or Device Configuration (TIA Portal) and verify Channel 0 measurement type is "RTD" or "R (resistance)" with "3-wire" connection selected. The default voltage mode will read 0x7FFF on any resistance input.
2. Wire break in Pt100 lead. Disconnect the Pt100 from the module and measure resistance between each pair of leads. The two matched-pair wires should show close to zero resistance between them (both connect to the same internal node). The lone wire should show ~107.79 Ω to each matched-pair wire at 20 °C. If the resistance to one of the matched wires is open (OL), the sensor element is broken.
3. Loose terminal screw. Re-torque each terminal screw to 0.5 N·m. Even slight loosening on analog terminals can cause intermittent open-circuit diagnostics.
4. Module input damaged. If the SM334 has been subjected to overvoltage on the inputs (e.g., by connecting a 24 V supply to a Pt100 terminal), the input protection diodes may have failed. Test by disconnecting the sensor and configuring the channel for voltage mode: the input should read approximately 0 mV with no sensor connected. A reading of ±5 V or random noise indicates damaged input.

Verification and Commissioning Procedure

After wiring and configuration, perform the following verification before placing the system in service. Each step builds on the previous one and isolates a specific failure mode.

1. Visual inspection. Verify all three terminals (2, 3, 4) for Channel 0 have correct screws torqued to 0.5 N·m. Confirm no stray strands bridge adjacent terminals. Confirm the shield is grounded at one end only.
2. Resistance measurement (sensor disconnected from module). With the front connector removed from the SM334, use a calibrated ohmmeter (4-wire measurement preferred) to measure:
   • Between matched-pair wires A and B: < 0.5 Ω (these connect to the same internal Pt100 node).
   • Between matched-pair wire A and lone wire: ~107.79 Ω ±0.5 Ω at 20 °C ambient.
   • Between matched-pair wire B and lone wire: ~107.79 Ω ±0.5 Ω at 20 °C ambient.
   If the two matched-pair-to-lone measurements differ by more than 1 Ω, suspect a high-resistance connection in the sensor or cable.
3. Module health check (sensor still disconnected). Power the PLC and monitor the SM334 input word with the programming software:
   • If configured for RTD 3-wire with no sensor: expect 0x8000 (wire break) or 0x7FFF (overflow). Both indicate the module is alive and detecting an open circuit.
   • If the input reads 0x0000 or a fixed mid-range value, the module may be damaged.
4. Sensor connection and live reading. Connect the Pt100 to the module. Within 1-2 update cycles (50-60 ms typical for 50/60 Hz integration), the input should report a value corresponding to ambient temperature. For a Pt100 in a 20 °C room, expect a value in the range 20350-20380 (linear interpolation between -200 °C = 0 and 850 °C = 27648).
5. Reference comparison. Place a calibrated reference thermometer (calibration certificate within 12 months) at the same measurement point as the Pt100. Compare readings after thermal equilibrium (typically 5-10 minutes for thermowell installations). Acceptable deviation:
   • Pt100 Class A: ±(0.15 + 0.002 × |T|) °C
   • Pt100 Class B: ±(0.30 + 0.005 × |T|) °C
   For a 100 °C measurement, Class A allows ±0.35 °C and Class B allows ±0.80 °C. If the SM334 reading exceeds these limits, suspect wiring error or sensor mismatch.
6. Diagnostic LED check. Confirm the SF (group fault) LED on the SM334 front is OFF. A lit SF LED indicates a configuration mismatch, wire break, or module fault that the input word alone may not reveal.
7. Channel-to-channel isolation test. If using multiple Pt100 sensors, verify each channel reads independently. Apply a known temperature (e.g., ice bath at 0.00 °C ± 0.05 °C) to one sensor while leaving the others at ambient. Each channel should track its own sensor, not cross-couple.

Common Wiring Mistakes and Module Protection

The original poster reported "I might have destroyed the card from all the different connections I tried." The two output channels (AO0 and AO1) on the SM334 are particularly sensitive to reverse wiring because their output stage can source ±10 V or 0/4-20 mA into a low-impedance loop. Common mistakes that lead to module damage:

To prevent module damage during commissioning:

• Always de-energise before changing wiring.
• Use a current-limited bench supply (≤ 50 mA) when first applying power to an unknown sensor circuit.
• Verify wire assignment with a multimeter before connecting to the module.
• Apply power incrementally: first to the PLC rack only, then to external sensors, then to actuators.
• Keep the original Siemens front connector keying (the polarisation pin in slot 20) to prevent the connector from being inserted upside-down.

If the SM334 outputs are damaged but inputs still function, the module can be repurposed for "read-only" applications where the analog outputs are not required. The analog input channels are electrically isolated from the output stage, so an input-side failure does not necessarily imply an output-side failure, and vice versa. Always document which channels are functional in the project documentation after a fault.

Comparison: 2-Wire vs 3-Wire vs 4-Wire RTD on SM334

The SM334 (6ES7 334-0KE00-0AB0) supports all three configurations but the engineering effort increases with wire count. For the majority of industrial installations (motor windings, bearing housings, process pipes, HVAC ductwork), 3-wire Pt100 on Channel 0 is the cost-effective and accurate choice. Switch to 4-wire only when the sensor is more than 50 m from the cabinet, or when the application requires Class A accuracy and a long cable run makes the 3-wire M- lead residual error significant.

For new installations, consider the SM331 RTD module (6ES7 331-7PF01-0AB0 or 6ES7 331-7PF11-0AB0) instead of the SM334. The SM331 provides 16-bit resolution, hardware-based linearisation, expanded diagnostic interrupts, and accepts the same Pt100 sensors. It occupies one slot and is recommended for new S7-300 designs where the additional slot cost is acceptable.

Frequently Asked Questions