Integrating SITRANS Probe LU HART Signal with Siemens S7-300 AI

Problem Description: Incorrect PLC Readings from a HART Loop-Powered Level Transmitter

A field engineer reports a recurring integration problem between the SITRANS Probe LU240 (and the original SITRANS Probe LU) ultrasonic level transmitter and a Siemens SIMATIC S7-300 PLC. The transmitter is configured for volume measurement, and the on-board LCD of the Probe LU correctly displays the engineered volume in cubic meters, liters, or gallons depending on the vessel curve selected. The 4–20 mA loop is wired to a standard SIMATIC S7-300 analog input module (an AI8 × 13-bit, type 6ES7331-1KF02-0AB0 or similar), and the raw count is scaled in the CPU with the standard "Scale" function (FC105 in STEP 7). Despite the LCD being correct, FC105 outputs erratic, drifting, or systematically biased values.

This is not a transducer fault, not a programming fault in the strict sense, and not a wiring polarity issue. The root cause is that a standard (non-HART) S7-300 AI module does not provide a low-impedance, HART-compliant loop interface. When the Probe LU is operated in HART mode, the module's input impedance, common-mode filtering, and lack of HART band-pass filtering distort the 4–20 mA analog signal that rides on top of the HART carrier. The transducer itself is fine — the analog acquisition path is wrong for the chosen device.

Root Cause: HART Carrier on a Non-HART Analog Input

The SITRANS Probe LU operating instructions specify a 2-wire, loop-powered 4–20 mA output with optional HART 7 communication. HART (Highway Addressable Remote Transducer) superimposes a 1.2 kHz / 2.2 kHz FSK signal at ±0.5 mA on the 4–20 mA DC loop. For a HART master to communicate, the loop must present a total impedance of 230–600 Ω at the HART frequencies, and the analog input of the receiving device must not shunt or filter out the HART band.

Standard S7-300 SM331 modules such as:

• 6ES7331-1KF02-0AB0 (AI8 × 13 bit)
• 6ES7331-7KB02-0AB0 (AI2 × 14 bit)
• 6ES7331-1KF01-0AB0 (AI8 × 13 bit, earlier revision)

are designed for clean 4–20 mA or ±10 V signals. Their input stages include filtering that attenuates the HART band, and their loop impedance is not characterized for HART communication. According to the Siemens Industry Online Support FAQ entry 27839768 ("What should you watch out for when using HART measuring transducers in conjunction with S7-300 analog input modules that do not support HART signals?"), HART transmitters connected to non-HART SM331 modules can:

1. Read systematically low because the module's input current is reduced by the HART coupling network.
2. Read high with overshoot or oscillation when the HART multidrop poll occurs (HART burst mode).
3. Read with increased noise when the long cable acts as an antenna in industrial environments.
4. Be unable to communicate with a HART handheld (375/475, FMR, or SITRANS MDS) unless a 250 Ω HART communicator resistor is added and the module is bypassed.

FC105 (the standard scale block in STEP 7 / STEP 7 V5.x) is not the source of the error. FC105 simply converts an integer (-27648 to +27648 for the unipolar 4–20 mA range) to a floating-point engineering value. If the raw input count is wrong because the HART signal is being attenuated or rectified by the module, FC105 faithfully produces a wrong engineering value. The scale block cannot fix a corrupted input.

Solution Path: Use a HART-Capable AI Module or Decouple the HART Layer

There are three proven solutions, in order of preference for new installations.

Solution 1: Replace the SM331 with a HART-Capable SM331 in ET 200M

The cleanest fix is to swap the non-HART AI8 for the HART-capable 6ES7331-7TF01-0AB0 (AI8 × 31 bit HART) in an ET 200M distributed I/O station. The module is documented in the Siemens manual SIMATIC ET 200M distributed I/O HART analog modules (entry 22063748).

The HART module 6ES7331-7TF01-0AB0 provides transparent HART pass-through to the PLC program. With the PDM (Process Device Manager) routing through PROFINET or PROFIBUS, the user can read primary variable (PV = level), secondary variable (SV = distance), tertiary variable (TV = volume), and quaternary variable (QV = temperature) directly from the Probe LU and still use the analog input for fast-loop control.

Solution 2: Add an External HART-to-Analog Isolator (Active Barrier)

If the S7-300 architecture must be retained and the slot cannot be changed, install an active HART isolator between the Probe LU and the SM331. Suitable products include:

• Phoenix Contact MACX MCR-EX-SL – 2-wire HART transparent isolator
• WAGO 857-980 – HART transparent signal conditioner
• Siemens SITRANS I300 – universal HART isolator / repeater

The isolator provides the 250 Ω HART termination, presents a clean 4–20 mA signal to the SM331, and supplies loop power if the transmitter is not loop-powered from the PLC side. The Probe LU remains HART-communicable with a 475 handheld across the isolator.

Solution 3: Disable HART in the Probe LU (Short-term only)

If the customer accepts losing HART diagnostics and remote configuration, set the Probe LU output to 4–20 mA only (HART multidrop = 0, burst mode off). The transmitter then emits a clean DC loop signal, and a standard 6ES7331-1KF02-0AB0 will read correctly. This is acceptable for simple level-only applications but removes the ability to do remote PV/SV reading, range changes, echo profile diagnostics, and Bluetooth configuration (Probe LU240 with Bluetooth option). Use this only when no future HART integration is planned.

Prerequisites Before Re-Wiring

1. Verify the Probe LU firmware and configuration by reading the on-board LCD; the value in the loop-test mode must match an independent reference (hand-held ultrasonic or manual tape measurement).
2. Read the Probe LU order code and confirm whether the unit has HART (option H) or HART with Bluetooth (option BT). The Bluetooth option is only available on the Probe LU240 family.
3. Identify the existing S7-300 module order number on the door label (the 6ES7 MLFB on the right side of the module).
4. Pull the STEP 7 hardware configuration and confirm whether the module is configured as 4-wire current, 2-wire current, or voltage. A Probe LU is a 2-wire loop-powered device and must be configured as "2-wire current" (4DMU in HW Config) for a standard SM331, or as "4DMU HART" for the 6ES7331-7TF01-0AB0.
5. Measure loop voltage at the Probe LU terminals. Loop voltage must be ≥ 12 V DC (Probe LU needs 12–30 V DC for the 4–20 mA/HART output). Anything below 12 V at the transmitter terminals indicates cable resistance is too high or the PS 305/PS 307 power supply is undersized.

Step-by-Step: Wiring and Configuring the HART SM331 (Recommended Path)

The following procedure assumes the user has STEP 7 V5.5 (or TIA Portal with S7-300 compatibility) and an ET 200M interface module such as 6ES7153-1AA03-0XB0 (IM153-1) on PROFIBUS DP.

Step 1 — Mechanical Installation

1. Power down the ET 200M station.
2. Remove the existing non-HART SM331 from the slot. Place a blank cover (6ES7393-4AA00-0AA0) over the unused slot if the module is relocated.
3. Insert 6ES7331-7TF01-0AB0 into the same slot. Tighten the two front-screws to 0.8 Nm.
4. Open the front cover and note channel 0–7. Connect the Probe LU positive lead to terminal I_n+ and the negative lead to terminal I_n–. With a 2-wire Probe LU the shield is terminated at the module's shield bar, not at the Probe LU housing (to avoid ground loops).
5. Add a 24 V DC supply to the module's power terminals (L+ and M).

Step 2 — STEP 7 Hardware Configuration

1. Open the SIMATIC Manager and the S7 project.
2. In HW Config, locate the ET 200M station and drag the SM331 AI8 HART (order 6ES7331-7TF01-0AB0) from the catalog to the correct slot.
3. Double-click the module to open Properties. For each used channel set:

   
     Measurement type:    4DMU (4-wire transmitter with HART) or 2-wire HART (Probe LU)
     Output range:        4 … 20 mA
     HART:                Enabled
     Channel diagnostics: Wire break, Upper/lower limit, HART failure

4. Save and compile the hardware configuration. Download to the CPU.

Step 3 — STEP 7 Program: FC105 with HART-correct Limits

For a HART SM331, the nominal input count is 0 to 27648 (unipolar) regardless of HART. However, the user can also access the digital HART variables in addition to the analog count. The recommended FC105 call for the analog count is:

  // ST (Structured Text) — STEP 7 V5.5
  // Inputs to FC105
      IN      := PIW 304;        // analog input word, channel 0
      HI_LIM  := 1.0E+02;        // engineering upper value, e.g. 100.0 % volume
      LO_LIM  := 0.0E+00;        // engineering lower value
      BIPOLAR := FALSE;          // 4–20 mA is unipolar
      RET_VAL := MD100;          // scaled result, REAL
      OUT     := MD104;          // duplicate output

  // Equivalent ladder: call FC105 with the same parameters.

If the HART module is being used to read the PV (level) directly in engineering units, the HART driver block (FB81 "HART_PV" for the older 6ES7331-7TF01, or the integrated HART driver in TIA Portal) returns the PV as a REAL. In that case FC105 is not required for the HART variable — but it remains correct for the analog count.

Step 4 — Verify Wiring Polarity and Loop Voltage

1. With the CPU in RUN, force the Probe LU output to a fixed value using the local push buttons (menu 1.4 "Loop Test") to 4 mA, 12 mA, and 20 mA.
2. At each setpoint, observe the PIW value in the STEP 7 Monitor/Modify table. PIW should step from 0 → 13824 → 27648 within ±2 counts.
3. If the PIW is off by more than 1 %, check for a ground loop, swapped polarity, or a 250 Ω resistor in the loop that wasn't there before.

Verification: A Field Commissioning Checklist

Parameter Reference: SITRANS Probe LU Output Settings

Important — Volume vs. Level mapping: When the SITRANS Probe LU is configured for volume, the loop current is still scaled to level (0 % empty, 100 % full). The volume conversion is performed internally and exposed on TV (tertiary variable) in HART. A user who wants volume in the PLC can either:

1. Recompute the volume in the PLC from the linear level using the same vessel curve; or
2. Read the HART TV (volume) directly through the 6ES7331-7TF01-0AB0 with FB81/FB101.

The local LCD on the Probe LU shows the configured primary variable, which can be level or volume depending on which is mapped to PV. Mismatches between LCD and PLC often originate here, not in the analog path.

Troubleshooting Matrix for SITRANS Probe LU + S7-300

Special Case: ET 200M vs. Central S7-300 Rack

The 6ES7331-7TF01-0AB0 HART module is mechanically a standard SM331 and can be installed in either a central S7-300 rack or an ET 200M distributed rack. The functionality is identical, but the diagnostic and configuration path differs:

• Central S7-300: Configuration is in HW Config of the CPU; HART PDM uses the CPU's MPI/PROFINET interface.
• ET 200M (IM153-1/-2): Configuration is in the same way, but HART PDM can be routed through the PROFIBUS or PROFINET interface module; multiple HART field devices can be serviced from a single PDM station.

For brown-field installations, the ET 200M option is the least disruptive: a new IM 153 plus a 7TF01 slot can be added next to the existing non-HART rack without changing the rest of the I/O.

Special Case: TIA Portal Migration

For users migrating from STEP 7 V5.x to TIA Portal, the 6ES7331-7TF01-0AB0 HART module is supported in TIA Portal V14 SP1 and later. The HART driver library "HART_PV" is replaced by the system block "HART_4AI" or "HART_8AI" depending on the firmware version. FC105 remains available as the scale block, or the user can use the TIA Portal "NORM_X" instruction. Scaling limits (HI_LIM, LO_LIM) and the analog PIW limits (0–27648 unipolar) are identical.

Safety and Functional Notes

Summary of Recommended Module Mapping

FAQ