ET200S 4AI 2-Wire Module: Wiring Active EX Barriers Correctly

Connecting an active EX barrier (e.g., Endress+Hauser RN221N) to a Siemens ET200S 4AI 2-wire analog input module (e.g., 6ES7134-4GD00-0AB0) is a recurring source of plant-floor failure. The combination produces a fixed 0 mA reading, an overflow (7FFFH), or a value stuck near 0x7FFF/0x8000 even though the same barrier drives an SM1231 input without complaint. The root cause is signal-source conflict: both the barrier output and the 2-wire AI channel are active current sources. The fix is module selection, not parameter tuning.

This reference covers the transducer-topology theory behind the failure, the ET200S AI module variants involved, two field-proven corrective paths (4-wire module swap and shunt-resistor workaround), and the Siemens S7-format scaling you must apply once the wiring is corrected.

1. Problem Definition

1.1 Symptoms

• AI channel value reads 0 mA, 4 mA, or 7FFFH (overflow) regardless of process variable.
• Same barrier and sensor operate correctly on an S7-1200 SM1231 4AI channel.
• Wiring, shield grounding, and PROFIBUS/PROFINET diagnostics show no fault.
• Channel diagnostic LED on the ET200S module shows "no fault," but the process value is unusable.

1.2 Affected Hardware

2. Root Cause: Two Current Sources in Series

2.1 Signal-Source Theory

Analog current loops follow the same Kirchhoff current law as any other series circuit. In a healthy 4-20 mA loop, exactly one device is the active source (it drives 4-20 mA into the loop) and the other device is the passive sink (it merely measures the current by developing a voltage across an internal sense resistor).

2.2 The Conflict

The ET200S 4AI 2-wire module energizes the loop with its integrated 24 V supply. The E+H RN221N active barrier also attempts to push 4-20 mA into the same loop. The two regulators fight each other: neither can establish a defined compliance voltage, the loop settles at a non-physical current, and the ADC either reads zero or saturates. The SM1231 avoids this because it has no internal loop supply - it is a pure passive sink.

3. ET200S AI Module Variants

Identifying the exact module in the cabinet is the first commissioning step. The 2-wire and 4-wire variants are mechanically and electrically distinct, but their faceplate labels can be ambiguous if the front cover is dirty or painted.

4. Solution A - Replace the 2-Wire Module with a 4-Wire Module

4.1 Mechanical Compatibility

ET200S analog modules share the same terminal-module footprint. A 2-wire channel module can be hot-swapped for a 4-wire module of the same channel count as long as the terminal module supports both. The 6ES7138-4CA01-0AA0 and 6ES7138-4CB01-0AA0 terminal modules accept either variant.

4.2 STEP 7 / TIA Portal Reconfiguration

1. Open the ET200S station in HW Config (STEP 7 V5.x) or the Device View (TIA Portal).
2. Right-click the slot containing the 4AI 2-wire module.
3. Select "Replace Object" > "6ES7134-4MC00-0AB0" (4-wire 4AI Standard) or "6ES7134-4MD00-0AB0" (4-wire 4AI HS).
4. Open the module properties. On the "Inputs" tab, configure each channel to Current measurement, range 0..20 mA or 4..20 mA.
5. Save, compile, and download the hardware configuration.

4.3 Wiring Change

5. Solution B - Voltage Mode with Shunt Resistor (Quick Fix)

If a 4-wire module cannot be sourced immediately, configure the existing 2-wire module for voltage mode (0-10 V) and place a precision shunt resistor across the AI terminals to convert 4-20 mA to 0.8-4.0 V. This violates the module's intended use of current mode but works as a stopgap. Use a 200 Ω / 0.1 % / 5 ppm/°C metal-foil resistor for an 0.8-4.0 V drop, or a 250 Ω / 0.1 % resistor for a 1-5 V drop.

6. Endress+Hauser RN221N Active Barrier

The RN221N is a single-channel active safety barrier designed to power and repeat a 4-20 mA signal from a 2-wire or 4-wire transmitter located in hazardous areas. It is not a passive Zener barrier - it is an active repeater that requires mains or DC power and produces its own active 4-20 mA output.

Because the output stage is an active current regulator, the RN221N must always be terminated into a passive sink. An ET200S 4AI 2-wire module is not a passive sink - it is an active source - hence the conflict.

7. Siemens S7 Format Scaling

Once the wiring is corrected, the analog value read into the S7 CPU follows the standard S7 format. For 0-20 mA on a 4-wire module configured for current:

For voltage measurement ranges (relevant to the shunt-resistor workaround), the 1-5 V range is also defined per the Siemens fail-safe / ET200 documentation:

For the shunt-resistor workaround (200 Ω, 4-20 mA → 0.8-4.0 V), the engineering conversion in the PLC is:

// Inputs
iRaw     : INT;   // PIW address of the AI channel
rScaleMin: REAL := 4.0;     // mA at 0% (4 mA live zero)
rScaleMax: REAL := 20.0;    // mA at 100%

// Linearization: 0.8 V -> 4 mA, 4.0 V -> 20 mA
rVolt    : REAL := INT_TO_REAL(iRaw) * 5.0 / 27648.0;   // assumes 0-5 V range
rMilliA  : REAL := rVolt / 200.0 * 1000.0;              // 200 ohm shunt
rPercent  : REAL := (rMilliA - rScaleMin) / (rScaleMax - rScaleMin) * 100.0;

8. Verification Procedure

1. With the barrier powered and the process at zero, read the raw value. Expect 0 (0-20 mA) or 0 (4-20 mA with scaling offset).
2. Apply a known current (e.g., disconnect sensor, inject 12.000 mA with a calibrator). Expect 0x3000 (11585 decimal) on a 0-20 mA scale.
3. Force the sensor to its maximum process value. Expect 0x6C00 (27648 decimal) at 20 mA.
4. Open the module's online diagnostics in STEP 7 / TIA Portal. Confirm no "channel fault" or "wire break" bit is set.
5. Check the SF LED on the ET200S head module - it must remain off.
6. Compare the PLC value to a calibrated multimeter reading in the same loop. The two must agree within 0.1 % of span.

9. Troubleshooting Matrix

10. Field-Proven Caveats

• The 2-wire vs 4-wire distinction is on the module, not the sensor. The same sensor can be 2-wire (loop-powered) or 4-wire (externally powered) depending on the manufacturer variant.
• The RN221N's output is always active. There is no DIP switch or jumper to make it passive.
• If you must keep a 2-wire AI module, the only way to connect an active source to it is via a current-to-voltage converter (signal conditioner) installed between the barrier and the AI channel. The shunt-resistor approach in Section 5 is the bare-minimum version of this; commercial isolators (e.g., Phoenix Contact MACX MCR-EX-SL) provide isolation, noise filtering, and a wider burden range.
• Some ET200S analog modules (e.g., 2AI TC, 2AI RTD) cannot measure current at all - do not attempt to use them with any 4-20 mA source.
• The SM1231 4AI input referenced in the field case is a passive sink; this is why it "just works." Do not generalize the SM1231 behavior to all S7 analog inputs.

11. Related Configuration in TIA Portal / STEP 7

When you replace the module in HW Config or TIA Portal, the I addresses shift only if the replacement module occupies a different number of input bytes. The 4AI Standard (4MC) and 4AI HS (4MD) both occupy 8 bytes of input area (4 channels x 2 bytes). The 2AI Standard (4MB) occupies 4 bytes. When swapping 2-wire for 4-wire within the same channel count, addresses remain stable.

For scaling in the user program, the standard Siemens FC105 / FB165 "SCALE" or "NORM_X / SCALE_X" library functions work without modification. For 4-20 mA live-zero applications, add a -27648 / 16384 offset to map the 4 mA point to 0 % engineering units.

12. References for Further Verification

For the S7-format measuring-range table, the Siemens fail-safe modules manual collection defines the 1-5 V range with the 5.704 V overflow threshold, 0x7FFF saturation, and the 0xED00 underrange values quoted in Section 7. Cross-check your project's hardware documentation against this range table before commissioning a live loop.