Connecting 2-Wire 4-20mA Sensor to LOGO! AM2 Analog Input

Problem Overview

When wiring a 2-wire 4-20 mA loop-powered transmitter (such as the JUMO MAERA S25 hydrostatic submersible level probe) to a Siemens LOGO! 8 AM2 expansion module (catalog number 6ED1055-1MA00-0BA2), the most common field symptom is a stuck-at-zero analog reading. The input value remains 0 even though the transmitter is properly powered, the polarity is correct, and the loop is closed. Engineers immediately suspect a faulty sensor, a blown input, or a wiring error. While all of those are real possibilities, the dominant root cause in the field is software-side: the LOGO! base module reserves the AI1 and AI2 addresses for its onboard analog inputs (which are repurposed from two of the digital input terminals), and the AM2 expansion's analog channels default to AI3 and AI4. If the application logic reads AI1, it samples a floating input that is not physically connected to the AM2 terminals — hence the constant zero.

This article walks through the full diagnostic and corrective workflow: equipment identification, the 2-wire current loop principle, the correct wiring, the address-conflict root cause, the resolution in LOGO! Soft Comfort, and the verification procedure. It is aimed at field technicians and panel builders who are commissioning a LOGO! 8 system with a loop-powered analog sensor for the first time, and at maintenance engineers who inherit a panel and need to understand why an AI block reads zero on a working loop.

Equipment Identification and Specifications

Three pieces of equipment are typically in play. Confirm the catalog numbers before troubleshooting — Siemens LOGO! has multiple analog expansion modules with different input ranges, and the wiring procedure below is specific to the AM2 (6ED1055-1MA00-0BA2). The AM2 RTD (6ED1055-1MD00-0BA2) is a different module for Pt100/Pt1000 sensors and must not be substituted for a 4-20 mA loop.

Siemens LOGO! Base Module

The host base module is one of the LOGO! 8 family (6ED1052-1xxx-0BA2 series). Common candidates are:

• 6ED1052-1MD00-0BA2 — LOGO! 8.3 12/24 RCE (relay outputs, Ethernet, 12/24 V DC supply)
• 6ED1052-2MD00-0BA2 — LOGO! 8.4 12/24 RCEo (same I/O, OLED display)
• 6ED1052-1CC01-0BA2 — LOGO! 8.3 230 RCE (relay outputs, 115/230 V AC supply)

All LOGO! 8 base modules share a defining feature: the first two physical input terminals can be configured as either digital or analog. By default, two analog inputs (AI1 and AI2) are enabled. This is the source of the address conflict that this article is built around. The default state is documented in the LOGO! 8 system manual available on the Siemens Industry Online Support portal at support.industry.siemens.com.

Siemens LOGO! AM2 Expansion Module (6ED1055-1MA00-0BA2)

The 6ED1055-1MA00-0BA2 is the LOGO! AM2 analog input expansion. Key specifications for a 4-20 mA wiring task:

• Analog inputs: 2 (terminals I1 and I2 on the module face)
• Input ranges per channel: 0 to 10 V DC, or 0/4 to 20 mA (software-selected per channel)
• Resolution: 10 bits (0 to 1000 raw counts over the selected range)
• Galvanic isolation: None between channels and the base module (common M bus)
• Power: 12/24 V DC supplied via L+ and M terminals on the module
• Sensor supply: The L+ terminal is intended to source loop power for a 2-wire transmitter; the M terminal is the loop return
• Maximum cable length to sensor: 50 m shielded (per Siemens manual), longer runs require shielded twisted pair with shield grounded at the panel end

The AM2 differs from the LOGO! AM2 RTD (6ED1055-1MD00-0BA2) in that it has no temperature-sensor inputs and no PT100/PT1000 support. Substituting the RTD module for a 4-20 mA sensor will produce a 0 reading and may damage the input stage if high common-mode voltages are present.

JUMO MAERA S25 (Hydrostatic Submersible Probe)

The MAERA S25 is a stainless-steel submersible probe used for continuous level measurement in water, waste-water, and rainwater wells. Relevant specifications for the wiring task (per the manufacturer datasheet at jumo.de):

• Output: 2-wire, 4 to 20 mA current loop (loop-powered, no separate supply pair)
• Supply voltage: 10 to 30 V DC at the transmitter terminals
• Load: ≤ (UB - 10 V) / 0.02 A — i.e., up to 1000 Ω at 30 V supply, 700 Ω at 24 V supply
• Reverse-polarity protection: Yes (built-in; loop survives reversed leads without damage)
• Overvoltage protection: Yes (TVS diode at the input)
• Connection: Permanently attached cable, typically 2-cond shielded (e.g., 10 m, 20 m, 30 m variants), with shield drain wire
• Calibrated span: factory-set to customer's specified level range (e.g., 0 to 10 mWC)

For a 0 to 10 m water column (mWC) calibrated probe, the loop draws 4 mA at 0 m level and 20 mA at 10 m level. The transfer function is linear across the span.

The 2-Wire Current Loop Principle

A 2-wire (loop-powered) 4-20 mA transmitter draws its operating current from the signal loop itself. There is no separate power pair. The transmitter behaves as a variable current regulator: it draws between 4 mA and 20 mA from the supply, where the current value is proportional to the measured process variable (level, pressure, temperature, etc.). The 4 mA lower bound serves two purposes: it is the "live zero" that distinguishes a working sensor from an open loop (0 mA), and it provides the minimum operating current for the transmitter's internal circuitry.

The fundamental loop equation, as documented in TI's 2-wire 4-20mA Transmitter Basic Design Considerations training material, is:

I_loop = 4 mA + (16 mA × (V_meas - V_LRL) / (V_URL - V_LRL))

Where:

• I_loop = Loop current in mA (4 to 20 mA)
• V_meas = Measured process variable in engineering units (e.g., meters of water column)
• V_LRL = Lower range limit (zero of the measurement, e.g., 0 mWC)
• V_URL = Upper range limit (span maximum, e.g., 10 mWC)

For a 0 to 10 mWC probe, a level of 5 m produces exactly 4 + 16 × (5-0)/(10-0) = 12 mA in the loop. At 7.5 m, the loop carries 16 mA; at 10 m, exactly 20 mA.

The receiver (LOGO! AM2 in this case) must provide a compliance voltage at least 10 V above the lowest expected loop voltage drop, while the transmitter requires a minimum terminal voltage of 10 V. The LOGO! AM2 supplies 24 V DC nominal at its L+ terminal; the transmitter drops a few volts internally and across its output stage, leaving the loop comfortably above the 10 V minimum.

Three things must be true for the loop to work electrically:

1. The L+ and M terminals of the AM2 must be energized (24 V DC) and share a common return with the LOGO! base M bus.
2. The transmitter's two leads must form a closed loop with the L+ and the analog input terminal — current must flow from L+ into the transmitter's + lead, through the regulator, out the - lead, into the AM2 I1 terminal, through the internal shunt, and back to the supply via the M bus.
3. The analog input channel must be configured for 0/4-20 mA mode, not 0-10 V mode. In voltage mode, the input is high-impedance and reads 0 V at the shunt, producing a reading of 0.

Wiring the JUMO MAERA S25 to the LOGO! AM2

The MAERA S25 cable typically contains two insulated conductors and a shield drain wire. The shield should be grounded at the LOGO! end only, to prevent ground loops. The two signal conductors are connected to the AM2 as shown in the table and diagram below.

Terminal-by-Terminal Wiring Table

Wiring Diagram (Inline SVG)

The complete signal path is: AM2 L+ (24 V) → MAERA S25 red lead → transmitter internal regulator → MAERA S25 black lead → AM2 I1 → AM2 internal shunt (typically 100-250 Ω) → AM2 M → back to supply L-. The 4-20 mA current flows in this loop, and the AM2 measures the voltage across its internal shunt to compute the digital value. If the loop is broken anywhere — a loose terminal, a broken conductor, a dead sensor — the AM2 reads 0 mA and produces a raw count of 0 (or, in some firmware versions, pegs to 1000 indicating out-of-range high; see the troubleshooting matrix below).

Root Cause: The Base-Module AI Address Conflict

Now we come to the most common reason an otherwise correctly wired 4-20 mA input reads 0 in LOGO! Soft Comfort, the LOGO! display, or a web server readout. The issue is not electrical — it is address-mapping.

LOGO! 8 base modules ship with two onboard analog inputs (AI1 and AI2) enabled by default. These are the result of repurposing two of the digital input terminals (the LOGO! 8 manual documents this as "analog input mode for I7/I8" on the base). The expansion modules downstream do not see those addresses as available; instead, the AM2's two analog channels are auto-numbered starting from the next free address: AI3 and AI4.

The full address map for a LOGO! 8.3 base module (6ED1052-1MD00-0BA2) with one AM2 expansion, in factory-default configuration, looks like this:

If the application logic uses an AI1 function block but the physical sensor is wired to the AM2's I1 terminal, the AI1 block is sampling the unused base-module analog input (which floats at 0 V / 0 mA when nothing is connected to the base I7/I8 terminals). The result: a constant zero on the LOGO! display, in Soft Comfort's watch window, and on any HMI or web server mapped to that variable — regardless of the actual loop current at the AM2 terminal.

This is exactly the situation in the field case: the sensor was wired to the AM2 expansion, but the LOGO! program referenced AI1. The actual value was on AI3. Without an oscilloscope, a current clamp, or a multimeter, the symptom is indistinguishable from a wiring fault or a dead sensor. The diagnosis is fast once the address map is checked, but the failure mode is invisible from outside the LOGO! programming tool.

The address conflict is not unique to the AM2. The same effect occurs with the AM2 RTD, the LOGO! AM2 AQ (analog output), and any other analog expansion. The renumbering is automatic and silent — no warning is displayed when an AI block is added in Soft Comfort pointing to a base-module address while the physical sensor is on an expansion channel.

Resolution: Step-by-Step Procedure

The fix is to either change the program to read AI3, or, preferably, to disable the base-module onboard analog inputs so that the AM2 channels renumber to AI1 and AI2. The second option is cleaner because it produces a contiguous, intuitive address map that matches the physical layout, and it frees the base I7/I8 terminals to be used as digital inputs again (which is often what the panel design intended).

Option A — Change the Program to Read AI3/AI4 (Quick Fix)

1. In LOGO! Soft Comfort V8.x, open the project file (.lsc or .lscpx).
2. Locate the analog-input function block (e.g., Analog Input block, threshold trigger, or comparator block referencing AI).
3. Double-click the block and change the address from AI1 to AI3 (or from AI2 to AI4).
4. Recompile: File → Compile → Program (Ctrl+B).
5. Download to the LOGO! via Ethernet (PC ↔ LOGO! direct cable or switch) or transfer via micro-SD card.
6. Verify the live value on the LOGO! base display or in Online → Watch mode.

Option B — Disable the Base Module's Onboard AIs (Recommended)

1. Open the project in LOGO! Soft Comfort V8.2 or later (V8.3 is current as of writing; V8.4 ships with LOGO! 8.4 base modules).
2. Navigate to Tools → Device Selection (or in older revisions: File → Properties → Hardware) and confirm the base module matches the physical hardware, e.g., 6ED1052-1MD00-0BA2, firmware FS:04 or later.
3. Select Tools → AI Allocation. In some revisions this is under Edit → Properties → AI Settings; the dialog is labeled "Analog Inputs" and lists each AI address with its source.
4. Uncheck the boxes for AI1 and AI2 on the base module row. This frees the onboard analog channels and reassigns the AM2's I1 and I2 to AI1 and AI2, respectively.
5. Save the project, recompile (Ctrl+B), and download to the LOGO!.
6. Verify in Online → Watch that the AM2 I1 channel now reports AI1 with a non-zero value when the sensor is excited. The expected raw count for a working loop at zero process variable is 200 (4 mA of 0-20 mA = 20% of span).

Option C — Map the Variable in the Web Server / HMI

If the reading is correct on the LOGO! display but the web server or connected HMI shows 0, the issue is in the variable mapping rather than the AI block itself. Open Tools → Web Server Editor (or the HMI tag editor if a third-party HMI is used) and confirm that the displayed variable is bound to AI1 (post-fix) or AI3 (pre-fix), not to an unrelated tag such as VW0 or a stale program symbol from a prior project version.

Verification Procedure

After the wiring is complete and the address is corrected, run the following checks in order. Each check has a specific pass/fail criterion; do not skip ahead — a pass on Step 1 is required before Step 2 makes sense.

Step 1: Loop Voltage Check (Sensor Disconnected)

With the LOGO! powered and the sensor cable disconnected at the AM2 end, measure the DC voltage between the AM2 L+ terminal and the AM2 I1 terminal. Expected reading: ~24 V DC (open-circuit voltage of the supply). If the reading is 0 V, the L+ rail is not energized — check the LOGO! power supply and the AM2 power input terminals. If the reading is present, proceed to Step 2.

Step 2: Loop Voltage Check (Sensor Connected)

Reconnect the sensor. Measure the DC voltage between the AM2 L+ terminal and the AM2 I1 terminal again. Expected reading: between 10 V and 22 V (the transmitter's minimum drop is 10 V; the AM2 supplies 24 V nominal). Typical reading: 18-22 V. If the reading is 0 V, the loop is short-circuited or the sensor is drawing excessive current. If the reading is ~24 V, the loop is open at the transmitter's internal regulator — the sensor is dead or the polarity is reversed on a sensor that lacks reverse-polarity protection.

Step 3: Loop Current Check

Insert a multimeter in current mode (mA DC, range 0-30 mA) in series with the black wire at the AM2 I1 terminal. Expected reading with the sensor exposed to atmosphere (zero level, V_meas = V_LRL): 4.0 mA ± 0.05 mA. With the sensor immersed to mid-scale (V_meas = 50% of span): 12.0 mA ± 0.1 mA. If the reading is 0 mA, the loop is not closing. If the reading is fixed at ~22-25 mA, the transmitter is in over-range or fault mode (above the calibrated URL). If the reading is correct here but the LOGO! still shows 0, the address issue is confirmed — proceed to the Resolution section.

Step 4: LOGO! Display Check

On the LOGO! base module's built-in display (or via Online → Watch in Soft Comfort), navigate to the analog input value. The raw count should now read between 200 (4 mA) and 1000 (20 mA). The displayed engineering value depends on the configured sensor scaling in the project; raw counts of 200-1000 are the default 0/4-20 mA representation, and the user can apply a scaling block (analog amplifier) to convert to engineering units.

Step 5: Process Response Check

Lift the submersible probe out of the water (or otherwise vary the process variable through at least 25% of the calibrated span) and confirm the LOGO! reading tracks smoothly with no step changes, no saturation at 0 or 1000, and no visible noise above ±2 raw counts. Excessive noise indicates a shield-grounding issue or a cable routed too close to a VFD output or contactor coil.

Troubleshooting Matrix

Engineering Notes and Best Practices

Address-management discipline. For any LOGO! 8 installation with one or more analog expansions, document the AI address map on the cabinet door or inside the panel lid. A simple sticker reading "AI1/AI2 = AM2 ch1/ch2 (base AIs disabled)" or "AI1/AI2 = base I7/I8, AI3/AI4 = AM2 ch1/ch2" eliminates a class of future troubleshooting and reduces commissioning time for the next technician. Siemens' own commissioning checklist for LOGO! 8 (in the system manual) recommends this practice for any installation with more than one analog input.

Consider a 4-wire transmitter if available. The JUMO MAERA S25 is also available in a 4-wire variant (separate supply and signal pairs). The 4-wire version eliminates the loop-voltage margin concern and removes any ambiguity about whether the LOGO! is sourcing or sinking the loop current. For long cable runs (>100 m) or installations with high electromagnetic interference, the 4-wire variant is generally more reliable.

Shield termination — single-ended ground. The MAERA S25 cable shield should be landed on the panel PE bar (or the AM2's M terminal, depending on local practice) and left floating at the sensor end. Do not connect the shield at both ends — that creates a ground loop and injects 50/60 Hz common-mode noise that biases the low-end 4 mA value, producing a stable but inaccurate reading (e.g., AI shows 250 raw counts at zero level when it should show 200).

Calibration verification before commissioning. Force the loop to known values using a calibrator (or a precision 4-20 mA simulator such as a Beamex MC5 or Fluke 754) before relying on the LOGO! reading for process control. A common field practice is to inject 4.000 mA, 12.000 mA, and 20.000 mA in turn and confirm the LOGO! displays 200, 600, and 1000 raw counts, respectively (after disabling any software scaling). If the LOGO! displays 196, 612, 988, the AM2 is within its ±2.5% accuracy spec and no adjustment is needed. If the error is larger, check the wiring and the sensor calibration certificate before assuming the AM2 is faulty.

Loop voltage margin for long cable runs. The MAERA S25 requires 10 V minimum. The LOGO! AM2 supplies 24 V nominal, but the actual voltage at the L+ terminal drops slightly under load and with long cable runs. For cable runs over 50 m, measure the voltage at the transmitter end (not the LOGO! end) to ensure the transmitter has its full 10 V headroom. A loop voltage below 11 V at the sensor terminals indicates a marginal loop — increase the supply voltage, reduce the cable resistance (use a larger conductor), or install a 4-wire variant.

Cross-platform note: the same address-conflict pitfall exists in other compact PLCs. Wago's 750 series, Allen-Bradley Micro820, and Schneider Electric's M221 all have analogous "onboard analog" features that can be enabled or disabled in software, and the same 0-reading symptom can occur if the program references the wrong channel. The general diagnostic approach — confirm the loop current with a multimeter, then check the address map — applies unchanged.

Safety — panel de-energize before any re-wiring. All wiring changes on the AM2 module should be performed with the LOGO! base module de-energized. The AM2's M bus is bonded to the LOGO! base M, and a wiring error while live can produce a short circuit that may damage the AM2 input stage or the transmitter's output stage. Power down, lock out, wire, verify, and power up.

Frequently Asked Questions