LOGO! AM2 AQ 4-20mA Output: Active vs Passive Signal Wiring

Overview

The Siemens LOGO! AM2 AQ analog output module is an active 4-20 mA current source. When the loop is broken, open, or terminated by a high-impedance probe such as a digital multimeter set to volts, the voltage between terminals Mx and Ix rises toward the module's compliance ceiling - typically 12-14 V at a 24 V DC supply. This behavior is mandated by the physics of a constant-current source and is not a defect. Misreading this voltage as a "fault" is one of the most common field issues with LOGO! AM2 AQ installations.

This reference explains:

• Why an open-circuit 4-20 mA output displays a non-zero voltage.
• How to measure a 4-20 mA loop correctly (burden resistor vs. series ammeter).
• Why two active 4-20 mA sources cannot be connected in parallel.
• Why a passive-only input such as the SmartBox sensor terminal cannot accept an active source.
• How to duplicate a passive 4-20 mA sensor value into a downstream device using the LOGO! and Modbus TCP.

Why a 4-20 mA Output Displays a Voltage

A current source is governed by Ohm's law:

Where:

• U_loop is the terminal-to-terminal voltage developed across the load (V).
• I_loop is the regulated loop current (mA).
• R_load is the total DC resistance the source must drive (Ω).

A DMM in voltage mode presents ~10 MΩ at the test leads. With AM2 AQ regulating to 4 mA, the terminal voltage tries to climb to:

The AM2 AQ cannot reach that - it is limited by its compliance. The output saturates at the compliance ceiling (about 13-14 V at 24 V supply, less at lower supply). This is exactly the reading the user observed: 13.49 V. The current source is operating correctly; the test method is wrong.

Voltage compliance window of the AM2 AQ

The AM2 AQ (Siemens part number 6ED1055-1MD00-0BA2, LOGO! 8 generation) is specified for 24 V DC supply on the logic/power bus of the LOGO! base module. At full 20 mA the source must keep the loop voltage within its compliance window so that the current remains regulated. Practical rules:

• Maximum load at 20 mA: roughly 500 Ω (so U_loop_max ≈ 10 V at 20 mA).
• Minimum load for stable regulation: typically 50-100 Ω to prevent the output stage from saturating against the compliance rail.

If U_loop climbs above the compliance rail the output leaves the constant-current region and the current droops below the programmed value. This shows up as "the current is right at low values but too low at 20 mA" when the wiring is too long or the burden too high.

Measuring a 4-20 mA Loop Correctly

There are three legitimate ways to verify what the AM2 AQ is actually doing on the wire.

Method 1 - Series ammeter (preferred for live verification)

1. Power down the LOGO! base and the AM2 AQ.
2. Disconnect the M or I wire at the field device.
3. Insert the DMM (in mA DC mode) in series, completing the loop.
4. Restore power. Read the current; it should track 4.000 mA at 0% and 20.000 mA at 100% of the analog range.

Method 2 - Burden resistor plus voltmeter (preferred for permanent verification)

1. Fit a precision 250 Ω, 0.1% resistor across Mx-Ix as a temporary or permanent burden.
2. Measure the voltage across the resistor.
3. Apply the conversion I (mA) = U (V) x 4. A 1.000 V reading = 4.000 mA; a 5.000 V reading = 20.000 mA.

This is the standard 4-20 mA scaling: a 250 Ω burden converts 4-20 mA into 1-5 V.

Method 3 - Loop calibrator or HART communicator

A loop calibrator reads and sources simultaneously; it lets you confirm the AM2 AQ's setpoint without breaking the loop. Useful during commissioning, but not always available on site.

Active vs Passive 4-20 mA Sources

The terminology "active" and "passive" refers to who supplies the loop energy.

The LOGO! AM2 AQ acts as an active source: it provides the compliance voltage and modulates the current. A passive input such as the SmartBox sensor channel sits at ~20 V DC and expects a sensor that simply modulates current as a function of the measured variable. The two roles are not interchangeable.

SmartBox 4-20 mA Input Behavior

The SmartBox accepts only a passive (loop-powered) 2-wire sensor on its analog input. Internally the input supplies ~20 V DC and presents a low-impedance current-sensing shunt (~250 Ω typical). The current loop must therefore originate inside the SmartBox and pass out through the sensor and back.

If you connect an active source such as the LOGO! AM2 AQ to a passive-only input, you create two supplies in series: the AM2 AQ's compliance rail and the SmartBox input's loop rail. Two outcomes are possible:

1. The higher-voltage rail wins and back-feeds the lower rail through the analog input stage of one device, potentially damaging it.
2. The lower rail clamps, and the current you measure is not the sum of two regulated sources - it is undefined.

Why Two Active 4-20 mA Sources Cannot Be Paralleled

Imagine two constant-current sources A and B, each trying to force a current through a shared load. The system has only one equation:

But current sources are designed to source current, not to absorb back-EMF. When you parallel AM2 AQ with another active source, each device tries to push the loop to its own compliance. Whichever has the lower compliance rail becomes reverse-biased and the other's output stage sees an undefined load. The result is unstable current and possible thermal stress on the output transistors.

The only safe way to share a 4-20 mA signal is:

• Use a passive signal isolator/splitter (e.g. Phoenix Contact MINI MCR, Wago 857, or similar 4-20 mA repeater) with one input and two isolated outputs.
• Convert the signal to digital (Modbus, IO-Link, HART gateway) and re-emit it downstream.
• Use a passive 250 Ω sense resistor in the original loop and read the voltage on a second, isolated input.

Duplicating a Passive 4-20 mA Sensor Signal

The user's use case is to take the value coming from a passive 4-20 mA sensor and re-emit it on the LOGO! AM2 AQ output for the SmartBox input. There are three viable approaches.

Option A - Modbus TCP/IP pass-through (recommended for LOGO!)

1. Read the 4-20 mA value on an AM2 analog input module connected to the LOGO! base (part number 6ED1055-1MD00-0BA1 or equivalent). This module is passive at the input (loop-powered by the sensor) and exposes the scaled value as an integer word in the LOGO! program.
2. Scale the value to 0-1000 or 0-27648 and write it into a holding register using the LOGO!'s built-in Modbus TCP server.
3. From another instance of the LOGO! (or any Modbus TCP client), read the scaled value and write it to the AM2 AQ output via Modbus TCP.
4. The second LOGO! instance uses a client block in LSC (LOGO! Soft Comfort) to read the word and drive AQ1 with it.

This avoids any analog wiring between the two devices and keeps both sides electrically isolated by the Ethernet isolation barrier of the LOGO! base module.

Option B - Hardware signal isolator/splitter

1. Place an active 4-20 mA input / two 4-20 mA output isolator (e.g. Phoenix Contact MACX MCR-UI-UI or Wago 857-402) in the loop between the sensor and the SmartBox.
2. Wire the sensor to the isolator's input.
3. Wire one isolated output to the SmartBox (still passive-source-compatible if the isolator is configured to source).
4. Wire the other isolated output to a LOGO! AM2 analog input for monitoring.

Option C - Voltage sense on a parallel branch

1. Place a precision 250 Ω, 0.1% resistor across the SmartBox sensor input.
2. The SmartBox sees 1-5 V across its input, exactly as if a passive 4-20 mA sensor were connected.
3. Tap the same 1-5 V signal into the LOGO! AM2 RTD/AI input (0-10 V range).

This is the lowest-cost approach and does not require a second LOGO!. It only works if the SmartBox input is truly voltage-tolerant (most 4-20 mA inputs scale from 1-5 V at 250 Ω, so a 0-10 V input on the LOGO! AI will read the same voltage safely).

Specifications - LOGO! AM2 AQ (6ED1055-1MD00-0BA2)

Commissioning Procedure for a 4-20 mA Loop with LOGO! AM2 AQ

1. Wire the AM2 AQ output as follows for current mode: I (source +) to the load +; M (source -) to the load -. Polarity is mandatory.
2. Confirm the supply to the LOGO! base is 24 V DC ±5%; the AM2 AQ derives all compliance from this rail.
3. Set AQ1 (or AQ2) to 4-20 mA in the LSC program and assign the scaling (e.g. 0-1000 → 4-20 mA).
4. Force AQ1 to 0.0 (0%). Measure with the multimeter in mA DC mode in series with the loop. Expect 4.000 ±0.020 mA.
5. Force AQ1 to 100.0 (100%). Measure. Expect 20.000 ±0.040 mA.
6. If the current droops at 100%, reduce the burden: the loop resistance at 20 mA must be ≤ 500 Ω for full-scale accuracy.
7. Restore the application scaling and download the project.

Verification Checklist

Troubleshooting Matrix

Signal Topology Diagram

Cross-References to Official Documentation

• Siemens Industry Online Support - LOGO! 8 manuals, firmware notes, and application examples.
• Texas Instruments SLVA545 - Reference on bipolar output amplifier behavior under load, useful background for engineers troubleshooting sourcing outputs.

FAQs