LOGO! AM2 4-20mA Signal Not Reading: Wiring & Diagnostics

Troubleshooting the Siemens LOGO! AM2 4-20 mA Input: Wiring, Loop Power, and Configuration

The Siemens LOGO! 8 AM2 analog expansion module (order number 6ED1055-1MA00-0BA2) is commonly misapplied because its input terminals look electrically similar to the AI1–AI4 channels on the LOGO! base unit but behave very differently. Engineers who have successfully read 4-20 mA on AI1–AI4 using a 500 Ω shunt resistor frequently conclude that the AM2 is defective when the same wiring technique is applied to its I1/I2 terminals. The module is not broken in most of these cases: the AM2 contains an internal current-sensing shunt and the resistor must be removed, and exactly one side of the current loop must source power.

This reference consolidates the wiring rules, configuration steps, LED diagnostics, and field-verified troubleshooting procedure for the AM2 4-20 mA input. It is written for first-time users of LOGO! 8 and for technicians migrating from AI1–AI4 to AM2 expansion I/O.

1. Why the AM2 Behaves Differently from AI1–AI4

The LOGO! base unit's onboard AI1–AI4 are voltage-input channels that share a single ground return and rely on an external 500 Ω resistor to convert the 4-20 mA loop current into a 2-10 V voltage that the ADC can measure. The AM2 expansion module is an entirely different circuit: each of its two input channels (I1, I2) contains an internal precision shunt resistor across the input terminals. The module's ADC measures the voltage drop across this internal shunt directly.

2. Active vs Passive 4-20 mA Loop Topology

A 4-20 mA current loop is a closed series circuit that requires exactly one device to act as the current source (active end) and the other device to act as the sink (passive end). The AM2 input is a passive sink — it has no internal loop power supply. The transmitter may be either active (self-powered, e.g., loop-powered by its own 24 V supply) or passive (requires the receiving instrument to power the loop).

2.1 Active Transmitter (Most Common Field Case)

The transmitter itself sources the loop current. The AM2 only measures it. Wire as follows:

• Transmitter + → AM2 terminal I1+
• AM2 terminal I1− → Transmitter −
• Transmitter is powered by its own 24 VDC supply; the AM2 input provides only the return path through its internal shunt.

2.2 Passive Transmitter (Loop Powered by LOGO!)

The transmitter requires the receiver to energize the loop. Use a dedicated 24 V supply tied to the AM2 input side:

• 24 VDC supply + → AM2 terminal I1+
• AM2 terminal I1− → Transmitter +
• Transmitter − → 24 VDC supply −

The LOGO! base 24 V supply (terminals L+ / M, or U1+ / U2+ depending on variant) can serve this role if its current budget allows. Confirm the transmitter's compliance voltage at 20 mA against the available supply voltage minus the AM2 internal drop (typically ≤ 2.5 V at 20 mA).

3. Required Hardware and Preparation

3.1 Prerequisites

• LOGO! 8 base module (any variant; firmware ≥ V8.0 for AM2 auto-detection).
• AM2 expansion module 6ED1055-1MA00-0BA2, properly seated on the right side of the base unit.
• LOGO!Soft Comfort V8.4 or later (V8.4.1 used in the field case). Available from the Siemens Industry Online Support portal.
• 24 VDC power supply sized for the LOGO!, AM2, and any loop-powered transmitter.
• Calibrated mA signal source or a verified 4-20 mA field transmitter.
• Digital multimeter (DMM) with mA current measurement capability.

3.2 Reference Documentation

• Siemens Industry Online Support — primary KB portal for LOGO! manuals, firmware, and EDS files.
• LOGO! 8 System Manual (entry ID 109751654) — covers AM2 wiring diagrams and analog input scaling.
• LOGO!Soft Comfort V8.4 Online Help — covers AI configuration and analog block parameter scaling.

4. Wiring the AM2 for 4-20 mA

1. Power down the LOGO! base and AM2 before wiring. Verify zero voltage on the 24 V supply rails with your DMM.
2. Connect 24 VDC to the AM2's power terminals. The AM2 draws power from the LOGO! bus; confirm the green PWR / RUN LED on the AM2 is lit.
3. Locate terminals I1+ and I1− on the AM2 (and I2+/I2− if a second channel is used).
4. Wire the transmitter or signal source in series with the AM2 input — do not place any external resistor in the loop.
5. Ensure exactly one side of the loop sources power. Verify polarity: red / brown to +, black / blue to −.
6. Power the LOGO! base and confirm both the base RUN LED and AM2 green LED are solid.
7. From LOGO!Soft Comfort, open Tools → Properties → AI Configuration and ensure the AM2 I1 and I2 channels are enabled and the type is set to 0-20 mA or 4-20 mA.
8. Insert an Analog Input block (B000 or similar) and route the AM2 channel to it.
9. Download the program and observe the live value on the LOGO! display or in LOGO!Soft Comfort's online view.

5. LED Indicators and What They Mean

The field case in the source post confirms that the AM2 LED is green, which means bus communication and module power are healthy. The fault therefore lies in the analog loop — not in module addressing or hardware failure.

6. LOGO!Soft Comfort Configuration Checklist

1. Open the program and select File → Properties. Under the AI Configuration tab, locate the AM2 channels.
2. Verify that AM2 I1 and I2 are set to Enabled.
3. Set the type dropdown to 0/4–20 mA. This selects the current input path through the internal shunt.
4. Set the type to 0-10 V by mistake → readings will remain at zero or near zero on mA sensors.
5. Add an Analog Input function block to your FBD diagram. The block dialog lets you bind it to AM2 I1, AM2 I2, or AI1–AI4 depending on the base variant.
6. Right-click the block → Block Properties → On Delay / Sensor → set sensor type to 4-20 mA so the block's 0-1000 scaling reflects 4-20 mA rather than 0-20 mA.
7. Save and download to the LOGO! base.

7. Step-by-Step Diagnostic Procedure

Use this matrix when the AM2 displays a fixed 0 or 4 mA reading despite a green LED and correct LOGO!Soft Comfort settings.

7.1 Step 1 — Isolate the Sensor from the PLC

Disconnect the field wiring from AM2 I1+ and I1−. Substitute a calibrated mA source in series with a 24 V supply and a known-good ammeter. Inject 12.000 mA and verify the LOGO! display reflects a proportional scaled value (e.g., 500 on a 0-1000 scaling). If the reading is correct, the sensor loop is the fault. If the reading is still 0, the AM2 input path is at fault.

7.2 Step 2 — Measure Loop Current with a DMM

Break the loop at the AM2 I1− terminal. Place the DMM in series (mA mode). Confirm the actual loop current:

• Reading < 4 mA: Loop is open, the active end is not supplying voltage, or the sensor has failed at minimum scale.
• Reading = 4 mA exact: Sensor is alive but at minimum output (often a sensor fault code for many transmitters).
• Reading = 20 mA exact: Sensor at maximum output — possibly saturating due to over-range input or a fault that drives the loop high.
• Reading tracks input correctly: Loop and sensor are healthy; suspect AM2 internal shunt failure.

This diagnostic method is the canonical procedure recommended for any 4-20 mA loop fault and is documented in manufacturer technical notes such as the AutomationDirect 4-20 mA troubleshooting guide.

7.3 Step 3 — Verify Compliance Voltage

Measure voltage at the transmitter's + terminal with respect to − while the loop is closed. For a typical 24 V supply, expect ≥ 12 V at 20 mA. If the voltage is below the transmitter's minimum compliance (often 10-12 V), the loop cannot deliver 20 mA and readings will top out prematurely.

7.4 Step 4 — Check Polarity

Reverse polarity is a common cause of zero reading on a current loop. Swap + and − at the AM2 terminal and re-test. Many sensors are protected against reverse polarity but still produce 0 mA when connected backwards.

7.5 Step 5 — Confirm AM2 Addressing

LOGO! addresses AM2 modules sequentially from the base unit outward. If a second AM2 or another expansion module sits between the base and the AM2 in question, the addressing shifts. In LOGO!Soft Comfort, use Tools → Hardware Catalog to confirm the AM2 slot position matches the physical order on the DIN rail.

8. Signal Range Mapping and Scaling

The scaling above assumes Sensor = 4-20 mA in the Analog Input block. If Sensor = 0-20 mA is selected, 4 mA will map to 200 and 20 mA will still map to 1000 — which produces a persistent 200-unit offset on every reading.

9. Common Fault Modes

10. Verification Procedure

After applying the corrective action, perform the following verification sequence before returning the system to production:

1. With the field transmitter disconnected, inject 4.000 mA, 12.000 mA, and 20.000 mA from the calibrated source and confirm the LOGO! AI block scaled value matches the table in Section 8 within ±1 unit.
2. Reconnect the field transmitter. Verify the live process reading matches a secondary reference (e.g., local display on the transmitter or a hand-held calibrator) within the sensor's stated accuracy.
3. Cycle power on the LOGO! base. Confirm the AM2 LED returns to solid green within 2 seconds and the AI value re-appears without re-configuration.
4. From LOGO!Soft Comfort, perform Online Test and capture a 60-second trend of the AI value. Confirm no dropouts, spikes, or step changes inconsistent with the process.
5. Document the corrected wiring diagram and the calibration points for the maintenance record.

11. Notes on Migration from AI1–AI4 to AM2

Engineers porting working 4-20 mA circuits from the LOGO! base unit's AI1–AI4 channels to an AM2 must:

• Delete the external 500 Ω resistor and any associated terminal block.
• Rewire the transmitter leads directly to the AM2 I1+ / I1− (or I2+ / I2−) terminals.
• Update the Analog Input block in LOGO!Soft Comfort from the AI1–AI4 source tag to the AM2 channel tag.
• Re-verify the sensor type setting (0-20 mA vs 4-20 mA) on the AI block.
• Re-test all four calibration points (4, 8, 12, 16, 20 mA) — wiring changes frequently introduce polarity inversions.

12. When to Suspect a Defective AM2 Module

The AM2 module is generally robust. A genuine module failure is rare but should be suspected only after ALL of the following are verified:

• Green LED solid (rules out power / bus fault).
• External wiring confirmed with a known-good loop and calibrated mA source.
• LOGO!Soft Comfort AI Configuration shows the channel enabled.
• AI block sensor type set correctly.
• Module firmware compatible with base firmware.
• Re-seated module on the DIN rail; verified mechanical latch.

If a substitute AM2 module from a known-good spare produces correct readings in the same slot, the original AM2 is defective and should be replaced with a new 6ED1055-1MA00-0BA2 unit.

Frequently Asked Questions