Resolving LOGO! AM2 6ED1055-1MA00-0BA2 Input Not Working

The LOGO! AM2 expansion module (Siemens part number 6ED1055-1MA00-0BA2) is one of the most frequently misconfigured analog input modules on the LOGO! 8 platform. The two most common reasons an AM2 input appears "dead" are (1) the AI address was never enabled in the LOGO! Soft Comfort project, and (2) the connected transmitter is 4–20 mA while the AM2 input is configured for the 0–10 V range. This article documents both root causes, the exact configuration path, and the scaling mathematics required when you wire a 4–20 mA sensor into a 0–20 mA input.

1. Problem Description

Symptoms reported in the field:

• The AM2 module powers up normally; the green Power LED is lit; no error (red) LED.
• LOGO! Soft Comfort online monitor shows the AI tag stuck at 0 regardless of the input voltage or current applied at the terminals.
• Connecting a 4–20 mA calibrator (Fluke 754, Beamex MC6, or similar) to the input has no effect on the program variable.
• The base module's display (if present on LOGO! 8 BM with text display) shows ---- for the expected AI tag.

In every case reviewed, the wiring was correct. The failure mode is configuration-driven, not hardware-driven.

2. Root Cause Analysis

2.1 AI Address Pool Not Activated

The LOGO! 8 family supports a maximum of eight analog input addresses, numbered AI1 through AI8. These addresses are not statically bound to physical channels. The mapping is dynamic and is determined entirely by the I/O settings you define in LOGO! Soft Comfort. If you do not declare the analog inputs in the project file, the analog amplifier / analog math blocks cannot resolve the Ax reference, and the AI variable remains undefined at runtime.

The base module can reserve zero, two, or four AI addresses depending on whether you have a base module with onboard analog inputs (LOGO! 12/24 RCE, 24 CE, etc.). The first AM2 expansion module picks up the first two unallocated addresses after the base's allocation.

2.2 Wrong Measurement Range Selected

The AM2 input is software-switched between two physical ranges:

A 4–20 mA transmitter driving an input configured for the voltage range will produce a near-zero reading because the internal sense resistor is not in the current path. Likewise, a transmitter driving an input configured for current but with the I/U jumper in the wrong position will read as a constant saturated or zero value.

3. Solution: AI Address Configuration

Follow these steps in LOGO! Soft Comfort V8.x or later.

1. Open the project file (.lsc or .lsc + .lma archive).
2. From the menu bar select File → Properties.
3. In the Properties dialog, click the Offline settings tab.
4. Click the I/O settings button (or sub-page, depending on Soft Comfort version).
5. In the Analog Inputs column for the base module, set the number of analog inputs the project will use on the base. Typical choices:
   • 0 — No base-module AIs. Use this if the base is a LOGO! 230 RCE or pure digital base.
   • 2 — Reserve AI1 and AI2 for the base (typical for 12/24 V bases).
   • 4 — Reserve AI1 through AI4 for the base (LOGO! 12/24 RCEo with optional AI4 expansion slot).
6. Click OK and confirm the prompt to update I/O addresses.
7. Place an Analog Amplifier block (or Analog Comparator) in your FBD program and select the correct AI tag:
   • If base is set to 0 AI: AM2 ch1 = AI1, AM2 ch2 = AI2.
   • If base is set to 2 AI: AM2 ch1 = AI3, AM2 ch2 = AI4.
   • If base is set to 4 AI: AM2 ch1 = AI5, AM2 ch2 = AI6.

4. Solution: 4–20 mA Scaling on a 0–20 mA Input

The AM2 has no native 4–20 mA mode. The input must be configured for the 0–20 mA range, and the resulting 0–1000 raw count must be scaled in software to remove the 4 mA offset and normalize to the engineering range. The transfer function is:

Engineering_Value = (Raw_Value − 200) × Full_Scale / 800

This is derived from:

• 0 mA → Raw 0
• 4 mA → Raw 200 (live zero, subtract this offset)
• 20 mA → Raw 1000 (full scale)
• Usable span → Raw 200 to 1000 = 800 counts

4.1 Worked Example: 0–16 bar Pressure Transmitter

4.2 Worked Example: 0–500 mmH2O Differential Pressure Level Transmitter

4.3 Implementing the Scaling in LOGO! Soft Comfort

Two practical methods exist.

Method A — Analog Amplifier with Gain and Offset:

1. Place an Analog Amplifier block (FBD → Special → Analog Amplifier).
2. Wire the AI raw tag to the input.
3. Set Gain = Full_Scale / 800.
4. Set Offset = −200 × Gain (i.e., negative of the live-zero count times the gain).
5. Add an Analog Math block downstream if clamping below zero is required.

Method B — Analog Math block expression:

Output = (AI_raw − 200) × Full_Scale / 800

Set the Analog Math block's operand set to Gain mode and disable any output clamping you do not want.

5. Wiring and Grounding

Incorrect grounding is the second most common reason for unstable or zeroed analog readings on the AM2. Siemens support document ID 109741041 — Connecting LOGO! inputs — explicitly addresses this. Key points:

• The encoder / transmitter supply ground must be bonded to the FE (functional earth) terminal on the expansion module, not left floating.
• The shield of a shielded signal cable should be terminated at the FE terminal at one end only (LOGO! end) to avoid ground loops.
• For current inputs, the transmitter positive terminal connects to I1 (or I2) and the negative to M on the AM2. Do not connect the loop return to PE; return to M.
• The I / U selector jumper behind the AM2 terminal cover must be set to I for 0–20 mA mode. With power applied, removing the cover to flip the jumper will not damage the module but will interrupt the loop.

6. Verification Procedure

After applying the configuration and scaling changes, verify the installation in this order:

1. Wiring continuity: With the loop powered, measure the loop current with a precision multimeter in series. Confirm it tracks the process variable within the transmitter's stated accuracy.
2. Jumper position: Power down, remove the AM2 terminal cover, and confirm the I/U jumper on the active channel is set to I.
3. Online monitor: In LOGO! Soft Comfort, switch to Online Test (or press F9). The AI raw tag must track the applied current:
   • 0 mA → raw 0
   • 4 mA → raw 200
   • 20 mA → raw 1000
4. Scaled output: Place a temporary Message Text block on the scaled engineering variable. Apply 4, 12, and 20 mA and confirm the displayed value matches the calculation in the tables above.
5. SD card / LOGO! Cloud: If using a LOGO! 8.3 BM with SD logging or LOGO! Soft Comfort Cloud, log the raw and scaled values over 60 seconds to confirm stability (jitter < ±2 raw counts is typical).

7. Diagnostic Matrix

8. Specifications Quick Reference — LOGO! AM2 (6ED1055-1MA00-0BA2)

9. Related Official Documentation

• Siemens Support — Connecting LOGO! Inputs (ID 109741041)
• Siemens Industry Online Support (search "LOGO! AM2")
• Siemens LOGO! Product Page

10. Field-Proven Caveats

• The AM2 is not galvanically isolated from the base module bus. If the transmitter ground is tied to a remote earth different from the LOGO! cabinet ground, install an isolated signal conditioner (e.g., Siemens 3RS70 series) between the transmitter and the AM2.
• The I/U jumper is a physical switch behind the removable terminal cover. Power-cycle the LOGO! after moving it so the analog front-end re-acquires the correct mode.
• LOGO! Soft Comfort V8.0 has a known behavior where the I/O settings dialog will silently keep the previous AI count if you cancel the dialog. Always close with OK and re-open to confirm the value persisted.
• If the project was migrated from LOGO! 6 or 7, the AI mapping is not auto-rewritten. Audit every Ax reference after migration.