Troubleshooting Siemens LOGO! 8 AM2 AQ: No Analog Output Fix

Troubleshooting Siemens LOGO! 8 AM2 AQ: Restoring a Dead Analog Output Channel

1. Problem Statement

A Siemens LOGO! 8 base module (6ED1052-1MD08-0BA0, LOGO! 8.3 / Firmware FS4) is mechanically and electronically mated to an AM2 AQ analog output expansion module (6ED1052-2MD08-0BA0 or -0BA1). The status LED on the AM2 AQ is illuminated green, the CPU is in RUN mode, and a test program has been downloaded that writes a raw value of 1000 to the analog output block. Despite these conditions, the voltage measured between terminals U1+ and M1 on the AM2 AQ is 0 V DC. The expected reading at a value of 1000 with a 0-10 V range is 10.00 V DC at the U1+ terminal referenced to M1.

This is one of the most common no-output scenarios reported with the LOGO! 8 / AM2 AQ combination. The root cause is almost never a hardware failure of the AM2 AQ module itself; it is overwhelmingly a configuration, firmware, wiring, or program-block wiring problem on the LOGO! 8 base. The troubleshooting matrix in this document covers all five categories and the verification procedure for each.

2. AM2 AQ Module Technical Reference

The AM2 AQ is the only analog output expansion module available for the LOGO! 8 family. Before troubleshooting the wiring, confirm the exact part in the panel by reading the article number on the front label of the module and the data matrix code on the side.

2.1 Terminal Layout (AM2 AQ top side, left to right)

1. L+ — 24 V DC supply positive (must be present)
2. M — 24 V DC supply negative (must be present, tied to base M)
3. U1+ — Voltage output channel 1, positive
4. M1 — Voltage output channel 1, negative / current return
5. I1+ — Current output channel 1, positive
6. U2+ — Voltage output channel 2, positive
7. M2 — Voltage output channel 2, negative / current return
8. I2+ — Current output channel 2, positive

A 0 V reading between U1+ and M1 with a correctly configured program and a present 24 V supply on L+/M means the module is either being held at 0 by the program, the analog output block is not being driven, or the firmware of the base does not match the project that was downloaded.

3. Five Root Causes of "No Analog Output" on AM2 AQ

Field data from LOGO! 8 commissioning shows the following distribution of root causes for the symptom "value of 1000 sent, U1+ measures 0 V":

Each cause produces the same external symptom (0 V at U1+/M1) but is fixed by a different action. The matrix below is the order in which a field engineer should work through them.

4. Step-by-Step Diagnostic Procedure

Step 1 — Confirm the catalog numbers and firmware versions

Open LOGO! Soft Comfort (LSC) and verify the following on the project that was transferred:

1. Menu Tools > Select Hardware > LOGO! Module: the base selected must read LOGO! 8.FS4 (6ED1052-1MD08-0BA0) or the specific article number printed on the label. If the selection is LOGO! 0BA6.Standard, 0BA7, or 0BA8.FS1–FS3, the program is not for the installed hardware.
2. Open File > Properties and check the LSC version. A LOGO! 8.3 / FS4 base requires LOGO! Soft Comfort V8.3 or higher to program the new function set. Older LSC can open the project but will only let you select older hardware targets.
3. On the physical base, navigate the on-device display: Menu > Diagnostics > Module and confirm the firmware version reported matches "FS4".

Step 2 — Verify the 24 V DC supply to AM2 AQ terminals L+ and M

1. With a calibrated multimeter set to DC volts (autorange, 30 V range minimum), measure between L+ and M on the AM2 AQ.
2. Expected reading: 20.4 V DC to 28.8 V DC for a LOGO! 8 base. Reverse polarity, missing wire, or a tripped external fuse will read 0 V or negative.
3. Confirm the source is the same 24 V DC rail feeding the LOGO! base. M of the AM2 AQ must be bonded to M of the base, otherwise the ground reference of the output floats.
4. Measure the same 24 V DC rail under load (e.g. while the base is in RUN); a sagging PSU under combined load can collapse the supply rail and the AM2 AQ will drop its output to 0 V as a protective behavior, even though its status LED remains lit from residual bus voltage.

Step 3 — Verify mechanical and bus engagement

1. Power down the LOGO! 8 base (24 V OFF, wait 5 s for the internal capacitance to discharge).
2. Slide the gray contact slider on the right side of the LOGO! base fully to the right (this exposes the expansion bus pins).
3. Mount the AM2 AQ onto the DIN rail and slide it left so its left-side pins fully engage the slider.
4. Push the contact slider on the AM2 AQ to the left to lock the bus connection to the next module (if any).
5. Re-apply 24 V DC and observe: the AM2 AQ status LED should be solid green, and on the LOGO! display Menu > Diagnostics > Module the AM2 AQ should now appear with its analog output icon (a triangle pointing out of the module).

If the AM2 AQ is not detected by the base, the on-device display shows only the base module. In that state the base cannot address the AM2 AQ outputs, and the analog value is not driven — exactly matching the symptom.

Step 4 — Verify program wiring of the analog output block

1. In LOGO! Soft Comfort, open the FBD editor and locate the block bound to AQ1. (For 0BA6-era projects, the block is labeled AM2 AQ with channel 1 / channel 2 selectors — re-create the block as an Analog Output block on the new FS4 target.)
2. Hover the input pin of the analog output block. It should show a wire connected to a value, a constant, or a function-block output.
3. If the input is unconnected, the block defaults to 0 — the symptom matches.
4. Confirm the gain / scaling: the raw value 0-1000 corresponds to the selected physical range. For 0-10 V, gain = 1.0; for 0-20 mA, gain = 1.0; for 4-20 mA, an offset of 200 (raw) is added by the block — meaning raw 0 yields 4 mA, raw 1000 yields 20 mA.
5. Download the corrected program using PC > LOGO! in LSC, watch for "Transfer complete" with no error code in the lower status bar.

Step 5 — Select the correct output type (voltage vs current) on the AM2 AQ

Unlike many analog modules, the AM2 AQ does not have a hardware jumper for V / mA selection. The selection is done on the on-device display or in the LOGO! TDE text panel:

1. On the base display: Menu > Setup > AQ1 (and AQ2).
2. Choose 0-10V, 0-20mA, or 4-20mA with the OK button.
3. Default after factory reset is 0-10V. If a previous program changed it to current mode, U1+ will read 0 V even with a perfect program — the output is being driven as a current source on the I1+ terminal, not as a voltage on U1+.

5. Program Snippet: Minimal Voltage-Output Test (LOGO! Soft Comfort FBD)

The following minimal program drives AQ1 with a constant 1000 (full scale on a 0-10 V output) and is a valid bench test for an AM2 AQ. Place an Analog Output block, set its address to AQ1, set its range to 0-10V, and wire a constant 1000 to its input. The transfer and run procedure is identical for any program.

Block 1:
  Type:        Analog Output
  Address:     AQ1
  Range:       0..10V
  Input pin X: Constant 1000
  Enable pin EN: tied to M28 (always-on flag) or hardwired to high

If the output on U1+/M1 is still 0 V after this program is in the base, the fault is not in the program — it is in hardware (Steps 2, 3, or 5 above).

6. Output Verification Procedure

1. With the corrected program downloaded and the base in RUN (RUN/STOP LED solid green, no flashing red error code), wait one full scan (≥ 100 ms).
2. Set the DMM to V DC, connect the red lead to U1+, the black lead to M1.
3. Expected reading: 9.95 V DC to 10.05 V DC at a raw value of 1000 in the 0-10 V range (the 10-bit DAC and the ±1 % output tolerance of the AM2 AQ account for the small spread).
4. Change the constant to 500 and re-transfer; expected U1+/M1 reading is 4.95 V DC to 5.05 V DC (mid-scale).
5. Change the constant to 0; expected reading is 0.00 V to 0.05 V DC. A reading above 50 mV at raw = 0 indicates a calibration error or a stuck DAC — return the module to Siemens support.

7. Error Codes and LED Patterns on LOGO! 8 / AM2 AQ

8. Common Pitfalls and Field Notes

• Target hardware in LSC: Always start a new project with File > New > LOGO! 8.FS4 when working with a 6ED1052-1MD08-0BA0 base. The dialog is sticky — once you save with the wrong target, subsequent opens default to that target.
• Analog output block naming: In 0BA6 the block was AM2 AQ; in 0BA7 it became AM2 AQ as well; in 0BA8 (FS1 through FS4) the block is Analog Output and is addressed by AQ1/AQ2 directly. If you copy-paste from an old project, the block may appear in the FBD but is treated as a no-op.
• SD card: If a program is loaded from an SD card on power-up and the SD card contains a 0BA6 program, the LOGO! 8 will run it. Delete the SD card or re-flash it with the FS4 image.
• Slot position: The AM2 AQ must be the last digital expansion (DM) module if one is in the chain. It occupies a slot in the expansion order; mis-ordering can prevent the base from detecting it.
• Current mode without load: If the program switches to 0/4-20 mA and the field wiring is on U1+/M1, the output stage is in current-limit and U1+ is not driven — the symptom is identical to "no output". Always toggle AQ mode in the on-device menu and re-measure.
• Common ground: A missing bond between the AM2 AQ M and the receiving instrument's signal ground produces a floating reading on a DMM. Bond M1 to the instrument's GND and re-measure.

9. When the AM2 AQ Itself is the Fault

If Steps 1 through 5 are all confirmed correct and the U1+/M1 reading is still 0 V at raw 1000 in 0-10 V mode:

1. Swap the AM2 AQ with a known-good spare.
2. If the spare produces 10 V on U1+/M1, the original module has a failed DAC stage or a cold solder joint at the output terminal — return to Siemens under warranty (article 6ED1052-2MD08-0BA0 or -0BA1).
3. If the spare also reads 0 V, the fault is in the base, the firmware, or the program — repeat Steps 1 and 4 with a fresh, known-good test project downloaded from the factory template.

10. Preventive Checklist for New LOGO! 8 / AM2 AQ Installations

• Print and attach the AM2 AQ article number to the cabinet door — saves 10 minutes of diagnosis later.
• Document the LOGO! Soft Comfort project version and the target hardware (FS4) in the program header comments.
• Use a fixed 24 V DC PSU sized for the sum of the base (typically 1.5 W) and the AM2 AQ (typically 1.0 W full load), plus all digital expansions — usually a 2.5 A PSU is sufficient for a 5-module stack.
• After first commissioning, perform the three-point test (raw 0 / 500 / 1000) and record the readings. Future drift indicates a hardware or scaling problem.
• Apply a ferrite to the analog output cable if the run is longer than 3 m to a noisy VFD or contactor panel — the 0-10 V output is single-ended and susceptible to common-mode noise.

11. Related Manuals and Documentation

• LOGO! 8 System Manual (6ED1050-1xx08-0BA1 / -0BA2) — chapter on expansion modules and AM2 AQ wiring
• LOGO! 8.3 (6ED1052-1MD08-0BA0) Product Information — confirms FS4 firmware level
• LOGO! Soft Comfort V8.3 Download and Documentation — for the FS4 target hardware
• LOGO! 8 Application Examples for Analog Signal Scaling — covers gain, offset, and the 0-10 V / 0-20 mA / 4-20 mA configuration