Wiring Siemens LOGO! AM2 AQ 0-10V Analog Output to a VFD

Overview

The Siemens LOGO! AM2 AQ analog output module (Siemens MLFB 6ED1 055-1MM00-0BA1) extends a LOGO! 8 base module with two independently configurable analog output channels. Each channel can be set to 0-10 V DC (voltage mode) or 0/4-20 mA (current mode), giving the LOGO! a closed-loop or setpoint capability typically used for proportional valves, chart recorders, and — the focus of this article — variable frequency drives (VFDs).

The most common field installation problem is that the LOGO! AM2 AQ wiring diagrams printed in the Siemens system manual only show the voltage output terminated across a load resistance of ≥5 kΩ. When an installer tries to map the U+ / M terminals directly onto a VFD that expects a 3-wire potentiometer input, the schematic can look ambiguous. This guide resolves that ambiguity with an explicit wiring procedure, including functional earth handling at X11 and DIP-switch setup per channel.

The signal kind selection for each channel can be specified at the device (via the underside DIP switch) and also via the LOGO! Soft Comfort software, as documented in the Siemens support article on AM2 AQ output behavior. Combining hardware and software configuration gives the AM2 AQ four usable modes: 0-10 V, 0-20 mA, 4-20 mA, and a reserved mode for diagnostics.

Module Identification and Ordering Data

Confirm the module on the bench before applying any wiring. The full Siemens order number is printed on the side label and must match the LOGO! 8 generation:

Terminal Layout

The AM2 AQ uses three removable terminal strips on top and one functional earth row on the bottom. The numbering follows Siemens convention for the LOGO! expansion family:

The two channels share a common 24 V ground reference and are not isolated from each other. If the VFD analog input is non-isolated and shares ground with its own DC bus, confirm that M1 (or M2) and the VFD analog-input common are at the same potential to avoid ground-loop currents.

Prerequisites

1. LOGO! 8 base module (any 12/24 V DC or 230 V AC variant) with firmware 8.0 or later, recognized by the expansion interface.
2. AM2 AQ module physically clicked onto the right side of the base. The right-side cover of the base must be removed before mounting the expansion; reinstall it after the last module.
3. 24 V DC power supply sized for LOGO! base load plus the AM2 AQ current draw. If the LOGO! base is a 230 V AC variant, the 24 V supply comes from the LOGO! internal bus rather than the field, but the AM2 AQ still draws its own current.
4. VFD with a differential or single-ended 0-10 V analog input. Verify in the drive manual that the input impedance is ≥ 5 kΩ — most modern drives present 20-100 kΩ, so the AM2 AQ loads them only minimally.
5. Shielded twisted-pair cable rated for the installation environment (e.g., Belden 8760, Lapp UNITRONIC LiYCY, or equivalent 2-core + drain).
6. LOGO! Soft Comfort V8.x or later installed on a programming PC with Ethernet connectivity to the LOGO! base.

Output Mode Selection — DIP Switch and Software

Each AM2 AQ channel has its own slide switch on the underside of the module housing. The two switches are physically independent and labeled "1" and "2" next to the channel number on the housing:

• Switch position "V" — Voltage output, 0-10 V DC, load ≥ 5 kΩ.
• Switch position "I" — Current output, 0/4-20 mA, load ≤ 250 Ω.

For a 0-10 V VFD reference, slide both switches to the "V" position. For 4-20 mA control (typical on current-loop drives or for noise-immune long cable runs), slide to "I" and adjust the LOGO! Soft Comfort analog-output block to map 0-1000 to 4-20 mA rather than 0-20 mA. The 0 vs 4 mA selection is a software decision: a normalized value of 0 produces 0 mA in current mode, but the block has a 4 mA offset option in the gain/scaling dialog. The Siemens support article describes this dual hardware/software mode selection in detail.

Wiring the AM2 AQ Voltage Output to a VFD

The 0-10 V channel of the AM2 AQ is a true two-wire source: the U+ terminal provides the variable voltage referenced to the M terminal. Unlike a 3-wire potentiometer output, the AM2 AQ does not need the VFD to source the excitation voltage. The VFD analog input behaves as a high-impedance load — the AM2 AQ drives the voltage, and the VFD only measures it.

Step 1 — De-energize

Remove all 24 V DC from the LOGO! base and the AM2 AQ module before making any field wiring changes. Lock-out / tag-out the 24 V supply and the VFD mains. Confirm absence of voltage at every terminal with a DMM set to DC volts.

Step 2 — Wire 24 V Power

Connect the LOGO! base 24 V to AM2 AQ terminals 1L+ and 1M. If a second expansion module follows the AM2 AQ, you may daisy-chain 24 V from 2L+/2M to that module's power input; otherwise leave 2L+ and 2M unused. Do not connect M1/M2 to 1M — they are not internally bonded and serve different functions.

Step 3 — Run the Shielded Cable

Pull a 2-core shielded cable from the AM2 AQ terminal block to the VFD analog-input terminals. Keep the run inside the same panel/conduit as the LOGO! supply and at least 200 mm away from VFD motor-side power cables. If the run must cross motor cables, do so at right angles.

Step 4 — Connect U+ to VFD Analog Input High

On the VFD, identify the 0-10 V input terminal (commonly labeled AI+, +V, AIN+, or REF+, depending on manufacturer). Connect the AM2 AQ U1+ (or U2+) terminal to this point.

Step 5 — Connect M to VFD Analog Common

Connect AM2 AQ M1 (or M2) to the VFD analog-input common (commonly AI-, COM, GND, or ACM). This is the path that completes the differential loop. The VFD input stage presents a high impedance between AI+ and AI-, so almost no current flows in this loop — but the return path is what defines the output voltage at the drive's input.

Step 6 — Bond the Cable Shield at X11

At the LOGO! end only, connect the cable shield drain wire to one of the two X11 functional-earth terminals. Do not bond the shield at the VFD end — single-point grounding at the source prevents shield-current loops that would otherwise inject noise into the 0-10 V signal. From the X11 terminal, run a short, heavy-gauge (≥ 2.5 mm² / 14 AWG) bonding jumper to the panel PE bar.

Wiring Schematic

Why the Siemens Schematic Shows a 5 kΩ Resistor

The Siemens wiring diagram in the LOGO! system manual shows a ≥ 5 kΩ resistor between U+ and M because the AM2 AQ voltage output stage requires that the load be at least 5 kΩ to remain within its compliance range. The resistor in the schematic is a representative load — your VFD analog input is the actual load. Most VFD analog inputs present 20-100 kΩ (Allen-Bradley PowerFlex 525 = 100 kΩ, Schneider Altivar 320 = 30 kΩ, Siemens SINAMICS V20 = 30 kΩ), so the AM2 AQ drives them easily. Do not install an external resistor across U+/M unless the VFD documentation specifically calls for one.

X11 Functional Earth Termination

The X11 row is a frequent source of confusion because it appears to be a second analog output terminal but is actually a dedicated functional earth (FE) clamp. It has two screw positions, used for the shield drain of one or two output cables. The two positions are internally bridged and can be used interchangeably.

Practical rules for X11:

• Use X11 only for cable shields — never for signal current or power return.
• If you run two output cables (one per channel), terminate each shield drain into a separate X11 position.
• Bond the X11 row to the panel PE bar with a dedicated short conductor. Do not daisy-chain FE through multiple modules back to a single bolt.
• Keep the bonding jumper ≤ 50 mm to minimize high-frequency impedance.
• Use a tinned copper braid or fine-strand wire (≥ 2.5 mm² / 14 AWG) rather than solid wire — solid wire breaks under vibration.

The X11 terminal has no internal electrical connection to any logic circuit. It is purely a mechanical termination for the shield drain. If you leave X11 unused, the shield drain should still be bonded to PE at the source end of the cable (at the AM2 AQ side, but at a different terminal block or clamp) for EMC compliance in industrial installations.

Software Configuration in LOGO! Soft Comfort

The AM2 AQ outputs are exposed in the LOGO! program as Analog Output tags AQ1 and AQ2. The block that drives them is the "Analog Output" block (function block B036 in LOGO! Soft Comfort V8.x), found in the Special Functions library under "Analog".

Placing the Block

1. Open your program in LOGO! Soft Comfort.
2. Drag the Analog Output block onto your FBD network.
3. Wire its input to the scaled setpoint (for example, the output of a PI controller, a math block, a multiplexer, or a numeric constant).
4. In the block properties, assign the block to AQ1 (or AQ2) and configure the gain/offset so that the engineering units you want map to 0-1000.
5. Download the program to the LOGO! base via Ethernet. The Analog Output block only becomes effective when the AM2 AQ module is recognized by the base; an unrecognized module leaves AQ1/AQ2 at 0.

Scaling Example — 0-60 Hz Reference

To drive a VFD from 0-60 Hz using a 0-10 V output, set the Analog Output block so that 0 LOGO! units maps to 0 V and 1000 units maps to 10 V. In LOGO! Soft Comfort this is entered as scaling with a low end of 0 V at value 0 and a high end of 10 V at value 1000. The relationship is linear: 1 LOGO! unit = 0.01 V. For a 0-50 Hz drive, the inverse gain is 20 Hz/unit (50 Hz / 1000 units = 0.05 Hz/unit, so the multiplier in the upstream math block is 20). The transfer function is documented in the Siemens support article on AM2 AQ output behavior.

Online Monitor

In Online mode (LOGO! Soft Comfort connected via Ethernet to the LOGO! base), right-click the AQ1 tag to read the live scaled value (0-1000) and the implied output voltage (0-10 V). If AQ1 reads 500 but the VFD receives 5.000 V, the scaling is correct. If the VFD receives something else, suspect the wiring, not the program. The online monitor also exposes the raw value before scaling for diagnostic purposes.

Block Enable vs. Disable

The Analog Output block (B036) has a digital enable input. When this input is LOW (logical 0), the AM2 AQ output holds at 0 V / 0 mA regardless of the analog input. Use this behavior for fail-safe coasting: if the LOGO! logic loses an interlock, force the enable LOW and the VFD will see 0 V (0 Hz reference) rather than the last commanded value. Always tie the enable to a known-good run-permissive, not just to the program scan.

VFD Parameter Configuration

Each VFD manufacturer uses different parameter numbers to configure the analog reference. The table below lists the most common settings for a 0-10 V speed reference. Always verify against the parameter manual for your specific drive firmware revision.

Verification and Commissioning

After wiring and programming, perform the following checks before applying full process load. Always work in coordination with the VFD commissioning engineer to avoid unexpected motor starts.

1. Power-up check: Apply 24 V to the LOGO!. Confirm the AM2 AQ green LED lights steady (no fault). A flashing red LED indicates a missing 24 V supply or a module that has not been recognized by the base.
2. Output voltage check: Force AQ1 to a known scaled value (for example, write 500 from LOGO! Soft Comfort online) and measure U1+ to M1 with a DMM. Expect 5.000 V ± 0.050 V. If the reading is 0 V or full-scale (10 V) regardless of input, suspect a DIP-switch mismatch or a missing 24 V supply at 1L+.
3. VFD input check: With the VFD in stopped state and reference source set to the AM2 AQ channel, monitor the drive's analog-input diagnostic. Most drives display the input as a percentage; 5 V should read as 50%.
4. Step-response check: Step AQ1 from 0 to 1000 in increments of 100, verifying the VFD speed tracks linearly. A non-linear response (jumps, dead band, saturation) usually points to the drive parameter (loss-of-signal scaling or input filter), not the LOGO!.
5. Ground-loop check: With the system running, measure AC millivolts between AM2 AQ M1 and VFD AI common. Any reading above ~50 mV AC indicates a ground loop — re-verify shield bonding (source-end only) and check that the panel PE bar is solid.
6. Noise check: Connect an oscilloscope (or DMM with min/max capture) to U1+ / M1 under normal motor operation. Peak-to-peak ripple should not exceed 50 mV. Higher ripple indicates either inadequate shielding or insufficient drive input filtering.

Troubleshooting Matrix

Edge Cases and Field Notes

Several less-common scenarios deserve mention for engineers retrofitting the AM2 AQ into existing panels.

Replacing a 3-Wire Potentiometer

If the VFD is currently configured for a 3-wire potentiometer (10 V excitation, wiper input), the AM2 AQ must replace the wiper only. The drive's 10 V excitation terminal stays open or is jumpered to common at the drive side per the drive manual. Wire AM2 AQ U1+ to the wiper input and M1 to the drive common. Reconfigure the drive to "2-wire voltage" reference rather than "3-wire potentiometer" — most modern drives auto-detect this, but check the parameter.

Output Isolation

The AM2 AQ does not provide channel-to-channel isolation. If you need to drive two VFDs from a single AM2 AQ and those drives are referenced to different grounds (e.g., one on a separate generator or on a different DC bus), you must add an external signal isolator on the second channel. A typical choice is a Phoenix Contact MINI MCR-2-UI-UI or a Weidmüller WAVEANALOG signal isolator, with 24 V DC loop power from the LOGO! panel.

Cable Length vs. Accuracy

The AM2 AQ voltage output is a true voltage source with finite output impedance (typically a few ohms). At very long cable runs (> 100 m) with high capacitance (typical of inexpensive shielded cable at 100 pF/m), the output may show a small attenuation or phase shift. For runs beyond 50 m, prefer current-loop output (4-20 mA) instead of voltage output, since current loops are immune to cable impedance. If voltage mode is required, use lower-capacitance cable (≤ 60 pF/m) and keep runs under 30 m.

Fail-Safe Behavior

If the LOGO! loses power or stops scanning, the AM2 AQ output drops to 0 V / 0 mA. This is the safe direction for most VFDs (zero reference = zero speed). However, some process applications require a defined minimum (for example, a cooling-fan drive that must stay at 30% minimum). For those cases, configure the VFD parameter "loss of reference" to a defined value (typically "minimum frequency" or "last value") rather than "stop", and accept the risk that a LOGO! failure will leave the drive at that minimum.

EMC Compliance

The AM2 AQ meets the industrial EMC requirements per IEC 61131-2 only when installed with shielded cable bonded at one end (the source end, as documented in this article). Verify that the shield drain path is continuous from the cable through X11 to the PE bar; any break in this path — including a shield drain wire wrapped around the cable jacket — defeats the EMC performance. Use proper shield-termination clamps (e.g., Weidmüller KLBÜ or Phoenix Contact S-RTN) rather than improvised wraps.

Frequently Asked Questions