Reading Siemens LOGO! AQ Output Values on HMI Display

Overview: Reading Siemens LOGO! Analog Output Values

The Siemens LOGO! 8 logic module (6ED1052 series) combines digital and analog I/O with a built-in HMI display and optional TDE (Text Display Expansion). Analog input signals enter through AI channels on the base unit or analog expansion modules (AM2 / AM2 RTD / AM2 AQ), are processed by function blocks in the LOGO! program, and are then routed to physical analog outputs (AQ1–AQ2 on the base or AM2 AQ expansion). The AQ function in LOGO! Soft Comfort is a write-only sink: the program value is pushed to the physical D/A converter, but the block exposes no feedback path that lets the application read what is actually being driven onto the wire.

As a result, an engineer who needs to mirror the same engineering unit (°C, bar, ppm, %) on the built-in LOGO! display, on a LOGO! TDE, or on a SCADA graphic must echo the value through an analog marker (analog flag) that the HMI can read back. This article documents the field-proven workaround used to display AQ values on a Siemens LOGO! HMI, with the additional step of simultaneously routing 4–20 mA to a SCADA system. Scaling math, function block selection, wiring notes, and verification tests are included for engineers commissioning a LOGO! 8 application where the analog output must serve both as a current-loop transmission and as a local display value.

Prerequisites

• LOGO! 8 base module with at least one AQ output, or a LOGO! 8 system with an AM2 AQ (6ED1055-1MM00-0BA2 family) expansion module configured for 0/4–20 mA.
• LOGO! Soft Comfort V8.2 or later installed on the engineering workstation.
• Analog input source wired to an AI terminal (e.g. 0–10 V temperature transmitter).
• 24 V DC loop supply sourced from the SCADA input for the 4–20 mA output.
• Either the built-in LOGO! display, a LOGO! TDE text display (6ED1055-4MH08-0BA0 family), or an external HMI / SCADA tag server that polls the LOGO! via S7 or Modbus TCP.
• Siemens LOGO! 8 system manual open for terminal assignments and AQ scaling conventions.
• Precision multimeter (true RMS, 0.05 % or better) for loop verification.

Why You Cannot Directly Read a LOGO! AQ Block

The AQ function block in LOGO! Soft Comfort presents a single output pin named AQ. In the compiled FBD / ladder program, this pin is bound to the physical D/A converter channel on the base module or AM2 AQ. The runtime does not expose a read-only echo of that value. Internally, the cycle reads the digital value from program memory, writes it to the D/A converter, and advances; the converter's output voltage or current is never sampled back into a program-accessible tag.

This behavior is by design across the LOGO! family and matches how a process analog output is defined at the engineering level: a digital-to-analog conversion with no defined read-back path. A separate analog input would be required to monitor the loop, but that defeats the purpose when the engineer already knows the requested value from the program and just needs to display it.

The Echo Workaround: Route the Value Through an Analog Marker

An analog marker (analog flag) is a 16-bit word that the LOGO! program can both write and read. By routing the same scaled value that feeds the AQ block into an analog marker, you create a tag the HMI display or SCADA can read. The marker becomes a shadow of the output value and updates every LOGO! scan cycle (typical cycle 1–10 ms depending on program size).

Available markers depend on the LOGO! 8 firmware version. In LOGO! Soft Comfort V8.x the project tree exposes Analog Markers with up to 64 words (AM1–AM64), plus 32 network markers (NA1–NA32) when network communication is enabled. Network markers are visible to other LOGO! devices and to external Modbus / S7 clients, which makes them ideal for SCADA integration.

Step-by-Step Implementation in LOGO! Soft Comfort

1. Open the LOGO! Soft Comfort project and identify the function block whose output drives the physical AQ (typically an Analog Amplifier block with output pin AQ).
2. Place an Analog Marker block on the diagram (toolbar: Marker → Analog Marker) and assign it the next free ID, e.g. AM01.
3. Wire the same scaled signal that feeds the AQ output to the input of AM01. Do not wire the AQ pin to AM01 directly; wire the upstream block output that drives AQ.
4. Configure AM01 as retentive only if the loop must retain its last value across power cycles; otherwise leave it non-retentive so the marker follows the live program value.
5. Compile and download to the LOGO! 8.

Scaling 0–10 V Input to 4–20 mA Output

The application in the source uses a 0–10 V temperature input on an AI channel and a 4–20 mA loop on AQ to a SCADA. The two ranges do not overlap natively: 0 V does not equal 4 mA, and 10 V does not equal 20 mA. A linear Analog Amplifier block performs the mapping, and the AM2 AQ hardware performs the voltage-to-current translation.

Internal Scaling Convention

LOGO! represents analog values as integers in the range 0–1000 for the default span. With the AM2 AQ configured for 0/4–20 mA, internal 0 typically corresponds to 4 mA and internal 1000 to 20 mA. Confirm the exact convention in the AM2 AQ device manual for your firmware build, as the offset interpretation has varied across firmware revisions (some legacy documentation treats 4 mA as internal 200 on a 0–1000 scale).

Linear Mapping Formula

The Analog Amplifier block implements the standard linear transform:

Out = ((In − InMin) × (OutMax − OutMin) / (InMax − InMin)) + OutMin

For a 0–10 V → 4–20 mA mapping with no dead-band compensation and a 1:1 engineering-unit pass-through, set:

• InMin = 0, InMax = 1000
• OutMin = 0, OutMax = 1000

The amplifier therefore reproduces the input value directly into the AQ channel, and the AM2 AQ module's hardware translates the internal 0–1000 value into 4–20 mA. If the SCADA expects a scaled engineering unit (e.g. 4.0–20.0 bar), apply the scaling after the AI amplifier rather than inside the AQ path so the local display and the SCADA share a single source of truth.

Example: Range 40–200 with /10 Display Divider

The field report uses a range of 40–200 representing a SCADA display value of 4.00 to 20.00 (bar, °C, or another unit). The reproducible pattern is:

1. AI 0–10 V → Analog Amplifier outputs integer 40–200 (i.e. engineering unit × 10).
2. The same integer 40–200 drives the AQ block, which the AM2 AQ converts to 4–20 mA.
3. For the local LOGO! display, divide the marker value by 10 using a math block, or display the marker with a decimal-point configuration so the operator reads 4.0–20.0.

The TI reference PLC Analog Output and Transmitter Introduction describes the parallel concept of pairing a 0–10 V / 4–20 mA output with a remote field element, which is the topology used here: a three-wire PLC output powering a remote SCADA / transmitter input.

Displaying the AQ Value on the LOGO! Built-in Display or TDE

Once the analog marker AM01 holds the scaled value, the LOGO! built-in display or a LOGO! TDE can render it through a Message Text function block.

1. Insert a Message Text block (Insert → Special → Message Text).
2. On the first line, enter a static label such as Loop mA: or Process:.
3. Open the text editor for the second line and drag the AM01 marker into the line. Configure the formatting to display two decimal places.
4. Configure the block's enable input to be always true (BQ=1) so the message is permanently shown, or trigger it from a digital input / cursor key on the TDE.
5. For bar-graph style indication, use the bar element option within Message Text and bind its length to AM01.

Sending the Same Value to SCADA via 4–20 mA

No additional programming is required if the AM2 AQ is already wired to the SCADA. The AQ pin in the FBD diagram carries the value that drives the converter, and the same upstream block feeds the analog marker used by the display. The loop therefore transmits the same engineering unit the operator reads on the HMI.

For SCADA that polls the LOGO! over Ethernet using S7 communication or Modbus TCP, expose AM01 as a holding register. The LOGO! 8 Modbus map places analog markers starting at the base of the holding register range when configured as a Modbus server. Consult the Siemens Industry Online Support LOGO! 8 communication manual for the exact register assignment for your firmware build, since register addresses have shifted between LOGO! 8 firmware versions.

Verification Checklist

1. Disconnect the field loop and connect a precision multimeter in series with the AQ terminal.
2. Force the AI input to the minimum (e.g. 0 V) and confirm the multimeter reads 4.000 mA within the AM2 AQ tolerance (typically ±0.5 % of full scale at 25 °C).
3. Force the AI input to the maximum (e.g. 10 V) and confirm 20.000 mA.
4. Read AM01 on the LOGO! display or TDE and confirm it tracks the AI value (e.g. 0.00 at 0 V input, 1000 at 10 V input).
5. If the SCADA is online, compare its PV against the multimeter reading; deviation should be inside the 4–20 mA loop accuracy budget (typically ±0.1 % of span for a modern SCADA analog input).
6. Power-cycle the LOGO! and confirm the displayed value returns to the live reading rather than a stale one if the marker was configured non-retentive.
7. Capture a 60-second trend of the marker on the LOGO! online monitor while the input is stable; any jitter larger than one LSB indicates scan-time interference or noisy AI wiring.

Troubleshooting Matrix

Field-Proven Patterns

Three patterns appear repeatedly in LOGO! 8 commissioning work where AQ must be both transmitted and displayed.

1. Single marker, single amplifier: One Analog Amplifier drives both AQ and AM. This is the cleanest topology and is recommended for new projects.
2. Dual amplifier for split ranges: Two amplifiers with different gains feed AQ and AM. Useful when the SCADA range differs from the local display range (e.g. 4–20 mA to SCADA, 0–100 % bar on the LOGO!).
3. Math-block division for engineering units: Place an Arithmetic block between the amplifier and the marker, dividing by 10 or 100 to obtain the operator-readable value while keeping the integer range for AQ.

Safety and Wiring Notes

• Never connect a voltage source to an AM2 AQ channel configured for current output; the internal shunt will be damaged.
• The AM2 AQ current output is passive on the LOGO! 8 side; the SCADA input must source the 24 V loop supply and provide a return path through the AQ terminal.
• Use shielded twisted pair for runs longer than 3 m and ground the shield at the panel end only to avoid ground loops.
• Confirm the AQ channel is configured for 0/4–20 mA in LOGO! Soft Comfort hardware configuration before energizing the field loop.
• For SIL or safety-relevant loops, add a dedicated AI feedback channel and a high / low alarm on the deviation between requested and measured current.

Frequently Asked Questions