Resolving LOGO!SoftComfort Analog Threshold Output Link Failures

Problem Description

When configuring a temperature sensor with the Analog Threshold Trigger function block in LOGO!SoftComfort, the output terminal of the block appears available, but attempting to drag a connection from that output to a digital output (DO) point on the LOGO! 8 base module or its expansion rack produces no link. The connector visually does not attach, and the program refuses to compile or download. The same symptom is observed with other analog function blocks (Analog Amplifier, Analog Multiplexer, Analog Comparator, PI Controller) when their outputs are targeted at a physical DO.

The user typically sees an input terminal on the block (an analog input connection, e.g., AI1) but reports that the output is not selectable, not present in the right-hand toolbox, or cannot be connected to any Q1, Q2, Q3, Q4 output or to an expansion module digital output.

Root Cause Analysis

LOGO!SoftComfort builds its I/O palette from the hardware configuration that the programmer declares in the Hardware Catalog tree on the left side of the project workspace. The software does not automatically scan for the physical modules connected to a real LOGO! 8. The I/O points that the analog block can target are the union of:

1. Digital and analog I/O integral to the selected LOGO! 8 base module.
2. Digital and analog I/O contributed by every expansion module that has been manually inserted into the hardware tree.

If the 8DI/8DO expansion module (a DM16 type) is physically wired to the base unit but is not declared in the LOGO!SoftComfort hardware tree, its eight digital inputs and eight digital outputs do not exist in the compiler's address space. Dragging the function-block output onto a nonexistent Q-point produces a link that is silently rejected, the output pad remains unbound, and the project either refuses to simulate or fails to download with a topology mismatch when the runtime controller checks against the actual rack.

The Analog Threshold Trigger itself is a purely virtual function block. It has one analog input and one boolean output. The boolean output can be routed to:

• Any digital output (Q) of the base module that has been declared.
• Any digital output of a declared expansion module (Q of DM8, DM16, EM…).
• A virtual marker (M) or open connector used as a boolean signal in further logic.

If none of these are visible, the hardware configuration is incomplete.

Affected Components and Versions

This issue applies across the LOGO! 8 family (LOGO! 8.2 and LOGO! 8.3 base modules, both pure and with Ethernet) and to every release of LOGO!SoftComfort V8.0 through the current V8.4 generation. The behavior is the same whether you are using the Standard, Long, or FBD / UDF editor.

Refer to the LOGO!SoftComfort V8.4 download and release notes for the exact firmware (LOGO! BM firmware) and editor compatibility matrix before re-flashing or upgrading a controller.

Prerequisites

Before editing the project, confirm the following:

1. The actual rack composition is known: which base module, how many expansion modules, and in what order (the LOGO! 8 supports a maximum of 24 digital inputs, 20 digital outputs, 8 analog inputs, and 8 analog outputs across the base plus up to 24 expansion modules, depending on the specific modules).
2. The expansion module's part number and order code are available (printed on the front of the device or on the side label).
3. LOGO!SoftComfort is installed and the project is opened.
4. The project is not password-protected against hardware changes, or the password is known.
5. The LOGO! base module is offline (project edited in the editor) unless you are prepared to push the corrected topology to the live controller.

Step-by-Step Solution

Step 1 — Open the Hardware Configuration

In the LOGO!SoftComfort project tree (left pane), expand the node LOGO! Module or right-click LOGO! 8 in the project workspace. The exact label depends on the editor locale, but the icon is the small LOGO! device.

Select Hardware Configuration (or Properties) from the context menu. The Hardware Catalog dialog opens, displaying the currently declared modules in a list with columns for slot, module type, and I/O count.

Step 2 — Verify the Base Module Selection

Confirm that the first row of the catalog matches the physical LOGO! 8 base module on the desk. The article number (e.g., 6ED1052-1CC08-0BA1 for the 12/24 RCE variant, or 6ED1052-1MD08-0BA1 for the 230 RCE Ethernet variant) must appear here. A wrong selection yields a different I/O palette at the start of the list.

If the wrong base is selected, double-click the row and pick the correct part from the device list. Click OK to commit.

Step 3 — Declare the DM16 (8DI/8DO) Expansion Module

The DM16 expansion module (also sold as a digital mixed module with 8 digital inputs and 8 relay or transistor outputs) is declared by clicking the Add Module button in the Hardware Catalog. From the catalog list, select the matching article number:

• 6ED1055-1CB00-0BA2 — DM8 24R (4DI / 4DO relay)
• 6ED1055-1NB10-0BA2 — DM16 24R (8DI / 8DO relay)
• 6ED1055-1FB00-0BA2 — DM8 230R (4DI / 4DO relay, 115/230 V AC)
• 6ED1055-1CB10-0BA2 — DM16 24V (8DI / 8DO transistor)

Drag the module into the next free slot, or use the Add arrow. The slot number increments automatically. The I/O column for the module shows the eight new digital inputs (Ix) and eight new digital outputs (Qx) that will now be available in the editor's toolbox.

Step 4 — Declare Any Analog Expansion (If Used for the Temperature Sensor)

If the temperature sensor is wired to an AM2 (6ED1055-1MA00-0BA2) or AM2 RTD (6ED1055-1MD00-0BA2) expansion because all four integral AI inputs are already in use, the AM2 must also be declared. The AM2 adds AI5 and AI6 (or AI5–AI8 for two AM2 modules) to the I/O palette. After the AM2 is added, its inputs appear in the toolbox and can be dragged onto the Analog Threshold Trigger input pad.

Step 5 — Re-open the Function Block Diagram

Close the Hardware Catalog and return to the FBD / ladder view. The I/O Toolbox on the right side of the editor (or the top, depending on the layout) now lists the digital outputs of the DM16 module as Q5 through Q12 (or whatever offset the slot sequence produces). These Q-points are real, addressable outputs that the Analog Threshold Trigger can drive.

Step 6 — Link the Analog Threshold Trigger Output

1. Click the output pad (the small box on the right side of the Analog Threshold Trigger block) until a "rubber band" connector follows the cursor.
2. Drag the connector over the desired Q-point in the toolbox. The Q-point highlights.
3. Release the mouse button. A solid line now connects the block output to the digital output.
4. To route to a virtual marker instead of a physical Q, drop the connector on M1 through M27 markers in the toolbox; the boolean state is then available for downstream logic without consuming a physical output.

Step 7 — Parameterize the Threshold

Double-click the Analog Threshold Trigger block to open its parameter dialog. The key fields are:

For a PT100 temperature sensor, set the sensor type to PT100 (only available when the input is connected to a hardware AI on the LOGO! 8 base or on an AM2 RTD). The block then linearizes the signal to degrees Celsius over the range -50 °C to +200 °C.

Step 8 — Compile and Simulate

Press F5 or click Simulation on the toolbar. The Simulation tool draws the analog input as a slider and the digital outputs as LEDs. Force the input above the on threshold and confirm that the Q-point connected to the Analog Threshold Trigger illuminates. If the LED does not light, re-check the threshold values and the wiring of the threshold trigger's input to the correct AI.

Step 9 — Download to the LOGO! 8

Switch from Simulation to Online mode via Tools → Transfer → PC → LOGO! (or press Ctrl+D). The editor sends the program and the topology. The LOGO! 8 enforces the topology match; a topology mismatch produces a download error. Once the transfer completes, the controller runs the new program with the declared DM16 in the correct slot.

Verification

To confirm that the fix is correct:

1. In LOGO!SoftComfort, open Tools → Online Test (or press Ctrl+I). The I/O table now shows the AI from the temperature sensor and the Q-point of the DM16 that the threshold trigger drives.
2. Raise the process temperature above the on threshold. The Q LED in the Online Test window turns on, and the corresponding relay LED on the physical DM16 module illuminates.
3. Lower the temperature below the off threshold. The Q LED turns off and the relay drops out.
4. Toggle to LOGO! Display mode on the physical base module and navigate to the I/O status screen. The same Q-point should show the active state.

Common Pitfalls and Workarounds

Advanced Notes on I/O Addressing

The LOGO! 8 address space is continuous across the base and its expansion modules. Digital inputs increment from I1 on the base through the modules in physical order. Digital outputs increment from Q1 on the base in the same fashion. Analog inputs and outputs follow the same rule, starting at AI1 / AQ1.

For a base module with four integrated outputs (Q1–Q4) and one DM16 expansion in slot 1, the DM16 contributes Q5–Q12. A second DM16 in slot 2 contributes Q13–Q20. A second expansion slot beyond the maximum topology produces an error in the compiler and the extra I/O are not generated.

The maximum supported topology for a LOGO! 8 base is documented in the LOGO!SoftComfort Online Help PDF and in the LOGO! 8 system manual on the Siemens LOGO! product page. Exceeding the limits silently drops the overflow modules from the palette.

Using the Analog Flag as an Alternative

If the analog value must be fanned out to multiple boolean consumers without re-reading the sensor, route the AI through an Analog Flag (block group: Analog → Analog Flag). The Analog Flag accepts an analog value as input and exposes the same value as a reusable analog signal in the toolbox. Connect the output of the Analog Threshold Trigger to a marker (M1–M27) and then drive multiple Q-points from that marker, rather than chaining the block to multiple Q-points directly.