Connecting LOGO! 8 Analog Input to Off-Delay Timer Fix Block

Problem Summary

A user commissioning a LOGO! 8 12/24RCE logic module (article number 6ED1052-1MD00-0BA8) — supplied as part of the KP300 Basic Starter Kit — reported that the analog amplifier output Ax could not be wired to the time parameter T of an off-delay timer. The connecting line could not be drawn between the two block terminals in LOGO! Soft Comfort V8.x, even though the equivalent example program downloaded from the official Siemens applications portal wired the two terminals correctly.

The symptom manifests as follows:

• The Ax pin on the analog amplifier (B001 family) appears visually inert when the cursor hovers; no "snap" highlight is drawn.
• Drag-and-drop from Ax to T on a downstream off-delay timer produces no connector line.
• The status bar at the base of LOGO! Soft Comfort reports "no compatible connector" or remains silent when the connection attempt is rejected.
• The same program behaves correctly when the affected analog amplifier and timer blocks are deleted and re-inserted as fresh blocks from the toolbar.

The fault is reproducible only on copies of blocks lifted from external sources (Siemens sample ZIPs, PDFs, screen captures rebuilt from raster artwork, or copy/paste between two LOGO! Soft Comfort instances). Fresh blocks inserted from the editor toolbar accept the wiring immediately.

Affected Hardware and Firmware

The 0BA8 generation increases the analog input count to four and adds Ethernet-based VM (Variable Memory) access over the integrated RJ45 port. The connector rules described below apply identically across V8.0, V8.1, and V8.2 Soft Comfort builds.

LOGO! 8 Block Types and Connector Rules

LOGO! Soft Comfort is a graphical function-block language. Every block exposes one or more connector pins on its perimeter, each of a specific data type. The editor enforces type compatibility at the wire-drop step — no override is possible.

The most-misunderstood row is the second: the T pin on a timer is not a wireable connector. It is an inline parameter exposed inside the timer block's body for visual continuity, but the actual time value is configured through the block dialog (double-click on the timer block > "Par" button > "Time").

Connector Reference Rules

• Analog-to-analog only. An analog connector such as Ax on the analog amplifier can drive another Ax pin (e.g. on a second amplifier, on an analog comparator Ax, or on a PI controller Par). It cannot drive a digital pin such as Trg on a timer.
• Threshold trigger. If the application requires an analog signal to start or stop a timer, route the analog amplifier output into an analog threshold trigger (block B007) or an analog comparator (block B008), whose digital output Q then drives the timer's Trg input.
• Time scaling. To convert a 0–10 V potentiometer value into a variable time constant (e.g. 0 V = 0 s, 10 V = 60 s), scale the analog value to time inside the timer's parameter dialog using the gain/offset fields, or use a math block (B015) followed by a parameterized timer.
• VM mapping. Variable Memory addresses VW0..VW850 (read) and VW1000..VW1850 (write) carry analog word values across the program. Any Ax output can be routed to a VW location and re-consumed anywhere in the program, including in arithmetic blocks. The VM mapping icon at the lower toolbar opens the address editor.

Why the Connection Fails: Root Cause Analysis

The reported "cannot connect Ax to T" failure has three plausible root causes; only one matches the user's symptom.

The user's resolution — "I copied that block from Siemens example. When I put a new block in everything works" — confirms the second root cause. The Siemens sample ZIP convert_value_into_time_format.zip was authored in a Soft Comfort build that, when reopened in a different build, regenerates the connector metadata inconsistently for blocks that were pasted rather than freshly instantiated.

Step-by-Step Recovery Procedure

1. Backup the project. File > Save As > potentiometer_timer_v01.lsc. Confirm a copy exists in the project folder before any modification.
2. Identify the faulty block. Click once on the analog amplifier that originates the Ax pin. Read the block number in the status bar (e.g. B001). Note its coordinates on the grid.
3. Delete the faulty block. Select the block, press Del. Confirm Soft Comfort removes any dependent connector lines (they will turn red before deletion).
4. Insert a fresh block. Drag the analog amplifier icon from the right-hand "Analog" toolbar onto the same grid coordinates. Soft Comfort assigns the next available block number (e.g. B007 if five blocks were deleted).
5. Recreate the connections. Hover over the Ax pin of the new amplifier; the editor draws a green highlight when the pin is active. Drag to the Trg pin of the off-delay timer and release. The connection is stored as a directed edge in the project file.
6. Set the time parameter. Double-click the off-delay timer. Click Par. Enter the desired off-delay in the format HH:MM:SS. Save the block properties.
7. Simulate. Press F3 to start the on-screen simulation. Drive the analog input slider (Tools > Simulation > AI1) and confirm that Ax transitions trigger the timer after the configured delay.
8. Transfer to LOGO! Connect the Ethernet cable between PC and LOGO! 8. Use Tools > Transfer > to LOGO!. Enter the IP address (default 192.168.0.10, subnet 255.255.255.0) or use the LOGO! discovery scan.

Alternative Wiring: Analog Comparator as Trigger Source

The most common engineering intent behind "Ax to T" is: scale a potentiometer position into a time constant. The clean implementation uses an analog comparator (B008) to convert the analog signal into a digital edge:

1. Wire the potentiometer wiper to AI1 (terminal I7 on the 12/24RCE base module).
2. Insert an analog amplifier (B001). Set gain = 1.0, offset = 0.0. Output Ax carries the raw 0–1000 internal value.
3. Insert an analog comparator (B008). Set the threshold (e.g. 500) and the hysteresis (e.g. 50). Output Q goes high when the amplifier value exceeds the threshold.
4. Wire the comparator's Q output to the timer's Trg input.
5. Double-click the timer, set the off-delay (e.g. 00:00:30 for 30 s).

This configuration guarantees a clean digital edge to the timer regardless of noise on the analog input, and the hysteresis eliminates relay chatter at the threshold.

Alternative Wiring: Variable Memory Mapping

When the application requires the analog value to be readable from multiple downstream blocks (or from the KP300 HMI screen), expose the amplifier output via Variable Memory:

1. Open the VM mapping dialog (View > VM Mapping or click the VM icon in the lower toolbar).
2. Add an entry: source = Ax of B001, target = VW100. This places the analog value into a 16-bit word at VM address 100.
3. From any analog-capable downstream block, select AM1 (analog marker 1) and assign it to VW100 via the analog marker dialog.
4. On the off-delay timer, the time parameter can be set as a reference value if the Soft Comfort build supports parameterized time constants from analog sources. Most 0BA8 firmwares do not permit T to be driven directly by an analog marker — verify against the function block description in the LOGO! manual.

VM mapping is the recommended pattern for HMI dashboards: the KP300 Basic panel polls the LOGO! base module over Ethernet using the S7-compatible protocol and reads VW100 directly.

Scaling 0–10 V to a Time Parameter

To convert the potentiometer voltage into a proportional off-delay value, the most reliable method is a math block plus a parameterized timer:

1. Insert a math block (B015). Input Ax from the analog amplifier (0–1000). Set the gain = 60 (max seconds) and offset = 0. Output AQV ranges 0–60000 ms.
2. Insert a pulse generator or a delay-on-make timer with parameter reference set to AM1 where AM1 is bound to the math block's output.
3. Verify in simulation that sweeping AI1 from 0 V to 10 V produces a 0–60 s delay.

For higher precision, chain a math block followed by an analog multiplexer (B013) to switch between two preset time constants based on whether the analog signal is above or below a comparator threshold.

Verification Procedure

After the recovery, validate the program with this checklist:

• Static check (offline). Tools > Information > Program Check. Soft Comfort reports any unfilled mandatory connectors or type-mismatched wires.
• Simulation. F3 to enter simulation mode. Use the AI sliders in the lower toolbar to drive AI1. Confirm the timer status lamp illuminates after the configured delay when AI1 is removed.
• Online test. Transfer the program to the LOGO! base module. Use a 10 kΩ potentiometer between +24 V (terminal V+) and GND, with the wiper on I7. Apply and remove the +24 V supply on AI1; the output relay should release after the configured delay.
• HMI cross-check. On the KP300 Basic, configure a numeric display tag bound to VW100. Sweep the potentiometer; the value must track 0–1000 in real time.
• Diagnostic dump. On the LOGO! base module, navigate Diagnostics > Module Information > Program. Confirm the block list includes the new amplifier and the timer with the configured time constant.

Troubleshooting Matrix

Related Best Practices

• Always save a project backup before importing Siemens sample circuits. Use File > Save As with a versioned filename (..._v01.lsc, ..._v02.lsc).
• Annotate blocks. Right-click each block > Properties > Comment. The comment is exported in the documentation and survives Soft Comfort version migration.
• Document the VM map. Maintain a separate spreadsheet of VW assignments. Variable Memory addresses are global within a LOGO! program and overlapping assignments silently overwrite each other.
• Prefer the analog comparator over direct analog triggering for any timing application. The comparator adds hysteresis that prevents the timer from re-triggering on small analog fluctuations near the threshold.
• Use the LOGO! Web Editor (V8.2 and later) to access VM addresses from any browser on the same Ethernet subnet during commissioning.
• Validate power budget. The 12/24RCE supplies 5 V at 200 mA on its sensor supply terminals (V+). A 10 kΩ potentiometer draws < 5 mA — well within budget. If you stack third-party analog sensors, sum their consumption.

Standards and Reference Documents

• Siemens Industry Online Support — official knowledge base for LOGO! 8 documentation, firmware updates, and application examples.
• SIMATIC S7-1200 Manual Collection — Processing of Analog Values — conceptual reference for analog normalization and scaling; the same 0.0–1.0 internal normalization applies to LOGO! 8 analog blocks.
• LOGO! Application Example — convert_value_into_time_format.zip — official Siemens sample referenced by the user.

FAQ