Siemens LOGO! 8 Softkey ?N7 Error: Block Number Workaround

Overview

The Siemens LOGO! 8 Softkey is a special function block that maps one of the physical cursor keys on the LOGO! base module's integrated display to a Boolean signal inside the logic program. The Softkey is widely used in compact, panel-less installations where the operator must start/stop a process, acknowledge an alarm, toggle a mode, or step through a state machine directly from the LOGO! faceplate. The block is one of the few blocks that interact with both the program (logic) and the user interface (HMI segment on the LOGO! display).

This article addresses a display anomaly where the Softkey function block, when assigned a block number above 200, fails to render its proper label on the LOGO! display and instead shows a corrupted token (commonly reported as "?N7"). The cause is a known display/initialization limitation in early LOGO! 8 firmware releases. The article documents the root cause, the affected versions, the exact workaround, and the long-term remediation path (firmware update).

Problem Description

When a Softkey function block is added to a LOGO! 8 circuit diagram in LOGO!Soft Comfort, the engineering tool assigns a free block number. By default, this number is the next available integer in the project's monotonically increasing block sequence. After a project is expanded, refactored, or imported, the Softkey may end up with a high number such as B325, B340, B388, etc.

Symptoms observed at runtime on the LOGO! display (and in the LOGO!Soft Comfort online view) include:

• The Softkey block label is rendered as ?N7 or another non-decodable token instead of the expected softkey descriptor.
• Pressing the assigned cursor key (Esc, OK, Up, Down, Left, or Right) does not toggle the signal as expected; the input behaves intermittently or stays LOW.
• The Message Text block that references the Softkey (e.g., for status visualization on the LOGO! display) fails to update, or shows a static placeholder.
• The "Resources used" page in LOGO!Soft Comfort (View > Resources) reports an unexpectedly high block count or an unused Softkey function block entry.
• The anomaly appears immediately after program transfer to the LOGO! base module, even when the offline simulation in LOGO!Soft Comfort runs without errors.

Affected Hardware and Firmware

The display anomaly with high block numbers is documented against the following Siemens product family:

Root Cause Analysis

The LOGO! 8 base module's integrated display is a small dot-matrix LCD that is driven by a fixed-width character set. Each function block rendered on the display is identified by a block number, and the internal block-number-to-label mapping has a display-side table that is populated and interpreted by the base module firmware. In early LOGO! 8 firmware builds, this mapping table contained entries for block numbers up to approximately 200, and any block number exceeding that range was not resolvable by the display driver, leading to the substitution of a non-decodable placeholder character sequence such as ?N7.

Three internal conditions trigger the fault:

1. Block-number overflow: The Softkey is assigned a block number above the display driver's resolvable range. In practice this manifests at block numbers in the 200-400 range, which is the practical maximum number of function blocks supported by LOGO! 8 firmware.
2. Display initialization race: When the LOGO! transitions from Stop to Run, the display driver initializes the on-screen labels using the internal block table. If a high-numbered Softkey is referenced before the table entry is resolved, the driver falls back to a placeholder.
3. Message Text linkage: The Message Text block (B001-Bnnn) references the Softkey's input signal by symbol name. A corrupted display label does not affect the logic signal path, but the user-perceived behavior suggests the Softkey is non-functional because the display feedback is missing.

The condition is purely a display/label rendering issue, not a logical fault in the program. The Softkey's Boolean output is still present in the program; it is the user-facing feedback that is corrupted, which often leads engineers to assume the block itself has failed.

Diagnostic Indicators

Use the following checks to confirm the issue and rule out related faults before applying the workaround:

1. Inspect the block number. In LOGO!Soft Comfort, click the Softkey block in the FBD diagram. The block number is shown in the block header (e.g., B325). Numbers above B200 are suspect.
2. Open Resources view. Navigate to View > Resources in LOGO!Soft Comfort. The "Blocks used" counter should not exceed the firmware capacity (400 blocks for LOGO! 8). If the Softkey appears as B325 or higher, the workaround applies.
3. Check the LOGO! display. With the program running on the BM, navigate to the message text screen that uses the Softkey. If the Softkey identifier is rendered as ?N7, the issue is confirmed.
4. Verify firmware version. From the LOGO! front panel: ESC > Diagnostics > FW Version. Confirm the firmware build is in the affected range (ES8 early builds, first- and second-generation LOGO! 8 BMs).
5. Test with the online monitor. In LOGO!Soft Comfort, switch to online mode (Tools > Online Test). If the Softkey output toggles correctly when the cursor key is pressed, the logic path is healthy and the issue is purely display-side.

Prerequisites

Before applying the workaround, gather the following:

• LOGO!Soft Comfort V8.x (matching the firmware generation) installed on a Windows PC.
• Active project file (.lsc / .lscprj) that contains the affected Softkey block.
• LOGO! base module with the original program loaded (or the ability to re-transfer after the workaround).
• Ethernet (RJ45) cable or PC cable for LOGO! programming port.
• Free block numbers in the low range (B001-B050) verified in the Resources view.

Step-by-Step Workaround

The workaround exploits the fact that LOGO!Soft Comfort assigns the lowest free block number when a new block is added to the program. By deliberately freeing a low block number, you force the next Softkey insertion to occupy that low slot, avoiding the display driver overflow.

1. Save the current project. Make a backup copy of the .lsc project file before editing. Use File > Save As with a versioned name (e.g., Project_pre_softkey_fix.lsc).
2. Identify a disposable low-numbered block. In the FBD diagram, locate a small-numbered block that is trivial to recreate. Typical candidates: an AND (B001), OR (B002), NOT (B003), or a simple constant/flag assignment. The chosen block must occupy a number in the B001-B050 range.
3. Insert a placeholder block. Drag a different simple block (an AND gate is ideal) from the toolbar and place it at an empty spot. Allow LOGO!Soft Comfort to assign it the next free low number. For example, if B007 is free, place the AND at B007.
4. Delete the original low block. Select and delete the original B001 (or whichever low number you chose in step 2). This frees the low number while keeping the program functionally equivalent (the placeholder block at the higher number performs the same role, or the new block can be wired accordingly).
5. Delete the affected Softkey. Select the Softkey block (e.g., B325) and delete it from the diagram. Make a note of its signal name and the cursor key it was mapped to (Esc, OK, Up, Down, Left, Right).
6. Re-add the Softkey. Drag a new Softkey from the logic catalog (Special > Softkey) and place it at the same wire position. LOGO!Soft Comfort will now assign the freed low number (e.g., B001 or B007) to the new Softkey.
7. Reconfigure the Softkey. Open the Softkey properties dialog and re-assign the same cursor key and the same output signal name as the original block. Verify the Softkey "On" / "Off" behavior matches the original specification.
8. Re-link the Message Text. If a Message Text block referenced the Softkey by its output symbol, re-link the reference to the new (low-numbered) Softkey output. In the Message Text configuration, select the Softkey output as the bar graph source or text condition.
9. Verify the block number. Click the Softkey block in the diagram. The header should now show a low number (B001-B050). Confirm in View > Resources that the Softkey occupies a low slot.
10. Compile and simulate. Run the LOGO!Soft Comfort offline simulation (Tools > Simulation). Toggle the cursor key and verify the output transitions correctly.
11. Transfer to the LOGO! base module. Connect to the BM and download the program (Tools > Transfer > PC → LOGO!). Allow the BM to restart.
12. Verify on the LOGO! display. Navigate to the message text screen on the BM. The Softkey label should now render correctly (no ?N7 token) and the cursor key should toggle the signal as designed.

Verification

After the workaround is applied, perform the following acceptance checks:

1. Display label check. On the LOGO! BM, navigate to the message text screen containing the Softkey. The label must be human-readable (e.g., "START", "STOP", "ACK") and not contain a placeholder such as ?N7.
2. Functional check. Press the assigned cursor key on the LOGO! faceplate. The Softkey output must transition to HIGH on press (for momentary) or toggle (for latching), and the connected logic must respond as expected.
3. Online monitor check. In LOGO!Soft Comfort online test, confirm the Softkey output wire turns green/red as the cursor key is pressed.
4. Resource count check. Verify in View > Resources that the Softkey block number is in the B001-B050 range and that total block count has not changed.
5. Power-cycle test. Power-cycle the LOGO! BM. The label must still be correct after a cold start (this rules out a transient display state masking the issue).

Permanent Resolution: Firmware Update

The block-number overflow issue was addressed in subsequent firmware updates by Siemens. The recommended long-term remediation is to update the LOGO! base module firmware to a build that includes the corrected display driver table. Steps:

1. Identify the current firmware version: LOGO! front panel > ESC > Diagnostics > FW Version.
2. Obtain the latest firmware for the BM model from the official Siemens Industry Online Support portal at support.industry.siemens.com. Search for the relevant LOGO! 8 base module article (e.g., 6ED1052-1MD08-0BA1) and download the firmware update package (typically a SD card image or a LOGO!Soft Comfort compatible upgrade file).
3. Apply the firmware update per Siemens instructions. The LOGO! 8 base module supports firmware updates via SD card (a 32 GB SD card formatted FAT32 with the firmware file dropped in the root) or via LOGO!Soft Comfort in the latest version.
4. After the update, verify the firmware version is in the affected-free range (ES9 with LOGO! 8.1 FS:03, or ES10+ for LOGO! 8.2 / 8.3).
5. Re-transfer the program. After the firmware update, the block-number workaround is no longer required and projects can use freely assigned block numbers within the firmware's full 400-block range.

Softkey Block Reference

The Softkey block in LOGO! 8 maps one of the BM's front-panel keys to a Boolean signal. The following table summarizes its parameters:

For the canonical description, consult the LOGO! 8 system manual, available on the Siemens Industry Online Support portal under the LOGO! product family entry.

Prevention and Best Practices

To avoid the high block-number display issue in future projects:

• Keep the block count low. Refactor FBD diagrams to use as few blocks as possible. Group simple conditions into single AND/OR blocks, use analog flag blocks (B001-B008) for shared state.
• Reorder blocks on creation. When adding a new Softkey, place it in a section of the diagram that has a low-numbered wire position. Use the LOGO!Soft Comfort function Edit > Renumber Blocks if available in your tool version to force a compaction of the block-number space.
• Document key mappings. Maintain a project notes file listing the LOGO! 8 Softkey assignments (key → signal name → block number). This reduces time spent diagnosing similar issues on related projects.
• Standardize on a single LOGO! generation. Where the application allows, use the latest LOGO! 8.3 base modules (6ED1052-1MD08-0BA4 family) with current LOGO!Soft Comfort. These are not affected by the display driver limitation.
• Run a pre-handover simulation. Before on-site commissioning, run the LOGO!Soft Comfort simulation for at least 10 minutes while exercising every Softkey. Capture a screenshot of each message text screen for the commissioning report.

Related Issues and Diagnostic Matrix

The following table maps symptoms that may be confused with the Softkey block-number overflow issue to their true root causes:

FAQ