Resolving LOGO! V8.4 M27 Message Acknowledgment Failure on the LOGO! TDE Display

This field reference documents a confirmed firmware defect in Siemens LOGO! 8.4 base modules (MLFB 6ED1052-2MD08-0BA2) running firmware V1.84.02, in which the Message Text function block (FB M27) cannot be acknowledged from a connected LOGO! TDE text display (MLFB 6ED1055-4MH08-0BA1, FW 1.83.01) while the M27 enable bit is in the ON state. The same program runs correctly in LOGO!Soft Comfort simulation and on the previous-generation LOGO! 8.3 (6ED1052-2MD08-0BA1) hardware. The defect is resolved in firmware V1.84.03, which is delivered as part of the LOGO! 8.4 SP1 (Service Pack 1) update.

1. Problem Description

The reported failure is reproducible on the following stack:

Symptom matrix observed in the field:

In the affected configuration, when the operator presses the ACK softkey on the TDE while M27 is in the ON state, the acknowledgment is not registered, the message text remains in the alarm banner, and the TDE logs the keystroke but does not transition the message to the acknowledged state. The same program downloads into LOGO! 8.3 hardware (6ED1052-2MD08-0BA1, FW 1.83.02) and acknowledges correctly. The same program also acknowledges correctly under LOGO!Soft Comfort simulation against the 8.4 target.

Inputs used in the failing reference application:

• I1 – used to enable / disable the M27 function block (EN input).
• I2 – used as the message trigger (e.g., the Boolean condition that the M27 message text reflects).

The defect is mode-sensitive: in manual mode (M27 disabled) the operator can acknowledge every message, but the moment the process transitions to auto mode (M27 enabled, EN = 1), the TDE ACK key becomes ineffective.

2. Root Cause

Siemens identified the symptom as a regression introduced between the LOGO! 8.4 initial release (FW 1.84.01) and the first public refresh (FW 1.84.02). The bug resides in the message-text handling of the LOGO! 8.4 base module firmware, specifically in the state machine that services the TDE ACK command when the corresponding M27 block is currently selected (i.e., its EN input = 1).

The Siemens Knowledge Base article that documents the fix is 109975995. The corrective firmware is V1.84.03, bundled in LOGO! 8.4 SP1. Engineers who attempted to apply the patch using the original 8.4 toolset and ended up at V1.84.02 reported no improvement, which is the expected outcome because V1.84.02 is the broken intermediate release. The fix is only present at V1.84.03 and later.

Key behavioral fingerprints that confirm this specific defect (and not, for example, a TDE-side issue, a character-set mismatch, or a wiring fault):

1. The TDE firmware version (1.83.01) is unchanged from the working 8.3 installation — the TDE is not at fault.
2. Simulation in LOGO!Soft Comfort acknowledges correctly — the program logic is correct.
3. Loading the same program into a 6ED1052-2MD08-0BA1 (LOGO! 8.3) base module with the same TDE works correctly — the application is portable.
4. Only the combination of LOGO! 8.4 base FW 1.84.02 with the M27 EN bit in the ON state reproduces the fault.

3. Affected Hardware and Firmware Versions

4. Resolution

The resolution is to update the LOGO! 8.4 base module firmware to V1.84.03 or later using the LOGO! 8.4 SP1 installer:

1. Download LOGO! 8.4 SP1 from Siemens Industry Online Support. The installer includes the new base-module firmware (V1.84.03) and the matching LOGO!Soft Comfort engineering revision.
2. Connect the PC to the LOGO! base module via Ethernet (the standard LOGO! 8.4 service port on the bottom of the module, default IP 192.168.0.3, subnet 255.255.255.0).
3. Open LOGO!Soft Comfort V8.4 SP1, go to Tools → Transfer → Update Firmware (menu name varies slightly by SP1 build), and select the target module.
4. Confirm the update; the base module reboots automatically. Do not power-cycle the module during the flash write.
5. After the module returns, verify the firmware revision on the TDE under Menu → Diagnostics → Module → FW Version or by reading the module information page in LOGO!Soft Comfort. The string must read V1.84.03 or later — V1.84.02 confirms the update did not take.
6. Re-download the application program (the M27 ladder / FBD) to the module.
7. Test the M27 acknowledgment path with EN = 1.

5. Verification Procedure

Use the following field checks to confirm the fix is active before signing off the work order:

1. Firmware string check. On the TDE, navigate to Menu → Diagnostics → Module. The base-module firmware line must show V1.84.03 (or later). If it shows V1.84.02, repeat the update — the flash was either skipped or interrupted.
2. EN = 0 acknowledgment. Force the M27 EN input LOW. Trigger a message by setting the message trigger (I2 in the reference application). Press ACK on the TDE. The message must clear from the alarm banner.
3. EN = 1 acknowledgment (the failing case). Force the M27 EN input HIGH. Trigger a message. Press ACK on the TDE. The message must clear from the alarm banner. This is the case that was broken on FW 1.84.02.
4. Toggle stress test. Cycle the EN input ten times in quick succession while messages are queued. Every message must be acknowledged on each cycle. This guards against any partial-state regression in future firmware revisions.
5. BM setting audit. Confirm that the TDE has all relevant BM (Betriebsmeldung / Operating Message) settings enabled, as in the original report. The fix is independent of the BM configuration, but leaving BM off will mask other acknowledgment symptoms and confuse root-cause analysis later.

6. M27 Message Text Function Block — Reference

The M27 Message Text block is the LOGO! user-facing alarm and status display primitive. It is widely used because it requires no additional HMI hardware beyond the TDE or the on-board LOGO! display, and because it supports both static and dynamic text (up to four lines, with embedded tag values from the LOGO! variable pool).

7. TDE Display BM Settings — Engineering Notes

The TDE has its own set of operating-message (BM, Betriebsmeldung) parameters that govern which messages appear, how they are prioritized, and which keys are bound to which actions. The relevant menu path is:

TDE → Menu → Setup → Message Settings → BM

Recommended baseline for an M27-driven alarm system:

8. Simulation vs. Hardware Behavior

One of the more confusing aspects of this defect is that the LOGO!Soft Comfort simulation environment does not reproduce it. Engineers investigating similar symptoms should follow this triage tree before assuming a firmware bug:

If the simulation already fails to acknowledge, the defect is in the application program (wrong M27 parameter, missing EN, character set mismatch, or TDE softkey mapping) and the firmware fix will not help.

9. Common Pitfalls During the Update

1. Stopping at V1.84.02. The most common cause of "I updated and the issue is still there" is that the engineer used the original 8.4 installer (which only knows V1.84.02) instead of the SP1 installer (which delivers V1.84.03). Always start the update from LOGO! 8.4 SP1.
2. Forgetting to re-download the program. Some early SP1 builds require a fresh program download to re-bind message-text tables. If the M27 messages still do not appear after the firmware update, perform a full program transfer (PC → LOGO!) and confirm in the on-line view that the M27 block is highlighted in the active message queue.
3. Power-cycling during flash. Interrupting the firmware write can leave the base module in bootloader mode. Recovery is performed by re-running the SP1 update — never by power-cycling repeatedly.
4. Wrong Ethernet subnet. The default IP of the LOGO! service port is 192.168.0.3. If the PC is on a different subnet (for example 192.168.1.x), the update utility will not see the device. Either change the PC's IP or change the LOGO!'s IP via the TDE (Menu → Network Settings).

10. Related Issues and Compatibility

The following items are commonly investigated alongside this defect and are listed here for engineering hygiene. None of these caused the failure in the field report, but each one has been misdiagnosed as the root cause at some point.

11. Field Commissioning Checklist

Use this checklist when commissioning a new LOGO! 8.4 + TDE stack with M27 message text. It isolates the M27-ON-state acknowledgment path early in the FAT, before the panel is shipped.

1. Confirm base module MLFB and firmware string. Reject any unit still on V1.84.01 or V1.84.02; require V1.84.03 or later.
2. Confirm TDE MLFB and firmware string. Accept V1.83.01 unless Siemens has published a mandatory TDE update alongside the SP1 release.
3. Download the program and verify M27 in the on-line view.
4. Force EN = 1 in the program, trigger a message, press ACK on the TDE, confirm the message clears.
5. Force EN = 0, trigger a message, press ACK, confirm the message clears.
6. Cycle EN ten times and re-verify.
7. Record the firmware string in the commissioning report — both base and TDE.

12. Standards and Reference Documents

The following Siemens Industry Online Support resources are the authoritative references for the LOGO! 8.4 family, the SP1 update, and the M27 function block. Engineers should consult the latest revision of each before commissioning.

• Siemens KB 109975995 — LOGO! 8.4 firmware update information (M27 TDE acknowledgment fix)
• LOGO! 8.4 product support page (manuals, firmware, FAQs)
• LOGO! 8.4 system manual (basic units, TDE, expansion modules)
• LOGO!Soft Comfort V8.4 online help (M27 Message Text function block reference)

13. Summary

The M27 acknowledgment failure observed on a LOGO! 8.4 base module (6ED1052-2MD08-0BA2) running firmware V1.84.02 is a known firmware regression. It is not a program fault, not a TDE fault, and not a character-set issue. The corrective action is to update the base module firmware to V1.84.03 using the LOGO! 8.4 SP1 installer, then re-verify the M27 EN = 1 acknowledgment path on the TDE. The TDE firmware does not require a corresponding change. Engineers should record the new firmware string in the commissioning report, particularly in safety-relevant applications where the acknowledgment path is part of the operator's corrective action.

FAQ