Siemens LOGO! Backlight Flag Conflict: M25/M26/M27 Fix Guide

Problem Overview

Siemens LOGO! logic modules (generations 0BA6, 0BA7, 0BA8, and 0BA8 FS4) integrate a text display with a tri-state backlight: white for normal operation, red for alarm states, and amber for warning states on 0BA8 hardware and later. The display exposes four special marker bits — M25, M26, M27, and M28 — that the user program can drive to override the default illumination behavior.

When a project drives M27 (red backlight flag) to indicate an alert, a common engineering mistake is to leave M26 (white backlight flag) undriven. Under that condition, the display shows red during the alert and then goes fully dark the moment the alert clears. Button presses produce no backlight response. The operator cannot read the screen unless a fault is actively present.

This is not a hardware defect. It is documented firmware behavior: the moment any of the four backlight markers is referenced in the user program, the built-in default of "white backlight on every key press" is surrendered to the program. With M27 referenced but M26 not driven, no logic source is available to illuminate the white backlight during normal operation.

Symptoms observed in the field:

• Display remains dark between alert events; the white backlight never activates
• Pressing ESC, OK, cursor keys, or function keys on the LOGO! or TDE produces no illumination
• Red backlight activates correctly during M27 events and clears when M27 clears
• No backlight is observed in the seconds or minutes after the alert clears, even with active key presses
• LOGO!Soft Comfort online status shows the correct M27 transitions, but no M26 transitions
• Forcing M26 HIGH in online mode immediately produces a white backlight, confirming the marker path is intact

Affected Hardware and Firmware Generations

Refer to the Siemens Industry Online Support LOGO! system manual, "Display functions" chapter, for the exact marker behavior on each generation.

Backlight Marker Architecture

The four backlight markers are reserved in the LOGO! internal I/O image. They are sampled once per display refresh cycle and produce the visual states below.

When M27 and M25 are both HIGH, M27 wins because the alert is the most operationally significant state. When M26 and M27 are both HIGH, M27 still wins. When M25 alone is HIGH, white is held continuously and the M26 path is irrelevant. The exact resolution between M25 and M26 is "M25 overrides M26" — once M25 is set, M26 cannot assert.

The default key-press white backlight is implemented in firmware as an implicit OR of "any key pressed" with M26. Once the program references M25, M26, M27, or M28, the firmware removes the implicit "any key pressed" OR, leaving the program to drive M26 explicitly.

Root Cause of the Symptom

In the affected project, the program network contains a coil or contact driving M27, but no path to M26. The display driver therefore sees:

• M27 toggling HIGH and LOW with the alert input
• M26 always LOW, because nothing in the program ever asserts it
• M25 unused, so continuous white is unavailable

The user sees red during alerts and dark at all other times because there is no source driving the white-backlight path. The expected key-press white illumination never occurs because the firmware no longer applies the implicit default once M27 is referenced. This is a long-standing design choice in the LOGO! firmware; the user program is expected to manage M26 explicitly when any of the four markers is in use.

Diagnostic Procedure

1. Open the project in LOGO!Soft Comfort (V8.x for 0BA8 modules, V7.x for 0BA7, V6.x for 0BA6).
2. From the View menu, open the constant/connector list. Locate the Special Markers folder and verify the presence and tooltip of M25, M26, M27, and (on 0BA8) M28. Place a marker on the workspace and inspect Properties (left mouse click → Properties) to confirm the meaning of each marker — the dialog shows the system role of the special markers.
3. Use Ctrl+F to search the program for M25, M26, M27, M28 references. Note which markers are driven by user logic.
4. Connect LOGO!Soft to the live module via Ethernet (or via the LOGO! USB cable for 0BA6/0BA7 without Ethernet).
5. Open Online → Status. Force M27 HIGH and confirm the display turns red. Release the force.
6. Press a key on the LOGO! or TDE while monitoring M26 in the online status. If M26 never goes HIGH, the missing M26 path is confirmed.
7. Force M26 HIGH in online mode. If the display turns white, the marker path is intact and the program simply does not drive M26 from any source.

Solution 1 — Drive M26 From a Key or OR-Gate

The most flexible fix is to wire M26 to a logical source that asserts during normal operation but stays LOW during a red alert. A typical pattern is to OR the cursor-key flags (C1, C2, C3, C4) and use the result, gated by NOT M27, to drive M26.

Example FBD network (key-press OR with alert suppression):

           +-------+
   C1 ---->|       |
   C2 ---->|  OR   |--+--------+      +-------+
   C3 ---->|       |  |        |  +-->|       |
   C4 ---->+-------+  |  NOT   |--+   |  OR   |----- M26
                    +->|       |      |       |
                    |  | M27   |      |       |
                    |  +-------+      +-------+
                    |                       |
                    +-----------------------+  (OR with M25 if continuous white is also desired)

The NOT M27 gate prevents M26 from asserting while the red alert is active, eliminating red/white flicker. The cursor-key OR provides momentary white illumination on any cursor press. Add a pulse relay or on-delay if the desired white duration must exceed the firmware's built-in timeout (default 1 minute on 0BA6/0BA7; configurable on 0BA8).

On the TDE (LOGO! Text Display External), the equivalent of C1–C4 is the F1–F4 function-key flags, accessed via Tools → TDE Configuration → Function Keys.

Solution 2 — Use M25 for Continuous Backlight

If LED wear is not a concern, drive M25 HIGH in the program. The display then holds white at all times, ignoring the M26 path. M27 still overrides the white state during alerts, so red remains visible. After M27 clears, white returns immediately because M25 remains HIGH.

Typical use case: a control cabinet door-mounted LOGO! in a brightly lit area where constant readability is essential and the module is replaced on a planned maintenance cycle every 3–5 years. In this scenario, LED lifetime is not the limiting factor for module service life.

Solution 3 — TDE Function Key Assignment

The LOGO! TDE adds four ergonomic function keys (F1–F4) that can be mapped to any marker, including the backlight markers. This is the recommended approach when operators interact with the LOGO! primarily through a TDE rather than the integrated display.

1. Open the project in LOGO!Soft Comfort.
2. Select Tools → TDE Configuration (or right-click the TDE icon in the project tree and choose Properties).
3. In the Function Key Mapping tab, assign F1–F4 to marker outputs as required. A common pattern:
   • F1 → M26 (toggle white backlight)
   • F2 → M27 (toggle red backlight)
   • F3 → a screen-jump or a reset pulse
   • F4 → a system-level acknowledge
4. Some TDE configurations require the user to press the function key in combination with ESC. Confirm the modifier behavior in the TDE properties and document it on the operator panel label.
5. Compile and download to both the LOGO! base module and the TDE. The TDE and the base LOGO! must be on compatible firmware generations for the marker mapping to take effect.

Mapping F1 to M26 in the TDE configuration gives the operator manual control of the white backlight. This is the cleanest fix when the application requires operator-driven backlight management rather than automatic key-press illumination.

LOGO!Soft Comfort Configuration Steps

1. Launch LOGO!Soft Comfort matching the target module generation (V8.0 or later for 0BA8, V7.0 or later for 0BA7, V6.0 or later for 0BA6).
2. Open the affected project (.lsc, .lsc7, or .lsc8 file).
3. In the programming surface, place a marker block by selecting the marker icon and dragging it onto an empty network, or use the marker constant from the connector list (Co → Special Markers).
4. Verify that markers M25, M26, M27, and (on 0BA8) M28 appear in the Special Markers folder with their tooltip descriptions intact. If tooltips are missing, the project file may be corrupted — restore from the most recent backup.
5. Wire the chosen source signal to M26 according to Solution 1, 2, or 3 above.
6. Compile the project. Watch for warnings about unused markers or unresolved references in the Output window.
7. Download the project to the LOGO! module (PC → LOGO! in the Tools menu).
8. Switch to Online mode (Tools → Online Test or F11). Force M27 HIGH and confirm red. Force M27 LOW and press a key to confirm M26-driven white. If both confirm, exit online mode and save the project.

Verification Checklist

• [ ] M27 transitions to HIGH exactly when the alert input is true
• [ ] M27 transitions to LOW when the alert input clears
• [ ] M26 transitions to HIGH on every key press in the absence of M27
• [ ] M26 does NOT transition to HIGH while M27 is HIGH (no white-vs-red competition)
• [ ] If M25 is used, M25 stays HIGH continuously and red still appears during M27 events
• [ ] TDE F-keys (if used) map to the expected markers per the TDE configuration
• [ ] After the alert clears, the display returns to dark within the configured timeout (default 1 minute on 0BA6/0BA7; configurable 1, 5, 10, 30, 60 minutes, or "always on" on 0BA8) instead of staying white indefinitely
• [ ] The project compiles with no marker-related warnings
• [ ] The downloaded program survives a power-cycle test (cycle power, confirm backlight behavior matches the no-power state)

Backlight Timeout Reference

On 0BA6 and 0BA7 modules, the backlight-on duration is fixed at 1 minute after the last key press when M26 is HIGH. The 0BA8 generation introduced user-configurable timeouts accessible from the LOGO! main menu under System → Display → Backlight. Refer to the LOGO! 8 system manual on the Siemens Industry Online Support portal for the exact menu path on your firmware version.

Troubleshooting Matrix

Power and Lifetime Considerations

The integrated LOGO! display draws additional current from the 24 V DC (or 12/24 V DC, depending on the variant) supply when the backlight is active. Continuous M25 operation therefore increases steady-state consumption. For solar- or battery-powered installations, prefer the M26 key-press approach to keep the average backlight duty cycle below 5%.

Industrial-grade LED backlights are typically rated for tens of thousands of hours of continuous operation. The exact rating for the LOGO! display module is documented in the system manual for the specific module variant. M25 held continuously accelerates wear proportionally to the duty cycle. M26 driven by a momentary key press or by a slow pulse keeps the duty cycle low and is the recommended long-life configuration.

When the display reaches end-of-life, the typical failure mode is reduced luminance rather than complete blackout. If the operator reports dim readings on a mature installation, plan a display replacement rather than a backlight-only service. The backlight is not field-replaceable on most LOGO! variants; the entire front display module must be exchanged.

Migration Between Generations

Programs written for 0BA6 transfer directly to 0BA7 and 0BA8 without marker changes. The M28 marker is silently ignored on 0BA6/0BA7 hardware. Conversely, an 0BA8 program with M28 used must be reviewed before loading onto 0BA6/0BA7 hardware to ensure no operational logic depends on the amber state.

When migrating a TDE configuration, the function-key mapping persists across generations. The modifier behavior (ESC + F-key) on 0BA6/0BA7 TDE hardware may differ from the 0BA8 TDE; verify the operator workflow in the new environment before commissioning.

Best Practices for Alert Backlight Design

• Always drive M26 explicitly when M27 is in use. Do not rely on the implicit key-press default once any backlight marker is referenced.
• Use NOT M27 to gate the M26 source to prevent red/white flicker during the alert transition.
• Prefer M26-from-key-press over M25-continuous in unattended or 24/7 installations to extend display service life.
• Reserve M28 (amber) for warning-level events that require operator attention but do not constitute a fault; use M27 (red) for hard alarms only.
• Document the backlight marker map in the project comments so future maintenance engineers can quickly identify which marker drives which visual state.
• When commissioning, cycle the alert condition three or more times to confirm M27 clears cleanly and M26 returns to key-press responsiveness.

FAQ