Siemens LOGO! 8 Message Text: I/O State Display Limits & Fixes

Problem Overview

A common commissioning issue on the Siemens LOGO! 8 platform is the silent failure of a message text block to render the state of digital outputs on the device's built-in LCD, even though the Soft Comfort V8.x simulation shows them correctly. In a representative incident:

• Hardware: LOGO! 8 24RCE (6ED1052-1CC08-0BA1) base module with one LOGO! AM2 (6ED1055-1MA00-0BA2) analog input expansion.
• Program: A counter block driven by Q1 and Q2, plus a message text block expected to print the live state of Q1 and Q2 alongside other telemetry.
• Observed: Soft Comfort renders the I/O values in the message text preview. The same program downloaded to the physical LOGO! renders every other line of the message text correctly but leaves the Q1/Q2 cells visibly empty.

Two well-documented constraints, both firmware features rather than defects, combine to produce the empty cells:

1. Display geometry — the LOGO! 8 base-unit LCD is fixed at 6 lines × 16 visible characters. The Ticker attribute extends each line buffer to 32 characters and scrolls the line horizontally, but anything past column 16 is hidden until the ticker has run for at least one full scroll period. A placeholder dropped at column 17 or later is invisible to the operator unless the ticker is enabled and the line has scrolled.
2. Operand type — message text blocks accept only enumerated I/O status names, bar graphs, counter/timer values, and arithmetic results as placeholders. A raw digital Q operand cannot be inserted into a text field; it can only be referenced through its named I/O identifier, a pre-scaled analog flag, or an arithmetic instruction that coerces the boolean to an integer.

Both behaviors are documented in the LOGO! 8 system manual and in the Soft Comfort V8.x online help under "Message Text Configuration", available through the Siemens Industry Online Support portal at support.industry.siemens.com. This article consolidates the two constraints into one diagnostic procedure and three resolution paths.

Hardware and Software Inventory

Verify the active MLFB and firmware revision against the Siemens Industry Online Support portal at support.industry.siemens.com before ordering spares or staging firmware upgrades; the AM2 has been superseded by AM2 AQ on some current shipping lots.

Display Geometry and Ticker Behavior

The LOGO! 8 base unit (12/24RCE, 24RCE, 230RCE) integrates a 6-line STN LCD with a native visible width of 16 characters per line. When the message text editor enables the Ticker attribute, each line buffer doubles to 32 characters and the firmware scrolls the line horizontally so cells 17 through 32 pass through the visible 16. The ticker is global per message text block; you cannot enable it for individual lines.

The practical implication: if a placeholder for Q1 or Q2 is dropped at character 17 or later, the operator never sees the value through the visible window unless the ticker is enabled and the line has had time to scroll. With the ticker disabled, the placeholders sit silently in the off-screen buffer and the only reliable on-device indicator becomes the dedicated I/O STATUS menu (top-level ESC → "I/O STATUS"), where every input, output, flag, and analog variable is shown by its configured I/O name regardless of message text contents.

Operand Restrictions Inside a Message Text

The editor enforces most of these restrictions. The greyed-out toolbar entries in Soft Comfort V8.x (View → Toolbars → Message Text) make the contract explicit: only "I/O Status Name", "Bar Graph", "Analog Input", "Counter / Timer Value", and "Message Text Variable" appear as valid insertions. A raw Q-tag icon does not exist by design.

Solution 1 — I/O Status Name

The I/O status name is the canonical mechanism for surfacing the on/off state of a digital output inside a message text frame. It exploits the I/O name table that Soft Comfort V8.x maintains for every input, output, flag, and analog variable in the project. Configure it once per project; the names then propagate to the LOGO! display and the LOGO! web server (V8.1+).

Configuration in Soft Comfort

1. Project tree → right-click I/O Names → Edit. The dialog lists every symbolic identifier Soft Comfort can render.
2. Locate Q1 and Q2 in the Outputs block. Default firmware names are "Q1" and "Q2" until the user overrides them.
3. For each, set the Display Name to a ≤4-character string readable on a 6 × 16 LCD. Examples: PMP1, VLVE, RN1, RN2. Exceeding four characters causes firmware truncation and space padding.
4. Save the project and re-open the Message Text block. With the text cursor inside the line where the Q-state belongs, choose Insert → I/O Status Name (or the toolbar button). Pick Q1, then Q2 from the tree.
5. The placeholder "Q1" (or the configured display name) now appears in the text. When Q1 is on, the placeholder shows the configured name; when Q1 is off, it shows "0" (zero) or blank depending on the "Show 0" preference in the message text dialog.
6. Repeat for Q2 in the same line or a separate line, taking care to keep each placeholder starting within columns 1–16 of its line if the ticker is disabled.

Behavior Summary

If "Show 0" is disabled and Q1 is off, the placeholder renders as spaces, which can look identical to a hidden cell. Always leave "Show 0" enabled during commissioning and only suppress it after the operator is trained on what an absent name means.

Solution 2 — Bar Graph via Scaled Analog Flag

The bar graph placeholder in message text is analog-only; it does not accept a digital operand directly. To coerce a Q-bit into a bar-graph-friendly signal, multiply the boolean by 100 in an arithmetic instruction and store the result in an analog marker (AM) slot that the editor treats as an integer.

1. Add an Arithmetic Instruction block (Soft Comfort: drag from the analog function palette).
2. Configure inputs:
   Operand A = Q1 (digital, type BOOL)
   Operator = ×
Operand B = constant 100
3. Route the arithmetic output to AM1 (analog marker). The block converts the boolean to integer 0 or 100.
4. In the message text block, insert a Bar Graph placeholder. Configure:
   Source: AM1
   Min: 0
   Max: 100
   Axis Position: bottom
   Bar Height: 1 cell
5. Save and download. With Q1=1 the bar fills to the right edge; with Q1=0 the bar collapses to the baseline.

Solution 3 — Arithmetic Result and Counter Values

For applications that already use a counter driven by Q1 and Q2 (the incident under discussion included a "count how many times Q1 and Q2 switch" use case), the counter's current value is itself a directly renderable message-text placeholder. Place a numeric placeholder bound to the counter and the device prints the count as a signed integer.

Recommended Approach for a Boolean Visible as 0 / 1

1. Keep the existing counter block unchanged.
2. Add a one-shot pulse that increments a second counter (C2) on every transition of Q1; do the same for Q2 → C3.
3. Render the parity of C2 mod 2 as the indicator. The arithmetic block "C2 mod 2" yields 0 when the count is even and 1 when odd, which directly equals the present state of Q1 regardless of how many transitions preceded.
4. Place an arithmetic-output placeholder in the message text with two decimal places suppressed, so the operator sees just "0" or "1".

The practical advantage of this method is debuggability on the device itself: the I/O STATUS menu shows the counter value live, so the field operator can correlate the visible integer with the underlying count without a PC.

End-to-End Configuration Procedure

1. Open the LOGO! Soft Comfort V8.x project. Confirm the base model is set to LOGO! 8 24RCE (or a display-equipped variant). Programs targeting a CE base (no display) drop the symptom entirely because there is no LCD; verify before debugging.
2. Project Tree → Program → Message Texts. Open the existing message text block.
3. Set the Activation input. Two modes exist: level-triggered (message is active while the trigger signal is high) and edge-triggered (message latches on a rising edge and acknowledges via the front-panel OK button). For a status display following Q1/Q2 continuously, use level-triggered.
4. Set Line Count (1–6). Each line is rendered when both data and the activation condition allow.
5. If any line needs to reference placeholders past column 16, enable Ticker at the message-text level. The ticker is global per block; it scrolls every line that exceeds 16 chars at the firmware-configured cadence (default ~1 second per char-shift).
6. Set Show 0 = enabled for each placeholder if the operator needs to distinguish an OFF state from a fault. Suppress "0" only after the operator is familiar with the screen.
7. Insert placeholders. Toolbar button "I/O Status Name" opens a tree to pick Q1/Q2 by their I/O display name. Toolbar button "Bar Graph" opens analog source selection.
8. Validate in Soft Comfort by pressing F5 (Start Simulation). The simulation pane is character-identical to the LOGO! LCD for placeholders that are valid in this firmware revision.
9. Transfer → PC → LOGO!. On the LOGO! unit, the front panel shows the message text only if the block is active; if not, navigate the system menu to verify the program is in RUN mode.

Device-Side Commissioning and Verification

1. Confirm RUN state. The green RUN LED is solid, the red ERROR LED is dark. If the error LED is solid or flashing, the firmware has rejected the download; investigate before re-rendering message text.
2. Confirm the message text is active. Use the four-way navigation to cycle through "Program → Card → Setup → I/O STATUS". When the message text is active the LCD displays it; when not, the LCD returns to the configured default screen. The active message text takes precedence over all other on-device screens except a fault.
3. Force Q1 = 1 via the simulation input. From Soft Comfort, Tools → Simulate Inputs. Toggle the input that drives the Q1 coil. Watch the LCD: the I/O status name placeholder should flip from "0" to the configured name.
4. Force Q1 = 0 and confirm the placeholder reverses. If Show 0 is enabled, the placeholder shows "0"; if disabled, it shows blank.
5. Check the I/O STATUS screen. Press ESC from the main screen, navigate to "I/O STATUS", and scroll to Q1/Q2. The status screen is independent of message text and is the ground truth.
6. Cycle the backlight. Press ESC from the message text with no menu selection to wake the screen. If the backlight has timed out (Setup → Backlight → Mode = OFF after 60 s default), the LCD may simply be asleep while the message text is still being executed — this is a different "I don't see anything" symptom and resolves by configuring Mode = ON1 / ON2.

Diagnostic Matrix

Firmware Compatibility and Field Edge Cases

Soft Comfort V8.0 introduced the modern message-text editor and the I/O status name. V8.1 added the LOGO! web server, where placeholders render with HTML/CSS geometry rather than the LCD 6 × 16 grid. V8.2 added improved bar graph axis alignment and the "Show 0" preference. A project authored with pre-V8.0 Soft Comfort downloaded to a LOGO! 8 base is rejected at compile time; that is not the present symptom.

LOGO! 8 base firmware has been V8.x throughout active production; behavior differences between V8.0 and V8.3 firmware are confined to the web server rendering and the LWE (LOGO! Web Editor) project sync. The 6 × 16 LCD geometry has been unchanged. Confirm the active revision under the LOGO! main menu → Diagnostics → FW (or on the start-screen text) before assuming the display geometry has shifted.

• LOGO! 8 CE variants (no display): programs targeting a CE base can include message text blocks but nothing is rendered on the device. Use the LOGO! TDE (6ED1055-4MH00-0BA0) for local visualization, or the LOGO! Web Server accessible via Ethernet.
• Multiple active message texts: the LOGO! 8 firmware renders only one message text at a time. If two activate simultaneously, the lowest block number wins (priority 1 is highest). This can mask a placeholder rendering correctly while the operator believes it is broken.
• ACK vs level-triggered: a message text configured as edge-triggered with ACK requires the operator to press OK to clear. Until clearance, the placeholder content is frozen; "Show 0" preference does not update while the block is waiting for ACK.
• Persistent M / AM markers: markers marked persistent retain state across power cycles. A Q coil coming out of the program (e.g., via a retained relay output) can leave the I/O status name placeholder showing a "1" until the coil is explicitly reset.
• LWE upload path: when using LOGO! Web Editor (V8.1+) the placeholder types match Soft Comfort, but the LWE hides the "CharSet" preference. Projects uploaded through LWE may render alt-character placeholders incorrectly on the LCD until the project's CharSet is synchronized through Soft Comfort.

FAQ