Suppressing -127°C Fault on LOGO! Display and Web Editor

1. Overview

Siemens LOGO! 8 (6ED1052) logic modules report a fixed out-of-range value of -127 °C when a PT100 or PT1000 sensor on an AM2 RTD expansion module is removed, broken, or short-circuited. The same code is propagated to the analog input word, to the built-in 6-line BM display, and to any custom page created in LOGO! Web Editor (LWE). Showing the literal string "-127" or a reading that flickers to that value is rarely acceptable in a user-facing dashboard.

This reference documents a deterministic technique to make the broken-sensor reading invisible on both the LOGO! BM/TDE display and the LWE page, while preserving normal numeric display when the probe is healthy. The approach relies on LOGO! Soft Comfort 8.x digital-state visibility, a one-pixel transparent PNG, and a comparator function block that flags the -127 °C value.

The technique applies to all LOGO! 8 generations (6ED1052-1MD00-0BA0, 6ED1052-1CC01-0BA0, 6ED1052-1HB00-0BA0, the 6ED1052-...08-0BA1 8.3 release) and to the LOGO! TDE external text display (6ED1055-4MH00-0BA0).

2. Why -127 °C Appears on Disconnect

The AM2 RTD (6ED1055-1MA00-0BA0 / -0BA2) and AM2 PT100 (6ED1055-1MD00-0BA0) modules measure resistance on a 2-, 3-, or 4-wire lead. The BM linearises this resistance to °C using the IEC 60751 curve, but the linearisation routine is bounded. According to the LOGO! 8 system manual, when the input resistance falls below the measurement window (broken wire, removed probe) the channel saturates to a fixed error value that the application can detect without additional hardware:

Detecting the fault therefore reduces to a single threshold comparison inside the LOGO! program: AI1 < -126. The comparator output is then used both to gate the message text on the BM display and to drive the visibility tag of an LWE overlay element.

3. Prerequisites

• LOGO! 8 base module: 6ED1052-1xxx08-0BA1 (recommended) or earlier 6ED1052-1xxx0x-0BA0 with firmware > 1.80. LOGO! 8 System Manual (Siemens Support, entry ID 109751604).
• LOGO! Soft Comfort 8.4 or later for the integrated Web Editor. Download from Siemens Support entry 109773404.
• AM2 RTD expansion module (6ED1055-1MA00-0BA0 or -0BA2) for PT100/PT1000 input, or an AM2 (6ED1055-1MA00-0BA0) with 0–10 V transducer.
• Optional LOGO! TDE (6ED1055-4MH00-0BA0) for the mirrored colour text page.
• A 1×1 pixel transparent PNG (or any small transparent image). A reference asset is bundled with LOGO! Soft Comfort 8.4 in the ...\Examples\WebEditor folder.
• Web browser supporting inline SVG / PNG (Chrome 90+, Edge 90+, Firefox 88+).

4. Method A — Suppressing the Reading on the LOGO! BM / TDE Display

The BM and TDE render a message line whenever a configured "message text" (function block B035) is active. Two text boxes can share the same line: one displays the live temperature, the other overlays a blank when the fault condition is true.

4.1 Configure the message text

1. Open LOGO! Soft Comfort, drag Message text (B035) from the special-functions palette.
2. In Input, wire the analog sensor value AI1 into parameter Aq (or use an arithmetic block to scale the integer to °C with one decimal: (AI1 - 200) / 10 on the default -50…+200 °C range).
3. Tick Show bar graph off; tick Acknowledge off so the line updates each cycle.
4. Enable the Message text input. Wire the inverted sensor-OK bit (see 4.2) into the enable line so the entire text block is blanked when the probe is bad.

4.2 Build the sensor-OK bit

Add an Analog comparator (B014) with:

• AI1 (raw PT100 reading) as AX.
• Constant -126 as AY.
• Function = |AX - AY| (mode 13, "difference > threshold") with a small gain, or mode 05: AX > -126.

The comparator output Q1 stays 1 whenever the live reading is in the valid range. Invert it (B017 NOT) and feed it to the enable input of the message text. With Q1 = 0 the line is empty; with Q1 = 1 the value renders normally. Place a second message-text block, tied to the same AI but with constant text "--- °C", and switch between them by enabling the "healthy" block on Q1 = 1 and the "placeholder" block on Q1 = 0.

4.3 Hiding the value while keeping the line visible

On the LOGO! 8 BM the line cannot be selectively transparent, but the B035 block allows the entire text to be replaced with whitespace. A pure-space string   (five non-breaking spaces for a 6-character field) renders as a blank bar. The same placeholder can be the white-on-white background colour, which on a LOGO! 8.3 BM (FS:04 and later) is selectable from BM setup → Display → Background.

5. Method B — Hiding the Reading in LOGO! Web Editor

LOGO! Web Editor (LWE) is the integrated HTML/SVG dashboard that ships inside the BM firmware. It is configured offline in LOGO! Soft Comfort 8.x and uploaded to the BM's microSD card as logo.bmp / logo.svg assets. The visibility of every element can be bound to a LOGO! variable, including digital markers, network inputs, and the result of a comparator block.

5.1 Bind visibility to a digital tag

1. Open Tools → LOGO! Web Editor.
2. Add a text field for the temperature reading and bind its Text to a LOGO! variable, e.g. VW0 (the integer holding the °C × 10 value).
3. Add a second text field with five space characters. Place it exactly on top of the first.
4. Open Properties → Visibility on the "real" text field and select Bind to variable → SensorOK (the comparator output from 4.2). Repeat for the placeholder text field with the inverted variable.

When the probe is healthy, SensorOK = 1, the real text shows and the placeholder is hidden. When the probe is removed, SensorOK = 0, the real text disappears and the blank string is shown instead. From the operator's perspective the value "fades out" rather than jumping to -127.

6. Transparent Overlay Technique

The LWE visibility binding is binary, so the placeholder is either fully visible or fully hidden. A more elegant dashboard is the two-image technique where the visible state is chosen by selecting between a background-coloured rectangle and a fully transparent one. The operator only ever sees the value, never the masking element.

6.1 Prepare the assets

1. Create a 1×1 px PNG. Fill the pixel with the EXACT background colour of the LWE page (sample with the colour-picker). Save as bg_color.png.
2. Create a 1×1 px PNG with alpha = 0. Save as transparent.png. Many image editors will save this; verify by re-importing — the preview tile must show a checkerboard pattern.
3. In LWE, drag the temperature value field onto the canvas. Drop bg_color.png behind it; align the rectangle exactly to the value field's bounding box.
4. Drop transparent.png on top of the same rectangle. Both rectangles share identical geometry.

6.2 Wire the assets to the sensor-OK bit

The mask is invisible because the colour of bg_color.png matches the LWE canvas background. The operator sees the value, sees it disappear, and never sees a square. This same technique is used by the LOGO! Soft Comfort sample project Tank_Level_Web.lsc shipped with the editor.

7. Sample LOGO! Program

The following FBD-equivalent ladder can be assembled in LOGO! Soft Comfort in under five minutes. Comments reference the block numbers so the program can be rebuilt from the description alone.

B001  AI1      (PT100/PT1000 raw, integer °C × 10, range -500…+2000)
B002  Arithmet.  VW0 = AI1 (passthrough, used as shared tag)
B014  Comparator  AX = AI1, AY = -1260, GAIN = 1, HYST = 5
       Mode 05 (AX > AY)         Q1 = SensorOK
B017  NOT(Q1)                 Q1 = SensorNotOK
B035  MessageText
       En    = SensorOK
       Text  = "Tank 1: " & VW0/10 & " °C"
B036  MessageText  (placeholder)
       En    = SensorNotOK
       Text  = "Tank 1: --- °C"
LWE   Bind "Tank 1: {VW0/10} °C" field to visibility SensorOK
LWE   Bind bg_color.png to visibility SensorOK
LWE   Bind transparent.png to visibility SensorNotOK

Wire the analog channel to AI1 in the BM configuration menu and select the correct sensor type (PT100, PT1000, Ni1000, or 0–10 V). The hysteresis of 5 in B014 prevents chatter when the wire resistance oscillates around the open-circuit threshold.

8. Verification

1. Healthy state: With the probe connected, the BM display shows the live value, e.g. "Tank 1: 23.5 °C", and the LWE page shows the same value with no overlay.
2. Simulated open circuit: Disconnect one leg of the PT100 lead at the AM2 RTD terminal. Within one cycle (typically 200 ms with the default AI smoothing) the BM line changes to "Tank 1: --- °C" and the LWE value field becomes blank.
3. Re-connect: Re-fit the wire. The display and LWE return to the live value within one cycle. No operator action is required; the system self-heals.
4. Edge case: Apply a deliberate short circuit across the PT100 terminals. The same -127 °C code is produced and the same masking logic activates.
5. Log check: From the BM menu Diagnostics → Web server access log, confirm that the LWE page refresh request returns the new state within the polling interval (default 5 s).

9. Edge Cases and Field Tips

• Multiple sensors: Build a sensor-OK bit for every PT input. Do not OR them together; the mask must be per-channel, otherwise one bad probe hides the others.
• Scaling with arithmetic block: The AM2 RTD default scaling is -50…+200 °C integer -500…+2000. A 0.1 °C resolution is achieved by dividing the AI integer by 10 inside an arithmetic block (B005) before passing to the message text and to the LWE tag.
• 2-wire vs 3-wire PT100: 2-wire mode is the most failure-prone; even a loose terminal produces the -127 °C fault. For long runs prefer 3-wire and a shielded cable to keep the false-trip rate low.
• BM display vs TDE: The TDE mirrors the BM message-text line. Masking logic applied to the BM propagates to the TDE automatically; no extra wiring is required.
• Memory cost: Each LWE page costs roughly 4 kB on the microSD. Four sensors plus a trend image fit inside the BM's 4 MB web project budget.
• Firmware compatibility: The LWE visibility binding for the "transparent PNG" technique requires LOGO! 8.2 (FS:02) firmware 1.82.01 or later. The technique works on every subsequent release including the 8.3 generation. LOGO! 8 firmware update notes (Siemens Support, 109773404).
• Browsers: Microsoft Internet Explorer 11 is the last version to ship with the LOGO! legacy web view. Use the LWE renderer (Chrome, Edge, Firefox) for the two-image overlay; IE11 collapses the alpha channel of transparent.png to white.
• Power-on transient: During the AM2 RTD's first scan after BM boot, AI1 may briefly return 0 before the lineariser settles. Mask the first 1 s with a TON (on-delay, B004) of 10 cycles on the SensorOK bit to avoid a 0 °C flash on the LWE page.

10. Comparison: Display vs Web Editor

11. Troubleshooting Matrix

12. Frequently Asked Questions