Reading LOGO! 8 BA5 Temperature Sensor Values via Message Text

Overview

This article documents the procedure to surface a live temperature sensor reading on the integrated 6-line display of a Siemens LOGO! 8.3 base module (the hardware family identified by the ...08-0BA5 article-number suffix), and to calibrate the sensor zero-point and gain offsets directly inside LOGO! Soft Comfort V8.3. The procedure applies to PT100 and PT1000 resistive temperature detectors (RTDs) connected natively to the analog inputs, as well as to 0-10V / 0-20mA / 4-20mA transducers such as the LM35 (10 mV/°C) and LM335 (10 mV/°K) that require software scaling. The only LOGO! tool that can render a live block value on the local display is the Message Text function block, so the calibration workflow is structured around how Message Text binds to an analog input value and to the output of an Analog Amplifier (B008) or Analog Math (B002) scaling block.

The canonical reference is the LOGO! 8.3 System Manual (Siemens ID 109769968) and the LOGO! 8 System Manual (109757154) on the Siemens Industry Online Support portal. The product landing page is at siemens.com/logo and the LOGO! Soft Comfort programming software page is at siemens.com/logo-soft-comfort. Application examples for temperature control are bundled with LOGO! Soft Comfort under File > Example Projects > Temperature.

System Identification and Prerequisites

The "BA5" suffix on a LOGO! 8 base module designates the LOGO! 8.3 generation hardware. Confirm the part number on the device label or in LOGO! > Diagnostics > Device Information before starting; the relevant article numbers for the LOGO! 8.3 family are:

On every 12/24 V and 24 V base module the four analog inputs share physical terminals with digital inputs I1-I8; this is documented in the LOGO! 8.3 system manual chapter "Wiring the LOGO!". PT100 and PT1000 RTDs can only be wired to AI3 (I7) and AI4 (I8) on the 12/24 V variant because the RTD bridge is hard-wired to those two channels. The 24 V variant does not include the RTD bridge and requires an external 0-10 V or 4-20 mA temperature transmitter. The 100-240 V variant supports 0-10 V only and has no RTD support on board.

Prerequisites for the calibration workflow:

• LOGO! 8.3 base module powered up and reachable on Ethernet (or via the micro-SD card programmer) with firmware FS:05 or higher, paired with LOGO! Soft Comfort V8.3 (article 6ED1058-0BA08-0YA1).
• Sensor wired to the correct terminal pair: PT100/PT1000 to I7/I8 (AI3/AI4) on a 12/24 V base module; 0-10 V transducer to I1/I2 (AI1/AI2); 4-20 mA transducer through a 500 Ω burden resistor to AI3/AI4 on a 24 V base module.
• Calibration reference: a stirred ice-water slurry (0.0 °C ± 0.1 °C) and either a certified thermometer or a Type K dry-block calibrator covering 0-100 °C.
• For remote display verification: a browser pointed at the LOGO! Web Editor (HTTP on port 80, default credentials LOGO / LOGO) or the LOGO! app for iOS / Android.

Understanding LOGO! 8 Analog Inputs

The Analog Input (AI) block in LOGO! Soft Comfort has three sensor-type modes that return values in different engineering units. Choosing the wrong mode is the single most common reason a user sees a meaningless value (such as "300" or "-273") on the Message Text. The three modes are documented in the LOGO! 8.3 system manual chapter "Analog value processing".

PT100 / PT1000 native mode

When the AI block is configured with sensor type PT100 or PT1000 on a 12/24 V base module, the LOGO! applies the IEC 60751 platinum curve internally and the actual value is returned directly in degrees Celsius (or Fahrenheit, if the unit toggle in the AI block properties is changed). The valid range is -50.0 °C to +200.0 °C. The value can be bound directly to a Message Text placeholder without any further scaling, and the zero-point / gain offset is applied in degrees C.

0-10 V mode

When the sensor type is 0-10V, the AI block returns a raw integer in the range 0 to 1000, where 0 corresponds to 0.000 V and 1000 corresponds to 10.000 V at the terminal (10 mV per count). A raw reading of 300 therefore represents 3.00 V, not 30.0 °C. To convert the raw value to an engineering unit, insert an Analog Amplifier (B008) or Analog Math (B002) block between the AI and any downstream consumer. The amplifier output, not the raw AI value, must be bound to the Message Text.

0-20 mA / 4-20 mA mode

On 24 V base modules the AI block supports 0-20mA and 4-20mA modes. The raw value is again 0 to 1000, with 0 corresponding to 0 mA (or 4 mA in live-zero mode) and 1000 corresponding to 20 mA. The same scaling requirement as 0-10 V applies, with the additional consideration that 4 mA live-zero is preserved as 0 and 20 mA as 1000, so a scaling block must also include the 4 mA offset subtraction if a true engineering unit is needed.

Step 1: Configure the Analog Input Block

The AI block is the first block to add to the program; the B-number is assigned automatically in the order the block is dropped onto the canvas. Double-click the block to open the properties dialog.

1. Open the project in LOGO! Soft Comfort V8.3 and select the correct device version under File > Properties > Hardware.
2. From the catalog tree on the right, expand Analog and drag the Analog Input block onto the program canvas.
3. Wire the input of the block to the desired physical input: AI1, AI2, AI3, or AI4. For PT100/PT1000 use AI3 or AI4 on the 12/24 V variant.
4. Open the block properties and set the Sensor type drop-down to PT100, PT1000, 0-10V, 0-20mA, or 4-20mA as appropriate.
5. For PT100/PT1000, set the unit toggle to °C (default). Switch to °F only if the application requires it; the offset is then in °F.
6. For the first calibration pass, set Zero point to 0.0 and Gain to 1.00. These are the values to be adjusted in Step 4.
7. For 0-10 V / 0-20 mA / 4-20 mA sensors, leave the AI block at default and plan to insert an Analog Amplifier (B008) or Analog Math (B002) block to perform scaling, as described in the LM35 / LM335 section below.
8. Click OK to apply the parameters.

Step 2: Insert a Message Text Function Block

Message Text is the only LOGO! tool that renders a live block value on the local 6-line display. Up to 50 Message Text blocks are allowed per LOGO! 8.3 project. To insert one:

1. From the LOGO! Soft Comfort toolbar, click the Message Text tool (keyboard shortcut F5 when the programming canvas is focused).
2. Click on the program canvas to drop the block. It receives the next free B-number automatically.
3. Double-click the block to open its properties dialog.
4. Tie the En (enable) input to a high constant 1 or to a marker that is true in RUN (e.g. the 1-Hz clock marker M8). With En = 1 the message is shown continuously.
5. Set the Priority in the range 0-9, where 0 is highest and 9 is lowest. For a single ambient temperature display, priority 9 (lowest) avoids conflict with alarm messages.
6. Enable or disable the Acknowledge option. With acknowledgement enabled the user must press the ESC key on the LOGO! to clear the message; leave it disabled for a continuously-running readout.
7. In the message text area, type the static label, e.g. Temp:, Sensor 1:, or T1=.
8. Position the cursor at the location where the live value should appear. The message editor toolbar shows a parameter-browser button. Click it to open the list of all available block values in the project.

Step 3: Select the Parameter to Display

The binding between the Message Text and a live value is performed through the parameter browser. The exact sequence is:

1. Open the Message Text properties and click into the message body at the position where the value placeholder should appear.
2. Click the parameter list icon in the message editor toolbar (or double-click the value position - this is the well-known two-click binding gesture on the LOGO!).
3. The parameter browser shows every block in the program, sorted by B-number. Switch to the All tab to see a flat list.
4. Select the parameter to display. Typical choices for a temperature readout are:
   • AIx actual value for a direct PT100/PT1000 reading in °C/°F.
   • B008 output for the output of the Analog Amplifier (the value to display for a scaled 0-10 V or 0-20 mA sensor).
   • B002 result for the output of the Analog Math block (for LM335 with Kelvin-to-Celsius conversion, or for compound span and offset).
   • B009 / B011 output for the on/off state of a threshold or comparator (useful to confirm a relay state next to the temperature).
5. Click OK in the parameter browser. A placeholder token (e.g. %B001) is inserted in the message text and the LOGO! display will replace it with the live value at runtime.

The Message Text supports up to four status values per line. To show two temperatures, place a static label, insert the first value placeholder, type a separator (e.g. | ), insert the second value placeholder, and so on. The Message Text also supports a horizontal bar-graph representation, which can be useful for a quick visual indication of how close the value is to a setpoint.

Step 4: Calibrate the Sensor Offset

Once the value is visible, the zero point and gain can be calibrated against a known reference. The procedure works for PT100/PT1000 RTDs and 0-10 V transducers, with the AI block's own offset and gain being the cleanest place to apply calibration because every downstream block (amplifier, math, comparator, Message Text) automatically picks up the corrected value.

Zero-point (offset) calibration

1. Place the probe in the low reference (0.0 °C ice-water slurry) and wait at least 5 minutes for thermal equilibrium.
2. Read the LOGO! display. Call this value M_disp.
3. Read the reference thermometer. Call this value M_ref.
4. Compute the new zero-point: offset_new = offset_old + (M_disp - M_ref). The result is in the engineering unit (°C for PT100/PT1000 and °C for a scaled 0-10 V sensor).
5. Open the AI block properties, enter offset_new into the Zero point field, and click OK. For a PT100/PT1000 sensor that is reading 27.0 °C in a 25.0 °C reference, the offset must be set to -2.0.
6. Download the project (PC > LOGO! > Ethernet) and confirm the LOGO! display now reads M_ref ± the inherent sensor tolerance.

Gain (span) calibration

Place the probe in a second reference at a different temperature (a Type K dry-block calibrator at 100.0 °C, or a boiling-water reference at 100.0 °C at standard atmospheric pressure). Wait for equilibrium, then apply:

gain_new = gain_old * (M_ref_2 - M_ref_1) / (M_disp_2 - M_disp_1)

where _1 is the low reference and _2 is the high reference. For a Class A PT100 or PT1000 the gain is normally left at 1.00; only the zero-point shift is required. For low-cost 0-10 V transducers the gain can deviate 5-10 % from nominal and a span correction is justified.

When to use the amplifier offset instead of the AI offset

Leave the AI block offset at 0.00 and place the calibration into the Analog Amplifier (B008) when the same AI channel is reused for two purposes (e.g. an unscaled readout for diagnostics and a scaled readout for control). Putting the offset on the AI block shifts every downstream consumer at once; putting the offset on the amplifier shifts only the amplifier's output. Pick the location that matches the application's data-flow intent.

Alternative Sensors: LM35 and LM335 on the 0-10 V Input

If the application cannot run a PT100/PT1000 cable to the LOGO!, the LM35 (10 mV/°C, 0-100 °C typical, 0-150 °C with the LM35DZ) and LM335 (10 mV/°K, -40 to +100 °C) are the two most common low-cost alternatives. Both output a low-voltage analog signal that must be scaled before display. The cleanest implementation is to insert an Analog Amplifier (B008) or Analog Math (B002) block between the AI block and the Message Text.

For an LM35 wired to AI1 on a 12/24 V base module, the AI block returns 0-1000 for 0-10.000 V. An LM35 at 25.0 °C outputs 250 mV, so AI_actual = 25 and Temp_C = 25 * 0.10 = 2.5. To make the display show 250 for 25.0 °C (i.e. the reading in tenths of a degree), set the Analog Amplifier gain to 10.00 with zero offset. The amplifier output is then bound to the Message Text, not the raw AI value.

For an LM335 the output is in Kelvin; the formula needs a -273.15 subtraction. This is most cleanly expressed in the Analog Math (B002) block. Open the block properties, set the formula type to (Input1 * Gain) - Offset, set Gain to 0.10 and Offset to 273.15, and wire the AI block's actual value to Input1. The block result is the temperature in degrees C and is bound to the Message Text. For sub-zero readings the Analog Math block supports negative output values, so the display will correctly show e.g. -15 for -15.0 °C.

Online Test and Verification

LOGO! Soft Comfort V8.3 includes an online test mode that overlays the live actual value of every block on top of the program diagram. This is the fastest way to verify the wiring, the scaling math and the offset calibration without physically reading the LOGO! display.

1. Connect the PC to the LOGO! via Ethernet (TCP/UDP) or insert the micro-SD card with the project into the PC.
2. In LOGO! Soft Comfort, select Tools > Online Test (or press Ctrl+T).
3. Click Go Online. The block diagram is now overlaid with the live actual values.
4. Hover the cursor over any block to see a tooltip with the block number, the engineering unit and the current value.
5. Apply a known temperature to the probe and confirm the AI block actual value matches expectations. For an LM35 at 0 °C the AI value should be 0. For an LM35 at 25 °C the AI value should be 25. For a PT100 at 0 °C the AI value should be 0.0 (or 0 if integer display).

For remote display verification, the LOGO! 8.3 supports the LOGO! Web Editor (built-in HTTP server on port 80) and the LOGO! app for iOS and Android. To enable web access, navigate to LOGO! Menu > Network > Web Access and tick the Enable checkbox. Open a browser and point it at http://<logo-ip>/; the default credentials are LOGO / LOGO (change them in Network > Web Access > Password for production). The web view shows every variable in the project, including the AI block actual value, the amplifier output and the Message Text currently being displayed.

For OPC UA integration with a SCADA package, the LOGO! 8.3 exposes a built-in OPC UA server (firmware FS:05 or higher) on port 4840. The variable list is auto-generated from the program. A SCADA client can subscribe to the AI block actual value, the amplifier output and the Message Text string. The OPC UA security modes are documented in the LOGO! 8.3 OPC UA server manual on the Siemens Industry Online Support portal.

Common Pitfalls: Output Relay Cycling

The most common secondary symptom reported on a temperature-control LOGO! program is the output relay (Q1, Q3 or any other) cycling on/off every few seconds or minutes when the measured value is close to the setpoint. Three root causes account for nearly every occurrence, and each has a different fix.

Pitfall 1: wrong sensor type, threshold compares raw voltage to °C setpoint

If a 0-10 V transducer is connected and the AI block is in 0-10 V mode, but the threshold block (B009) is comparing the raw 0-1000 value against a °C setpoint (e.g. 25), the comparator will see "25 vs 25" and toggle continuously with even minor noise. Fix: insert an Analog Amplifier (B008) between the AI block and the threshold block, scale the raw value to °C, and wire the amplifier output to the threshold input. The threshold setpoint and the AI block now live in the same engineering unit.

Pitfall 2: no hysteresis on the threshold

The Analog Threshold Trigger (B009) has a single setpoint and a single on/off decision. Without hysteresis, a 1 LSB noise spike crosses the threshold and the relay toggles. Fix: replace B009 with the Analog Comparator (B011), which accepts separate On threshold and Off threshold parameters. Set the off-threshold 1-2 °C below the on-threshold to obtain clean switching. A typical configuration for a 25 °C setpoint is On = 25.0, Off = 23.0 (2 °C hysteresis band).

Pitfall 3: sensor noise from poor wiring

Long cable runs on a PT100/PT1000 sensor without shielded twisted pair, or a 0-10 V transducer sharing a cable with 230 V relay outputs, can introduce switching noise large enough to cross the threshold repeatedly. Fix: use shielded twisted pair, ground the shield at one end only (typically at the LOGO! terminal), and physically separate analog and digital wiring by at least 100 mm. On PT100/PT1000 sensors use 2-wire connection as documented in the LOGO! 8.3 system manual; a 3-wire RTD can be wired to a 12/24 V LOGO! by tying the third wire to the I7 or I8 terminal block ground return.

Diagnostics and Troubleshooting Matrix

The following matrix maps the most common reported symptoms to their root cause and corrective action. Use the online test (Ctrl+T) as the first diagnostic step in every case.

Advanced Calibration Patterns and Multi-Channel Displays

For applications that need the temperature shown in tenths of a degree with a custom sign convention (e.g. sub-zero environments, or a calibrated offset that must be edited by the operator), leave the AI block at its default (zero = 0.0, gain = 1.00) and put the calibration into the Analog Math (B002) block. The formula is:

Result = (Input1 * Gain1) + Offset

With Gain1 = 1.00 and Offset = the desired shift in °C. The original AI reading is preserved in the AI block (useful for diagnostics and the online test), and the calibrated value lives in the Analog Math block. The Message Text binds to the Analog Math result, and the offset is editable in a single place. This pattern is also useful when the same AI channel is reused for two purposes (e.g. an unscaled readout for the operator and a scaled readout for the comparator).

For projects with more than one temperature channel, the LOGO! 8.3 supports up to two PT100/PT1000 sensors (AI3 and AI4) on a 12/24 V base module, or up to four 0-10 V / 0-20 mA / 4-20 mA channels on a 24 V base module. Each channel needs its own AI block, its own amplifier / math block (if scaling is required) and its own value placeholder in the Message Text. A single Message Text line can show up to four values, for example:

T1=%B001 | T2=%B008 | T3=%B012 | T4=%B016

For the second temperature channel displayed on a separate Message Text line (so the operator can switch between them with the cursor keys on the LOGO! display), use the message line break feature: insert a line break in the message text, type the second label, and bind the second value placeholder. The LOGO! display supports up to 6 lines per message text and 50 message texts per project.

For SCADA integration via Modbus TCP, the LOGO! 8.3 exposes a Modbus TCP server on port 502. The AI block actual values, the amplifier outputs and the comparator states are all readable as holding registers. The Modbus register map is generated automatically by LOGO! Soft Comfort and exported under Tools > Modbus Address Table. This is the most common integration path for an existing SCADA package that does not yet speak OPC UA.

End-to-End Commissioning Workflow

1. Identify the sensor (PT100, PT1000, LM35, LM335, 0-10 V, 4-20 mA) and wire it to the correct terminal pair. For RTDs use I7 / I8 (AI3 / AI4) on a 12/24 V base module; for 0-10 V use I1 / I2 (AI1 / AI2) on any variant; for 4-20 mA use AI3 / AI4 on a 24 V base module with a 500 Ω burden.
2. Drop the AI block onto the program canvas, configure the sensor type, and leave the zero / gain at the default (0.0 / 1.00).
3. If the sensor is a 0-10 V / 0-20 mA / 4-20 mA transducer, add an Analog Amplifier (B008) or Analog Math (B002) block to convert the raw value to the engineering unit. Skip this step for PT100 / PT1000.
4. Add a Message Text block, tie En to a constant high, and bind the value placeholder to the AI block (for RTDs) or to the amplifier / math output (for 0-10 V / 0-20 mA / 4-20 mA sensors).
5. Download the project to the LOGO!, run the online test (Ctrl+T), and verify the displayed value against the reference thermometer.
6. Apply the zero-point offset (in °C) to compensate for sensor and wiring error. Re-verify.
7. Apply the gain correction (only if the sensor has a non-unity span error; rare for PT100/PT1000, more common for low-cost 0-10 V transducers). Re-verify.
8. Connect the comparator (B011) input to the calibrated value, set on / off thresholds with at least 1 °C of hysteresis, and confirm clean switching at the output relay.
9. Optionally enable the LOGO! Web Editor, the LOGO! app, or the Modbus TCP server for remote monitoring; configure the OPC UA server for SCADA integration.

For the canonical Siemens documentation, see the LOGO! 8.3 system manual on the Siemens Industry Online Support portal, the LOGO! 8 system manual at 109757154, the LOGO! product page at siemens.com/logo, and the LOGO! Soft Comfort V8.3 product page at siemens.com/logo-soft-comfort. The function block list with all parameter definitions is in the LOGO! 0BA5 / 0BA6 / 0BA7 manual on the same portal.