Siemens LOGO! LSC Analog Sensor Type Configuration Reference

Overview: Analog Sensor Selection in LOGO! Soft Comfort

Siemens LOGO! logic modules expose their analog inputs to user programs as a uniform 0-1000 integer range. To make that raw range usable, LOGO! Soft Comfort (LSC) attaches a sensor-type attribute to every analog block that has an Ax terminal in its parameter dialog. The selection is informational scaffolding that pre-fills the block's minimum/maximum/sensor offset fields; it does not change the firmware A/D conversion math, which always returns 0-1000 regardless of what the user picks.

The five selections the editor exposes are:

• No sensor
• 0 - 10 V
• 0 - 20 mA
• 4 - 20 mA
• PT100 / PT1000 (clipped to RTD-capable blocks)

Understanding which selection actually changes the downstream math, and which only changes the editor's hint text, is the difference between a clean amplifier cascade and a debug session chasing phantom scaling errors.

LOGO! Hardware and Analog Input Channels

Sensor selection in LSC is only meaningful where there is real analog hardware behind it. The channel map for the current LOGO! 8 generation (6ED1052-x*x08-0BA1 base modules, 0BA8 firmware family) is:

The base-module AI on I7/I8 is always 0-10 V hardware; the 4-20 mA and 0-20 mA choices are only physically usable when an AM2 (current-mode DIP selected) or external 250 Ω shunt + AM2 voltage input is wired. Wiring a 4-20 mA transmitter into the base I7/I8 without a shunt will saturate the input at the rail voltage of the transmitter loop supply.

Reference the LOGO! 8 system manual for the AI terminal assignments on each base unit, the AM2 DIP-switch positions, and the maximum loading of the integrated 24 V sensor supply (typical 100 mA total across I7/I8 and any expansion module).

Native A/D Conversion: Always 0-1000 Units

Every LOGO! analog input, regardless of physical range, is converted to a 10-bit unsigned integer by the firmware A/D converter and presented to the program as a value in the closed interval [0, 1000]. The mapping is linear:

• 0 V at the terminal → 0 units
• 10 V at the terminal → 1000 units
• 0 mA at the terminal → 0 units
• 20 mA at the terminal → 1000 units
• -50 °C on PT100 → 0 units
• +200 °C on PT100 → 1000 units

The sensor-type dropdown does not change that mapping. What it changes is the editor pre-fill of the block's gain/offset fields and the on-line display shown in the LOGO! TD or LOGO! TDE. Setting "No sensor" leaves the gain at 1.00 and the offset at 0, with display in raw units. Setting "4-20 mA" pre-fills a small offset so that a typical industrial scale (4 mA = 0 % span, 20 mA = 100 % span) lines up with the visual representation.

Default Scaling Values per Sensor Selection

The following table documents what LSC pre-fills when each selection is chosen on a block whose gain and offset are at default. Verify against the live Block Properties dialog on the actual installation, because LSC has shipped minor tweaks between V8.0, V8.1, V8.2, and V8.3.

For the three selections "No sensor", "0-10 V", and "0-20 mA", the gain and offset pre-fill to identical values (1.00 and 0). Functionally the program cannot tell them apart at runtime; the difference is purely cosmetic in the LSC diagram and the LOGO! TD online view.

Only "4-20 mA" and the RTD selections alter the pre-filled offset so that the displayed value matches what an installer expects from a live-zero current loop or a thermocouple/resistance sensor.

Why "No Sensor" Exists

The "No sensor" option is provided so the engineer can tell the editor (and any future maintainer of the program) "do not pre-fill anything; this block is downstream of a sensor-aware block and is operating on already-conditioned units". The canonical use case is the Analog Amplifier cascade:

1. Raw AI (e.g. I7) → Analog Amplifier (sensor type = "0-10 V", gain 1.00, offset 0; or sensor type = "4-20 mA" with the proper offset).
2. Amplifier output → Analog Comparator threshold reference.
3. Amplifier output → Analog Multiplexer input.

Once the first amplifier has applied the sensor-specific gain and offset, every downstream block receives already-scaled engineering units. Re-applying a sensor-type offset to those downstream blocks would double-scale the value and corrupt the result.

Setting the downstream blocks to "No sensor" is therefore not just "an equivalent choice"; it is the correct choice when the upstream amplifier has already performed the sensor-specific conditioning. The engineer documents intent and prevents accidental double-application of the offset.

Analog Amplifier Cascade Rules

The single most important rule for sensor selection in a multi-block analog signal chain is:

Configure the sensor type on exactly one block per physical input — the block closest to the physical terminal. Set every downstream block to "No sensor".

Practical cascade patterns and the correct sensor setting on each block:

The "sensor type" dropdown is independent per block. It is the designer's responsibility to set exactly one block in the cascade to the sensor-matching value, and all downstream blocks to "No sensor". LSC will not warn about double-scaling.

When to Choose Each Sensor Type

Use the decision flow below when configuring a new block. The output is the recommended sensor-type setting for that single block (assuming the cascade rule above is observed).

A common anti-pattern is to set every analog block in the diagram to "4-20 mA" because the field wiring uses a 4-20 mA transmitter. This silently adds the -250 offset twice or three times and the displayed reading is wildly wrong. Resist the urge.

Step-by-Step Configuration in LOGO! Soft Comfort

1. Open the project in LSC and locate the analog block bound to the physical AI terminal (typically named AI1, AI3, AI7, AI8, or as configured by the user).
2. Double-click the block to open Block Properties. Switch to the Parameters tab.
3. In the Sensor dropdown, select the value that matches the physical input:

• Base I7/I8 wired 0-10 V → 0..10V or No sensor (identical result).
• AM2 input wired to a 4-20 mA transmitter → 4..20mA.
• AM2 RTD input with a PT100 probe → PT100.

4. Confirm Gain and Offset reflect the desired engineering units. The dropdown pre-fills these; you can leave them at the pre-fill or edit them.
5. For every downstream block fed from this block's output, open its Block Properties and set the Sensor dropdown to No sensor. Confirm the downstream gain/offset remain at 1.00/0 unless further scaling is required.
6. Save the project with Ctrl+S, then transfer to the LOGO! base module (Tools → Transfer → To LOGO! or Ctrl+F8 in LSC V8.3 and later).
7. On the target module, enter RUN and observe the AI value via the LOGO! TD/TDE or the LOGO! Web Editor (LOGO! 8 with Ethernet base).

For software-only simulations, use Simulation mode (LSC Tools → Simulation) and force the AI value with the slider under Tools → Simulation Parameters. The simulated cascade will exercise the gain and offset pre-fills so you can confirm the sensor-type choice behaves as expected before going on-site.

Verification

After transfer, verify the cascade with three independent checks. Each one should pass before commissioning is signed off.

Check 1 — Single-block correctness

Force a known input. For a 0-10 V source, apply 5.00 V from a calibrator and confirm the AI block reads approximately 500 units (or 500 °C if the block is configured as PT100 with default scaling, scaled by a downstream amplifier as needed). Tolerance: ±2 % of full scale is acceptable for a 10-bit converter with reference tolerance.

Check 2 — Cascade continuity

With the input forced to two distinct values (e.g. 25 % and 75 % of span), confirm that the downstream block's value moves proportionally and identically to the upstream amplifier's output. If the downstream value moves by a different ratio, a sensor-type offset has been applied twice and the downstream block must be set to "No sensor".

Check 3 — Live-zero behaviour (4-20 mA only)

Reduce the current to 3.8 mA (below the 4 mA live zero). A correctly configured "4-20 mA" block reports a negative value (approximately -50 units) or flags a sensor fault. A misconfigured "No sensor" block will simply read the raw value (approximately 190 units) without warning. If the application needs the under-range alarm, confirm the block's Sensor error output is wired to a message text or to a digital flag in the program.

Edge Cases and Field Notes

Several scenarios that look like sensor-selection issues turn out to be hardware issues; recognising them saves a service call.

Mixing 0-10 V and 0-20 mA on the same AM2 input

The AM2 (6ED1055-1MA00-0BA2) has a hardware DIP switch per channel selecting voltage vs current mode. The LSC sensor-type dropdown is not a substitute for the DIP. If the DIP is in voltage position and the transmitter is wired to inject 4-20 mA through a 250 Ω shunt, the reading will be wrong by a factor of 4; if the DIP is in current position and the source is a true 0-10 V signal, the reading will read near full scale and saturate. Always verify the DIP before troubleshooting the LSC sensor-type setting.

Sensor type changed but gain/offset left at old values

LSC will not reset manually-entered gain and offset when the sensor-type dropdown is changed. If the program was developed for a 4-20 mA loop (gain 1.00, offset -250) and the sensor type is later changed to "0-10 V" without resetting the offset, the displayed value will be off by 250 units across the full scale. Always pair a sensor-type change with a manual review of gain and offset.

PT100 wiring in 2-wire mode

The AM2 RTD supports 2-wire, 3-wire, and (with limitations) 4-wire modes. In 2-wire mode, lead resistance adds directly to the measured resistance and the displayed temperature will read high by typically 1-3 °C depending on lead length. The sensor-type selection is correct ("PT100") but the installation compensates by adding a fixed negative offset to the amplifier. Document the offset in the program comments; do not bake the correction into the sensor-type selection.

LOGO! 0BA6 vs 0BA8 compatibility

Projects saved in LSC V8.x target LOGO! 0BA8 modules. Programs targeting 0BA6 (LOGO! 6) modules use LSC V7.x and the sensor-type dialog has slightly different pre-fill defaults. If a project is opened in the wrong LSC version, the gain/offset will silently round to the version's defaults and the program may behave differently on the older hardware. Match LSC version to LOGO! hardware generation.

Firmware update changes defaults

Siemens has revised the 4-20 mA pre-fill offset between firmware releases (most notably between LOGO! 0BA7 and 0BA8). When upgrading the LOGO! base module firmware in the field, re-verify any 4-20 mA loop after the update against a calibrator; do not assume the pre-update calibration holds.

Teach-in / laser distance sensor alternative

For applications where the analog input comes from a laser distance sensor with a Teach-In button (for example, a 45CPD-series analog laser sensor installation per Rockwell's 45CPD-IN001), the physical sensor's analog output is still 0-10 V or 4-20 mA into the LOGO! AI. The Teach-In button on the sensor does not change the wiring; it changes the sensor's internal setpoint behaviour. The LOGO! sensor-type selection follows the electrical signal (0-10 V or 4-20 mA), not the Teach-In state.

FAQ