Configuring LOGO! AI4 Input and AQ1 Output Scaling in LOGO!Soft

1. Overview of LOGO! Analog I/O Architecture

The Siemens LOGO! is a compact logic module that integrates digital and analog signal handling into a single base module. Unlike a rack-based PLC, LOGO! exposes analog channels directly on the base module (BM) or through attached expansion modules (EM). For an engineer building a closed-loop regulation around a 0–10 V sensor feeding a proportional actuator, the entire data path is contained inside the LOGO! footprint:

sensor (0–10 V) → onboard analog input (I1–I4) → AI analog block in LOGO!Soft Comfort → function blocks (PI regulator, comparator, threshold switch) → AQ analog output block → onboard analog output (AQ1 or AQ2) → actuator or downstream controller.

Throughout this chain the LOGO! firmware never exposes raw volts to the user program. Instead it presents analog signals as integer counts in the range 0 to 1000, where each count equals 0.01 V (10 mV) of physical voltage. The mapping is fixed for the 0–10 V hardware range:

• 0 counts = 0.00 V
• 250 counts = 2.50 V
• 500 counts = 5.00 V
• 1000 counts = 10.00 V

This convention holds whether the signal originates from an onboard terminal, an analog expansion module (LOGO! AM2 / AM2 RTD), or a network analog input. The user does not configure the scaling explicitly; the runtime enforces it. The user only configures the engineering units (°C, bar, %, etc.) inside the AI block parameter dialog if the connected sensor is linear and requires remapping.

For the 0–10 V generic sensor described in the source scenario, the AI block "Sensor" parameter is set to "0–10 V" (no rescaling) and the AI block passes raw counts straight through to the PI regulator and on to AQ1. Pressing the simulator slider from 0 to 1000 therefore moves AQ1 from 0.00 V to 10.00 V in 10 mV steps. That observation is correct, not a unit error, and is the behavior LOGO!Soft Comfort is designed to produce.

2. LOGO! 8 Base Module Compatibility Matrix

Not every LOGO! 0BA8 (LOGO! 8) base module ships with onboard analog inputs and outputs. Analog-capable base modules use 12 V or 24 V supply and route the low-level signals directly into the CPU. The 230 V AC base modules cannot accept analog on the onboard terminals because their input conditioning network is designed for line-voltage digital signals only.

For onboard analog outputs (AQ1, AQ2) the population is narrower still. According to the LOGO! 8 system manual, onboard AQ channels exist only on the LOGO! 24CEo, LOGO! 12/24RCE, LOGO! 12/24RC, and LOGO! 12/24RCo variants and on the LOGO! 24 / 24o variants in the 6ED1052-1HB / 1FB series. If the target hardware is a pure 24 RC or 230 V module without the "12/" in its part number, you must add a LOGO! AM2 AQ analog output expansion module to obtain AQ channels. Mixing an AM2 with a base module that already has onboard AQ is supported, but the program must reference the correct channel number: AM2 AQ outputs are addressed as AQ3 and AQ4 in the firmware addressing map, regardless of where the AM2 sits physically in the stack.

3. The 0–1000 Internal Scaling Convention

The internal count space is the same whether the underlying hardware is voltage, current, or PT100/PT1000 RTD. The AI block converts the engineering range to the count range automatically; the user-visible value at every block pin is an unsigned integer between 0 and 1000. The conversion is strictly linear:

V_physical = Count / 100
Count = V_physical × 100

For a 4–20 mA current loop the LOGO! resolves 4 mA to 0 counts and 20 mA to 1000 counts, giving an effective resolution of 16 µA per count. For PT100/PT1000 the count range encodes the temperature span configured in the sensor block. To convert counts back to current or process units, multiply by the appropriate range constant:

I_mA = 4 + (Count / 1000) × 16 (for 4–20 mA inputs)
T_engineering = T_min + (Count / 1000) × (T_max − T_min) (configured sensor span)

When you simulate the program in LOGO!Soft Comfort and slide I2 from 0 to 1000, you are not moving a voltage cursor – you are pushing raw counts into the AI block, which forwards them at face value to downstream blocks. If the AI block is configured as "0–10 V generic" (no sensor-specific linearization), counts pass through unchanged; if a sensor linearization is configured, the AI block remaps the count into your engineering units first.

4. Prerequisites and Required Components

1. LOGO! 0BA8 base module with onboard analog — recommended part numbers 6ED1052-1MD08-0BA1 (LOGO! 12/24RCE, Ethernet) for networked installs or 6ED1052-2MD08-0BA2 (LOGO! 12/24RCo, no display) for OEM builds. Order the LOGO! 8 system manual alongside the module.
2. LOGO!Soft Comfort V8.x or later. Confirm Help → About shows a build that matches the BM firmware; LOGO!Soft Comfort V8.3 supports BM firmware through V1.82.x and V1.83.x as documented in the LOGO!Soft Comfort release notes.
3. 0–10 V sensor with 24 V DC loop power. Examples: pressure transducer such as the SICK PBT-RB or Honeywell 13C, ultrasonic distance sensor, or potentiometric linear position sensor.
4. 24 V DC regulated power supply rated for the LOGO! BM (typical 0.5 A max for the 12/24 variants) plus the sensor load. Use a Siemens SITOP 6EP1334 or equivalent industrial supply.
5. Twisted-pair shielded cable for analog runs longer than 1 m. Ground the shield at one end only — the panel end, on the ground bar — to avoid ground loops.
6. Optional: LOGO! AM2 analog input module (6ED1055-1MA00-0BA2) if the application requires more than four analog inputs.
7. Optional: LOGO! AM2 AQ analog output module (6ED1055-1MB00-0BA2) if the BM lacks AQ channels, or AM2 RTD (6ED1055-1MD00-0BA2) for PT100/PT1000 inputs.

5. Wiring the Analog Sensor to Input I2

On the LOGO! 12/24RCE base module the upper terminal block carries the supply and digital I1–I4/I5–I8 lines, while the lower terminal block carries analog-conditioned I1–I4 plus the M (ground) reference. LOGO!Soft Comfort exposes the analog terminals as AI1, AI2, AI3, AI4. To use I2 as analog, double-click the input flag in the program editor and switch its type from "Digital input" to "Analog input"; the BM terminal then routes through the analog path.

For a 0–10 V three-wire sensor (most pressure and distance transmitters), wire as follows:

• Sensor V+ (typically brown) → 24 V DC supply positive
• Sensor GND (typically blue) → 24 V DC supply negative (and to LOGO! M terminal)
• Sensor OUT (typically black) → LOGO! terminal I2
• LOGO! M terminal → 24 V DC supply negative (same star point as sensor GND)

For a two-wire 0–10 V sensor the OUT terminal carries both signal and supply return; route into I2 and tie the sensor ground to LOGO! M so the analog input reference sits at the correct potential. Failure to tie M to the supply negative is the single most common reason for AI inputs wandering 5–10 counts when the sensor load changes.

Suppress noise on long analog runs by routing the analog cable at least 200 mm from VFD output cables, contactor coils, or welding leads, and terminate the shield at the panel ground bar using a 360° pigtail no longer than 50 mm. Twisted-pair shielded cable of type LiYCY 2×0.34 mm² performs well for runs up to 30 m in industrial panels. For runs beyond 30 m use a 4–20 mA sensor instead and add a 250 Ω sense resistor at the BM M–I2 pair — the voltage drop is negligible compared to the noise immunity gain.

6. Configuring the AI4 Analog Input Block

In LOGO!Soft Comfort drag the AI (Analog Input) block from the catalog tree onto the program canvas. Connect its input pin to the AI2 flag — this flag appears automatically in the input list once I2 is set to analog type. Double-click the block to open its parameter dialog. The dialog offers two principal sensor modes plus several specialty sensor modes:

For most process-control applications the engineer selects "0–10 V with offset/amplification". The dialog then asks for:

• Sensor low value — the engineering quantity at 0 V (e.g., 0 bar for a 0–10 bar pressure transducer)
• Sensor high value — the engineering quantity at 10 V (e.g., 10 bar)
• Lower limit / upper limit — clamp the output to ignore failed sensors. Lower limit 0 and upper limit 1000 are typical defaults.
• Gain — a multiplicative scaling factor on the input span (default 1.00; raise if the engineering span is wider than 10× the voltage span)
• Offset — an additive shift on the input span (default 0; negative value raises the displayed value when the sensor reads minimum)

The AI block now emits a value already converted to engineering units but still tagged as integer counts in the program. Internally, LOGO! uses 0 to 1000 in the engineering-unit space when min/max are unset; when min/max are set to non-zero values the counts still occupy 0 to 1000 but represent the configured engineering span. To display process units in the LOGO! onboard message text, multiply the count by the unit-scale and divide by 100, or set up a calculated block to convert on the fly.

For the simple pass-through case (volts → volts in the same count range), leave the engineering min/max at 0 and 1000. The block becomes a transparent 0–10 V repeater, which is exactly the configuration the original question describes. The AI input tag visible from the program editor will read AI2 and the value will track the simulation slider or live terminal 1:1.

7. Adding and Configuring the PI Regulator Block

The PI regulator block is found in the regulator library. It accepts an SP (setpoint) and PV (process variable) pin, both in count units 0 to 1000 (or in the engineering-unit space if AI is configured with min/max). Wire AI2 to the PV pin. Wire a constant block, analog multiplexer output, or an analog arithmetic result to the SP pin to drive the setpoint from the operator panel.

A reasonable start-up configuration for a heating loop with 0–10 V output to a solid-state relay (SSR) is KC = 1.0, TI = 0:30 (30 seconds), DIR = +, output upper limit 1000, output lower limit 0. Tune further on-site using the bump-test method:

1. Switch the regulator to Manual and force the output to 200 counts (2.0 V).
2. Wait until the PV slope stabilises; record ΔPV per minute.
3. Set KC = 200 / ΔPV_scaled_to_counts.
4. Set TI ≈ 1.5 × process dead time, measured from output step to first observable PV change.
5. Switch back to Auto and apply a small setpoint step (e.g., 50 counts); observe the response and trim KC downward if overshoot exceeds 10 %.

8. Configuring the AQ1 Analog Output

The AQ block is found at the output side of the program. Connect its input to the PI regulator output. The AQ block has no scaling dialog: it simply re-emits the incoming count at the physical terminal. AQ1 thus produces 0 V when fed 0 counts and 10 V when fed 1000 counts. The minimum update time is the LOGO! cycle time, typically 10–20 ms with no Ethernet activity and no more than 200 blocks loaded (per the LOGO! 8 system manual cycle-time specification).

If the controlled device expects a 4–20 mA signal rather than 0–10 V, the AQ output must be conditioned with an external V/I converter such as a Phoenix Contact MINI MCR-SL-UI-UI-NC or a WAGO 857 series signal conditioner. The converter's input span is 0–10 V and its output span is 4–20 mA, mapping 0 counts to 4 mA and 1000 counts to 20 mA. The LOGO! never explicitly supports a 4–20 mA output stage on its base module or AM2 AQ module; both produce voltage only.

For PWM-style devices (slow SSR control of a resistive heater) the AQ output can drive a 0–10 V analog input on the SSR controller. Many modern SSRs accept 0–10 V or 4–20 mA command and produce time-proportional output internally; the LOGO!'s AQ1 becomes the setpoint and the SSR handles the duty cycling. This is the most robust way to operate a heating element in the 1–10 kW range from a LOGO! without using a relay output directly.

9. Sensor Linearization and Scaling Conversions

When a sensor is not 0–10 V at the engineering high range, or when the engineer wants engineering units directly inside LOGO!, configure the AI block's "Sensor" parameter to "0–10 V with offset/amplification". The offset and gain fields perform the linear transformation:

PV_engineering = Offset + (Count / 1000) × Amplification

Count is the raw count coming in from the AI block. Amplification scales the range, and Offset shifts it. Worked examples:

0–10 bar pressure transducer, 0–10 V output, want bar on the AI pin:

• Offset = 0
• Amplification = 10
• At 5 V (500 counts): PV = 0 + (500/1000) × 10 = 5 bar

1–5 V transmitter, −10 to +100 °C:

• Amplification = 110 (full engineering span)
• Offset = −110 (subtract the dead-band and the negative part)
• At 1 V (100 counts): PV = −110 + (100/1000) × 110 = −99 °C approx, close to true −100 °C
• At 5 V (500 counts): PV = −110 + (500/1000) × 110 = −55 °C approx, which is wrong — instead pick Offset = −55 and Amplification = 155 so that 100 counts → −100 °C and 500 counts → +100 °C, then capture the dead region with a lower clamp

More generally, for any sensor, define:

• V_min_sensor at the electrical minimum (1 V for a 1–5 V transmitter, 0.5 V for a 0.5–4.5 V ratiometric)
• V_max_sensor at the electrical maximum (5 V or 4.5 V respectively)
• X_min, X_max in your process unit (°C, %, ppm, etc.)

Then:

Count_at_min = (V_min_sensor / 10) × 1000
Count_at_max = (V_max_sensor / 10) × 1000
Slope_counts = Count_at_max − Count_at_min
Amplification = (X_max − X_min) × (1000 / Slope_counts)
Offset = X_min − (Count_at_min / 1000) × Amplification

Enter offset in count units if your firmware asks for raw counts; otherwise enter engineering-unit values directly. Refer to the LOGO!Soft Comfort online help for the exact entry mode in your firmware revision; in V8.3 the offsets are entered as count units directly. With these parameters the AI block returns a value that maps directly to engineering units without further scaling downstream.

10. Program Simulation and Verification Procedure

LOGO!Soft Comfort includes a software PLC simulator that exercises the program without any hardware connected. Use it to confirm block wiring and scaling before deploying the program to the BM.

1. Select Simulation → Simulation Mode (or press F8). The toolbar swaps to simulation color and the input panel becomes interactive.
2. Click on I2 in the program canvas to expose a slider control. The slider covers 0 to 1000 counts.
3. Drag the I2 slider from 0 to 1000 counts. The AI block tooltip changes in lockstep from 0 to 1000.
4. Track the wire running from AI2 to the PI regulator's PV pin. The PV value should mirror the AI value when no scaling is configured, or the engineering-unit value when scaling is configured.
5. Set the SP constant to 500 counts. Observe the AQ1 block value. With PI gains KC = 1.0, TI = 0:30, the output ramps from 0 toward steady state within 30 s of simulation time. Watch for first overshoot at the AQ pin as the integral term winds up.
6. Use the cursor on the simulation trace to inspect any time stamp. Verify AQ1 ↔ PV convergence, with the steady-state offset driven by the integral term only (no offset bias should remain).
7. Click Stop Simulation to return to editor mode.

When the simulation behaves correctly, transfer the program to the BM using Tools → Transfer → PC → BM (or click the BM icon in the toolbar). Power down, wire the physical sensor, power up, and observe the actual AQ1 voltage with a calibrated DMM. Confirm that the displayed AI2 block value (View → Online View) equals the measured voltage × 100 counts. For a 5.00 V input the AI2 block must read exactly 500 counts; ±5 counts of variation is acceptable jitter from ADC noise and sensor ripple.

11. Expansion Modules for Additional Analog Channels

If the BM is fully populated with sensors and a fourth PI loop is required, attach a LOGO! AM2 expansion module to the right of the BM. Stack the modules with the integrated bus connector on the bottom and clip them to the DIN rail. Power the BM last so the module discovery sequence completes cleanly.

After attaching the AM2, the inputs become AI5 and AI6 in the program. For analog outputs the AM2 AQ provides two 0–10 V outputs labelled AQ3 and AQ4. Channel numbering follows the global AQ ordering regardless of which physical module the channel resides on. The maximum count of expansion modules stacked to the right of the BM is 24 per the LOGO! 8 system manual, subject to the BM's 24 V power budget; practical installs stop at 4 or 5 modules. Each expansion module draws ~20–40 mA from the bus, and the BM must be able to feed the worst-case current or else an "EXT FAULT" appears.

12. Troubleshooting Matrix

Frequently Asked Questions