Configuring LOGO! 8 Onboard Analog Inputs: AI and Digital Sharing

Overview

The Siemens LOGO! 8 logic module consolidates digital and analog I/O into a single 71 mm wide base module. Unlike a conventional PLC where analog and digital channels are physically isolated, the LOGO! 8 base module shares its analog input terminals with selected digital inputs. AI1 and AI2 share the same physical screw terminals as I7 and I8, while AI3 and AI4 share terminals with I1 and I2. The sharing is configurable in LOGO! Soft Comfort (LSC) through the AI Configuration property, allowing the engineer to enable zero, two, or four analog channels depending on the application's I/O budget.

The most common field question is: If I enable 2 AIs, can I still treat I8 as a normal digital input? The answer is yes. The AI enable flag only reserves the analog address space inside the LOGO! 8 program; it does not disable the corresponding digital input. The same terminal can therefore be read simultaneously as a digital bit and scaled as a 0-10 V analog value, provided the voltage lies inside the supported analog range and the program processes both representations without conflict.

This article documents the exact terminal-to-AI mapping, the voltage thresholds, supported base module variants, the LSC configuration path, and verified wiring examples so that the AI sharing feature can be deployed without losing digital I/O.

LOGO! 8 Hardware Variants and AI Availability

Analog input capability is offered only on the DC-powered base modules. The 230 V AC variant does not expose onboard analog channels because its input circuit topology is designed for mains-voltage switching rather than low-voltage measurement.

Confirm the printed part number on the side label of the module before designing the AI wiring. The AI Configuration property is only displayed by LSC when the online module or selected hardware in the project navigator is a DC variant. Refer to the Siemens Industry Online Support portal for full ordering data and firmware-specific manuals.

AI Configuration Options in LOGO! Soft Comfort

The number of available analog inputs is set inside the LSC project, not on the physical module. There are no DIP switches or jumpers on the LOGO! 8 base module for AI selection.

Configuration path (LSC V8.0 - V8.2):

1. Open the project in LOGO!Soft Comfort.
2. Select File > Properties.
3. Open the AI Configuration tab.
4. Choose one of three options:
   • Enable 0 AIs – no analog channels exposed.
   • Enable 2 AIs – AI1 and AI2 available on I7 and I8.
   • Enable 4 AIs – AI1, AI2 on I7, I8 and AI3, AI4 on I1, I2.
5. Click OK and re-compile the program.

Configuration path (LSC V8.3 and later):

1. Select Tools > AI Settings from the menu bar.
2. The dialog shows the current AI mode; select 0, 2, or 4.
3. Click Apply. The program is recompiled automatically.

The choice is stored inside the program file (.lsc / .lma / .lmp depending on LSC version) and is downloaded to the LOGO! 8 as part of the project. After a successful download, the AI channel becomes available in the analog function block palette.

Terminal-to-AI Mapping and Electrical Sharing

The hardware sharing between analog and digital inputs is fixed by the input stage design. Each shared terminal routes the incoming voltage simultaneously to two ADC/digital comparator paths inside the LOGO! 8 ASIC.

The implication of the threshold table is critical for mixed-mode designs:

• If the analog voltage is below 5 V, the digital input reads 0.
• If the analog voltage is above 8.5 V, the digital input reads 1.
• If the analog voltage is between 5 V and 8.5 V, the digital state is undefined and may toggle.

This means a 0-10 V analog signal on AI1 (I7) can produce phantom edges on I7 every time the analog value crosses the digital threshold. In practice, avoid relying on I1, I2, I7, or I8 as reliable digital inputs when those same terminals are wired to a continuously varying analog source. Use those digital inputs only for clean 24 V DC switching signals or use the spare digital channels (I3-I6) for switching logic.

Digital Input Function While AI is Active

The AI enable flag is purely a software address-allocation setting. It does NOT install any series resistance, optocoupler, or relay in the input path that would isolate the digital comparator from the analog ADC. Therefore:

1. Digital inputs are never disabled by enabling AI. The I1, I2, I7, I8 screw terminals always remain electrically functional as 24 V digital inputs.
2. Both digital and analog values are simultaneously updated. The LOGO! 8 ASIC samples each shared terminal at the analog sampling rate and forwards both the scaled analog value and the digital comparator result to the runtime engine.
3. You can use I8 as a digital input while AI2 is enabled on the same terminal. The LSC program simply references I8 for digital logic and AI2 for analog scaling; both work concurrently.
4. Use 0 AIs only when no analog measurement is needed. Setting AI count to 0 removes AI1-AI4 from the address map but does not change any electrical behavior on the terminals. The reason to set 0 AIs is purely to reduce program memory and prevent accidental use of an analog block on a non-existent channel.

Voltage Thresholds and Analog Scaling

The onboard analog inputs use a 10-bit successive-approximation ADC. The full-scale value of 10 V maps to an integer of 1000 in the program, giving a usable resolution of 10 mV per LSB. The input impedance of the analog circuit is high enough that most 0-10 V transducers (10 kΩ potentiometers, 4-20 mA transmitters with 250 Ω shunt, 0-10 V outputs from sensors) can be connected without loading errors.

Analog scaling formula:

The internal representation is:

AI_raw = (V_in / 10 V) × 1000

For a 4-20 mA transmitter with a 250 Ω shunt resistor, the voltage across AI is 1-5 V. Convert this to engineering units using the LSC Analog Amplifier (B001) or Analog Multiplexer (B013) function blocks. A typical scale for 0-100 °C using a 4-20 mA / 0-100 °C transmitter:

• AI at 1 V → 0 °C
• AI at 5 V → 100 °C

Configure the Amplifier block with Gain = 25 and Offset = -25 (or use the linear equation T = (V - 1) × 25). Verify the result by applying a known mA source and reading AI from the LSC online monitor.

Wiring Topologies and Sensor Connections

The shared nature of the terminals requires care when connecting mixed signal types. The following topologies cover the most common field installations.

Topology 1 – Potentiometer (3-wire) on AI1:

1. Connect one end of the 10 kΩ potentiometer to terminal M (0 V reference of LOGO! supply).
2. Connect the wiper to terminal I7 (shared with AI1).
3. Connect the opposite end to terminal +24 V (LOGO! supply).
4. Set AI Configuration to "Enable 2 AIs" or higher.

The wiper voltage swings 0-10 V linearly as the knob is rotated; AI1 reads 0-1000 scaled. Because the wiper is also connected to the digital comparator, I7 will read 0 when the wiper is below 5 V and 1 when above 8.5 V. Use the AI1 block for proportional control and ignore the I7 digital reading, or implement hysteresis in the digital logic to ignore the ambiguous mid-band (5-8.5 V).

Topology 2 – 4-20 mA transmitter (2-wire) on AI3:

1. Install a 250 Ω, 0.1 % precision resistor between terminal I1 (AI3) and terminal M (0 V).
2. Connect the transmitter positive lead to terminal +24 V.
3. Connect the transmitter negative lead to terminal I1.
4. Enable 4 AIs in LSC; use AI3 in the program.

The 4-20 mA loop develops 1-5 V across the 250 Ω shunt; AI3 reads 100-500 raw. I1 will read 0 for the entire 1-5 V range, so the digital function on I1 remains quiet and can be used freely as a regular digital input without conflict.

Topology 3 – Clean digital input on I8 while AI2 is enabled:

1. Wire the 24 V DC switching contact (pushbutton, sensor output) directly to I8 and M.
2. Enable 2 AIs in LSC; reference AI2 for an unrelated analog measurement, or leave AI2 unused.
3. Use I8 in the digital logic; the contact transitions remain clean because the source is a true 24 V switching signal.

Programming with AI Blocks in LOGO! Soft Comfort

Once AI is enabled and wired, the analog channels appear in the LSC block palette under the Analog tab. The fundamental blocks that read AI1-AI4 are:

Example program (FBD): Read AI1 (I7 terminal) as a 0-10 V signal and use the same I7 terminal as a digital enable flag.

Network 1 (Digital logic):
  I7 (digital) ---[ NO contact ]--- Q1 (output relay)

Network 2 (Analog logic):
  AI1 (raw) ---[ B001 Analog Amplifier ]--- Gain = 0.1, Offset = 0
                |
                +-- MW100 (engineering value, 0.0 - 100.0)

The Analog Amplifier with Gain = 0.1 and Offset = 0 scales the raw 0-1000 value to 0.0-100.0 for engineering display. To convert to physical units (e.g., °C for a 0-100 °C probe), set Gain = 0.1 and Offset = 0; for a -10 to +50 °C probe, use Gain = 0.06 and Offset = -10.

Example program (Ladder):

|     I7      B003 (Threshold)      Q1   |
|------[ ]-------|>|-----[/]--------( )---|
                  |  On=80  Off=20

|     AI1     B001 (Amplifier)         |
|------>AI-----| x0.1 +0 |-->MW100     |

This ladder diagram uses B003 to provide a digital function at a specific analog value, and B001 to expose the scaled AI value at memory word MW100 for use elsewhere in the program or for display on the LOGO! TDE.

Expansion Modules vs. Onboard AI

When the application needs more analog channels than the base module provides, or when the sensor type is not 0-10 V (e.g., Pt100 RTD or 4-20 mA isolated), an expansion module can be added on the right side of the LOGO! 8 base.

Expansion modules are addressed as AI5, AI6 (for the first AM2) and AI7, AI8 (for a second AM2) in the LSC program. They do not interact with the onboard AI1-AI4 configuration; you can run "Enable 4 AIs" on the base and still add AM2 modules for additional channels. The total analog input count per LOGO! 8 station is limited by the number of expansion slots supported by the base module (typically up to 24 modules).

Troubleshooting Matrix

Verification Procedure

After completing the wiring and downloading the program, run this verification sequence to confirm correct AI configuration:

1. Online monitor. In LSC, click Tools > Online > Start Online Test. The analog block should display a live value, not "---".
2. Sensor substitution. Disconnect the field sensor and apply a known voltage from a calibrated source (e.g., 5.00 V from a precision reference). Confirm the AI raw value reads 500 ± 15.
3. Digital independence check. Toggle the shared digital input (e.g., I8) via a 24 V test signal while monitoring AI2. The AI2 reading must remain stable regardless of I8's digital state, confirming that the digital input does not disturb the analog measurement.
4. Threshold verification. Slowly ramp the analog voltage from 0 V to 10 V while watching the digital input. Confirm I7 transitions to 1 at approximately 8.5 V and back to 0 at approximately 5 V.
5. Save the program to onboard SD card (if installed) and to the LOGO! 8 flash memory to ensure the AI Configuration setting persists across power cycles.

Successful verification confirms that the AI Configuration, wiring, and program are mutually consistent. Document the AI Configuration setting in the project file's Project > Properties > Comment field so that future maintenance engineers understand why the AI mode was chosen.