LOGO! 12/24 Input Voltage Thresholds: Sub-Threshold Detection

The Siemens LOGO! 12/24 logic module is the most widely deployed micro-PLC in industrial and building automation, used for everything from gate control to HVAC sequencing to small machine control. The "12/24" suffix identifies the integrated 12 V DC / 24 V DC power supply variant of the LOGO! 8 family. Despite this name, the most common question in field engineering forums is: "Can I connect 24 V DC to a digital input without exceeding the rating, and what is the minimum voltage that will register as a logic high?" This reference documents the exact digital input threshold levels (5 V low / 8 V high), the absolute maximum continuous input voltage, the 0-10 V analog input scaling, and the field-proven technique for detecting voltages between 0 V and 8 V by using the analog input with the Analog Threshold Trigger block in LOGO! Soft Comfort.

1. LOGO! 12/24 Module Variants and Input Inventory

The LOGO! 12/24 family is the low-voltage DC input member of the LOGO! 8.4 platform. Three main variants are in production:

All three variants share identical digital and analog input electrical characteristics. Of the eight digital inputs, I1 through I4 are dual-function pins: they default to digital mode but can be reconfigured as analog inputs AI1 through AI4 (0-10 V DC, 10-bit resolution) by placing an Analog Input (AI) block in the program. Inputs I5 through I8 are digital-only and cannot be used as analog inputs. Refer to the Siemens LOGO! 8 system manual on the Siemens Industry Online Support portal for full pinout and electrical specifications.

2. Digital Input Voltage Thresholds

The digital inputs of the LOGO! 12/24 are designed to read both 12 V DC and 24 V DC signaling per IEC 61131-2 type 1 digital input requirements. Per the LOGO! manual (edition 05/2006, page 32) and confirmed in the current LOGO! 8 system manual, the input recognition levels are:

The minimum voltage guaranteed to register a valid digital "1" is 8 V DC. The maximum voltage guaranteed to register a valid digital "0" is 5 V DC. The transition band between 5 V and 8 V is a guaranteed undefined region where the input may be read as either state. This 3 V of built-in hysteresis is intentional and provides noise immunity for installations with 12 V DC signaling where cable voltage drop and contact bounce are common. For 24 V DC signaling, the input is comfortably within the high band even after typical 5-10% supply sag and 1-2 V of cable drop across 100 m of 0.5 mm² cable.

The 12/12RC (the older 12 V-only LOGO! variant) specified a slightly higher high-threshold of 8.5 V DC for additional margin on 12 V supply rails. The current 12/24RCE family uses the unified 5 V / 8 V thresholds regardless of supply voltage.

2.1 Input Current, Impedance, and Pull-Down Behavior

Each digital input draws current through an internal pull-down network to the module's 0 V reference. Typical input currents are:

This makes the inputs directly compatible with standard sinking NPN sensor outputs and dry mechanical contacts wired to the +24 V (or +12 V) supply rail. The input impedance is high enough (typically 12-16 kΩ) that 5 V TTL logic families with weak drive strength may struggle to pull the input above the 8 V threshold; in that case use a discrete NPN pull-up transistor, a 5 V to 24 V logic level translator, or the analog input + threshold trigger method described in Section 8.

2.2 Response Time and Debouncing

The digital inputs include a hardware debounce filter of approximately 1-2 ms, which suppresses contact bounce on mechanical switches and short noise spikes on solid-state sensor outputs. The total recognition latency is the debounce time plus one program scan time (typically 10-20 ms for an 8-block program). For high-speed signals (above 100 Hz), the LOGO! 12/24RCE offers dedicated high-speed counter inputs on I1-I4 (5 kHz per channel) that bypass the debounce filter.

3. Maximum Input Voltage and Overvoltage Considerations

The LOGO! 12/24 module can accept 24 V DC at any digital input continuously without damage. The absolute maximum continuous voltage is 28.8 V DC (24 V DC × 1.2) per the IEC 61131-2 type 1 input specification. Applying sustained voltages above 30 V DC stresses the input clamping Zener structure and accelerates aging; sustained voltages above 35 V DC can permanently damage the input stage.

For relay output variants (LOGO! 12/24RCE), the 4 relay contacts are rated 10 A at 250 V AC / 30 V DC resistive, 3 A inductive, with a mechanical life of 10 million operations. For the transistor output variant (LOGO! 12/24RCEo), each of the 4 outputs is rated 0.3 A at 24 V DC with overload and short-circuit protection; a 1 A fast-blow fuse per output is recommended for branch protection.

4. Analog Input Specifications (0-10 V)

Inputs I1 through I4 are dual-function pins: digital by default, but reconfigurable as analog inputs AI1 through AI4 by placing an Analog Input (AI) block in the LOGO! program. The analog specifications are:

The analog-to-digital conversion uses 10 bits of resolution mapped to the 0-10 V input range. Because the LOGO! presentation is scaled to 0-1000 raw counts (rather than the raw 0-1023 binary), the linear conversion is:

V_analog = (raw_count ÷ 1000) × 10 V

For example, a 3 V signal at AI1 returns raw count 300, a 5 V signal returns 500, a 7.5 V signal returns 750, and a 9.99 V signal returns 999. This clean 1:1 scaling (10 mV per count) is the foundation of the sub-threshold detection method described in Section 8.

4.1 Enabling the Analog Input

The AI block is enabled in the LOGO! program (not in the module's hardware configuration). In LOGO! Soft Comfort, drag an AI block from the Special function palette, double-click to open its properties, select the desired channel (AI1-AI4), and the corresponding terminal (I1-I4) is automatically switched from digital to analog mode at the next program download. To revert the terminal to digital mode, delete the AI block from the program and re-download. Note that the digital status of I1-I4 is not available while the AI block is active; only the analog scaled value is accessible to the program.

5. The 5 V to 8 V Undefined Band: Why It Exists

The 3 V undefined band between 5 V (guaranteed LOW) and 8 V (guaranteed HIGH) is a deliberate design choice by Siemens to comply with IEC 61131-2 type 1 digital input requirements. The standard mandates a minimum 5 V of noise margin on a 24 V input, and a minimum 2 V of noise margin on a 12 V input. Siemens implements a 3 V band that satisfies both requirements with a single threshold pair, simplifying the input stage and reducing cost.

For 24 V signaling, the 3 V band is irrelevant — a healthy 24 V signal with 10% sag is still at 21.6 V, well above the 8 V high threshold. For 12 V signaling, the 3 V band provides critical noise margin against the 12 V supply's typical 5-10% regulation tolerance and 0.5-1 V of cable drop. The band is therefore not a flaw but a feature for noise immunity.

The problem arises when an external sensor outputs a voltage between 0 and 8 V that must be detected as a logic state. Examples include 3.3 V microcontrollers, 0-5 V transducer outputs, battery monitor signals (2-4 V), and photodiode amplifiers (4-7 V). For these cases, the analog input + threshold trigger method is the correct solution.

6. When to Use the Analog Input Method

Use the analog input + threshold trigger method when any of the following are true:

• The input signal is between 0 V and 8 V (below the digital HIGH threshold)
• The input signal is from a low-voltage logic family (3.3 V, 2.5 V, 1.8 V) without a level translator
• The input signal is from a 0-5 V or 0-10 V transducer that must be evaluated as a boolean
• The input signal is a battery or DC-link voltage that must be detected at a specific level
• The input signal has slow rise/fall times that pass through the 5-8 V band slowly, risking undefined reads
• The application requires programmable hysteresis around a decision point

For 12 V or 24 V signals from standard industrial sensors (inductive proximity, photoelectric, mechanical limit switches), use the digital input directly — the analog input is unnecessary and reduces the available digital input count from 8 to 4.

7. The Analog Threshold Trigger Block (B027)

The Analog Threshold Trigger is a special function block in LOGO! Soft Comfort (catalog number B027 in the special function library) that converts a continuously varying analog input into a clean digital output with programmable ON and OFF thresholds.

7.1 Block Parameters

7.2 Block Behavior

The Analog Threshold Trigger operates as a Schmitt trigger with two thresholds:

1. When the analog input rises above the ON threshold (A), the block output transitions from 0 to 1.
2. When the analog input falls below the OFF threshold (B), the block output transitions from 1 to 0.
3. When the analog input is between B and A, the output holds its previous state (Schmitt trigger behavior).

This Schmitt trigger behavior is the key to suppressing noise-induced toggling. The output only changes state when the analog signal crosses one of the two thresholds, so any noise with peak-to-peak amplitude smaller than the A-B gap is filtered out.

7.3 The Analog Differential Trigger (B028)

For applications that need to trigger on a change in the analog input (e.g., detect a pressure spike of 1 bar above the running baseline), the LOGO! Soft Comfort library includes the Analog Differential Trigger (B028). This block fires its output when the difference between two AI values (or between an AI value and a reference) exceeds a programmable delta. It is commonly used for rate-of-change detection and trend monitoring.

8. Worked Example: Detecting a 3 V Logic-Level Signal

Requirement: A 3.3 V microcontroller outputs a 3.0 V HIGH signal (and 0.2 V LOW) on a general-purpose I/O pin. The signal must be detected as a boolean input to the LOGO! program.

1. Route the microcontroller output to terminal I1 on the LOGO! 12/24RCE. Note that I1 doubles as AI1 when reconfigured.
2. Add an Analog Input block (B001) to the LOGO! program. In its properties, select AI1 as the source.
3. Add an Analog Threshold Trigger block (B027) to the program. Wire the AI1 output to the trigger's input.
4. Set the ON threshold (A) = 300. This corresponds to 3.0 V on the 0-1000 scale (300/1000 × 10 V = 3.0 V).
5. Set the OFF threshold (B) = 250. This corresponds to 2.5 V and provides 50 counts (500 mV) of hysteresis.
6. Wire the trigger's output (Q) to the next logic element (e.g., a counter, a flip-flop, or directly to output Q1).

Result: When the microcontroller outputs ≥ 3.0 V, the trigger output goes HIGH and the LOGO! program sees a logic 1. When the microcontroller outputs ≤ 2.5 V, the trigger output goes LOW. Voltages between 2.5 V and 3.0 V hold the previous state, which suppresses any noise in this band. The detection latency is 50 ms (AI sample period) plus 10-20 ms (program scan time), giving 60-70 ms worst-case response.

9. Worked Example: Converting a 4-20 mA Loop to a Boolean

Requirement: A 4-20 mA pressure transmitter must trigger a LOGO! output when the current exceeds 12 mA (50% of full scale). The LOGO! 12/24 has 0-10 V analog inputs, not 4-20 mA, so a precision resistor is required.

1. Connect a 500 Ω, 0.1% precision resistor (Vishay VPR221 or equivalent) in parallel with the AI1 input. At 20 mA, the resistor drops 10.0 V (within the 0-10 V range). At 4 mA, it drops 2.0 V (the "zero" of the loop). At 12 mA, it drops 6.0 V.
2. Place an AI block on AI1 in the LOGO! program. Enable the sensor type "0-10 V" (default).
3. Place an Analog Threshold Trigger block. Set ON threshold (A) = 600 (6.0 V = 12 mA). Set OFF threshold (B) = 580 (5.8 V = 11.6 mA).
4. Wire the trigger output to the desired function. With 12 mA, the output fires. With current below 11.6 mA, it releases.

Note: The 500 Ω resistor must be a precision type (0.1% or better) with low temperature drift (≤ 10 ppm/°C) for accurate loop current measurement. A standard 5% carbon-film resistor will introduce up to 5% scaling error plus 5-10% drift over temperature, which is too much for most process applications.

10. Recommended Hysteresis Values by Signal Environment

Hysteresis (the A-B gap in the threshold trigger) must be wide enough to absorb peak-to-peak noise on the analog signal without toggling, but narrow enough to release the trigger cleanly when the signal falls. The LOGO! 12/24's 9.8 mV LSB makes this an exercise in choosing the right number of counts:

11. Configuring Threshold Triggers in LOGO! Soft Comfort

LOGO! Soft Comfort V8.4 (or later) is the engineering software for programming the module offline and online. The configuration flow for an analog threshold trigger is:

1. Open or create a circuit diagram. The toolbar under the "Special" group contains the Analog Input (AI), Analog Threshold Trigger, and Analog Differential Trigger blocks.
2. Drag the AI block onto the diagram, double-click to open its properties, and select the channel (AI1-AI4) and sensor type. For 0-10 V signals, leave the sensor type at "0-10 V" (the default).
3. Drag the Analog Threshold Trigger block to the diagram. Wire the AI block's output (Q) to the trigger's input.
4. Open the trigger block properties. Enter the ON threshold (A) and OFF threshold (B) as decimal raw counts in the range 0-1000 (or as scaled engineering units if Gain/Offset is used).
5. Wire the trigger's output to the next logic element or directly to a digital output (Q1-Q4).
6. Press F5 (Simulation) to verify behavior. The Simulation tool includes a manual slider for the AI block, allowing you to drag the input voltage from 0 to 10 V and observe the trigger output in real time.
7. Press Ctrl+D or click the download button to transfer the program to the connected LOGO! 12/24RCE module via Ethernet or the LOGO! USB cable.

12. On-Device Configuration (No PC Required)

For installations without a PC, the threshold trigger can be configured from the LOGO! 12/24RCE's integrated display and keypad. The procedure is:

1. From the main menu, navigate to Program → Add Block → Special Function → Analog → Analog Threshold Trigger.
2. Select the AI source (AI1-AI4).
3. Set the ON threshold (A) by scrolling to the value and using the cursor keys.
4. Set the OFF threshold (B) by scrolling to the value and using the cursor keys.
5. Press OK to confirm. The block is now active in the program.
6. Use the cursor keys to navigate the program and verify the trigger output in the I/Q status display.

The on-device configuration supports the same parameter ranges as LOGO! Soft Comfort (ON/OFF thresholds 0-1000 raw counts). The on-device editing is slower than PC-based configuration but is invaluable for field adjustments when a laptop is unavailable.

13. Wiring and EMC Best Practices

Because the LOGO! 12/24 input thresholds are tight (3 V band between 5 V and 8 V), induced noise on the input cable can push a borderline signal in or out of the high band. The following wiring rules maintain signal integrity:

• Route signal cables in dedicated cable trays separated at least 100 mm from any 230/400 V AC power cable. Cross AC and DC trays at 90°.
• Use shielded twisted-pair cable for any cable run longer than 3 m, especially in panels with VFDs or soft starters. Ground the shield at one end only (the panel end) to avoid ground loops.
• Keep the +24 V supply rail stable. If the supply is shared with inductive loads (solenoids, contactor coils), install a 24 V DC buffer capacitor (≥ 1000 µF, 35 V minimum) at the LOGO! power terminals.
• For analog inputs routed to AI1-AI4, add an RC filter (10 kΩ + 100 nF, fc ≈ 160 Hz) right at the terminal to suppress RF noise. The 50-80 kΩ input impedance of the AI pin makes this an effective noise filter without significant signal attenuation.
• When interfacing 3 V or 3.3 V logic, place a 1 kΩ series resistor and a 100 nF capacitor to ground at the AI terminal. This gives a 1.6 kHz low-pass filter that prevents microcontroller switching noise from being detected as a 3 V DC level.
• For signals that may exceed 28.8 V (e.g., 24 V nominal with 25% overshoot from a poorly regulated supply), install a 27 V Zener diode (1 W minimum) in parallel with the input terminal for transient clamping.

14. Verification and Commissioning Procedure

After wiring, validate the digital input recognition and the analog threshold behavior with a calibrated source:

1. Power the LOGO! 12/24 module from a stable 24 V DC supply verified to be within 20.4-28.8 V at the LOGO! terminals.
2. Connect a precision DC source (calibrator, decade box, or a 24 V PSU with a digital voltmeter and a 10-turn potentiometer) to I1.
3. In LOGO! Soft Comfort, open the online monitor (Ctrl+M) and observe the signal status of I1.
4. Sweep the input from 0 to 12 V in 0.5 V steps. Record the actual transition voltage at which the status bit changes from 0 to 1. Expect 7.5-8.5 V; values outside this range indicate a wiring or supply problem.
5. Switch the input to AI1 by adding an AI block to the program and downloading. Observe the raw count. Sweep the input and verify the count tracks linearly: 0 V = 0 ± 5, 5 V = 500 ± 8, 10 V = 1000 ± 15.
6. Insert the Analog Threshold Trigger and verify the ON/OFF transitions at the programmed raw counts. Use a multimeter or the LOGO! on-device display to confirm the trigger output state.
7. Toggle the input repeatedly between 0 V and 5 V (within the LOW band) to confirm I1 reads 0. Toggle between 8 V and 24 V (within the HIGH band) to confirm I1 reads 1. Toggle between 5.5 V and 7.5 V and confirm the status is undefined or read as 0 — this is the expected band behavior.
8. Document the measured transition voltage, the analog linearity error, and the trigger hysteresis behavior in the commissioning report.

15. Troubleshooting Matrix

16. Cross-Reference: LOGO! 8.3 and 8.4 Behavior

The analog threshold behavior described in this article applies uniformly across the LOGO! 8, 8.1, 8.2, 8.3, and 8.4 firmware generations. Firmware-specific differences are limited to:

• LOGO! 8.3 (FS:03+): Adds the LOGO! Web Editor (LWE) for browser-based programming. AI block behavior is unchanged.
• LOGO! 8.4 (FS:04+): Adds MQTT publish from the LOGO! 8 BM (Base Module) for direct cloud push; AI block behavior is unchanged.
• All versions: The 5 V / 8 V input recognition thresholds are hardware-defined and do not change with firmware updates. Threshold values are not user-adjustable; only the analog-to-digital conversion range is configurable via the AI block properties.

For the official specifications, refer to the Siemens LOGO! 8 system manual on the Siemens Industry Online Support portal and the LOGO! product page on siemens.com. The IEC 61131-2 standard (Programmable controllers — Part 2: Equipment requirements and tests) defines the type 1 digital input characteristics that the LOGO! 12/24 module complies with.

17. LOGO! 12/24 vs LOGO! 230R: Input Topology Comparison

Engineers sometimes confuse the LOGO! 12/24RCE (low-voltage DC inputs) with the LOGO! 230RCE (high-voltage AC/DC inputs). The recognition thresholds are completely different:

Use the 12/24RCE for any 24 V DC industrial sensor network, and the 230RCE for 230 V AC signaling or where isolation from the 24 V DC rail is required. The two are not interchangeable, and using a 230RCE for a 24 V signal will fail because the 79 V AC / 88 V DC high threshold is far above 24 V.

18. Advanced: Variable Memory (VM) and UDF Blocks

For complex threshold logic, the LOGO! 12/24 provides Variable Memory (VM) addresses that can be shared between the on-device program and LOGO! Soft Comfort. VM addresses allow you to compute a threshold in one part of the program and use it in another, enabling adaptive thresholds that follow process conditions. For example, a "set threshold" button could overwrite VM0 with the current AI1 value, and a comparator block could use VM0 as the trigger level.

User Defined Function (UDF) blocks let you encapsulate a sub-program as a reusable block, which is useful for standardizing threshold trigger configurations across multiple LOGO! modules. UDF blocks are configured in LOGO! Soft Comfort and downloaded to the module. The 12/24RCE supports up to 16 UDF blocks per program, each with up to 8 inputs and 8 outputs.

19. Networking and Data Logging

The LOGO! 12/24RCE includes an integrated Ethernet port supporting Modbus TCP and S7 communication. This allows the AI1-AI4 raw counts and the threshold trigger output states to be polled by an external HMI or SCADA system. For data logging, the LOGO! 8.4 BM supports microSD cards (up to 32 GB) for CSV data logging of analog values, threshold transitions, and event timestamps. The microSD card is inserted in the slot on the right side of the module and formatted as FAT32.

For remote monitoring, the LOGO! 8.4 with the LOGO! BM (Base Module) can publish MQTT messages directly to a broker, enabling cloud-based dashboards for AI values and threshold events. Refer to the LOGO! 8.4 system manual for MQTT topic configuration and broker setup.

20. Frequently Asked Questions