This article identifies the logic level voltage thresholds as well as the input resistance for a PLC’s digital input.
Key Takeaways
- There is a defined voltage boundary for
TRUEandFALSEas shown in Figure 1. Those boundaries are 11 to 24 VDC and 0 to 9 VDC respectively. - There is a hysteresis region between
TRUEandFALSE. - The PLC input has a measurable resistance to ground.
- The values are for the tested Crouzet PLC. These are not universal. Always refer to the datasheet for your chosen PLC.
- Do not assume NPN and PNP sensors are interchangeable.
Please refer to this prerequisite article to better understand the distinction between NPN and PNP sensors.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Applied Theory
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 01 Sep 2026
Figure 1: Diagram showing the PLC’s logic thresholds where 11 VDC and above is a logic-1 while 9 VDC and below is a logic-0.
Logic Thresholds
The physical test setup is shown in Figure 2. The Crouzet Millenium Slim PLC was chosen as a representative component. Two power supplies are required. The one on the right provides 24 VDC to power the PLC. The variable supply on the left is used to provide a test voltage to the PLC’s input pin. The PLC is programmed to turn on the green indicator lamp when the digital input is above the logic-1 threshold.
By changing the variable supply voltage, we can determine the PLC’s turn-on and turn-off points by observing the green indicator lamp. The results are presented in Figure 1. All voltages below 9 VDC are recognized as a logic-0 while voltages above 11 VDC is recognized as a logic-1. The band between 9 and 11 VDC displays state-dependent memory.
This is a form of hysteresis where it takes 11 VDC to turn the device on while the voltage must drop to 9 VDC to turn it off. This is desirable as it provides a crisp transition and likely prevents indeterminate (chattering) digital input detection.
Tech Tip: You may have encountered hysteresis in a class dedicated to op amps. Recall that a comparator circuit uses positive feedback to make an output that is “sticky.” The output of the op amp-based comparator stays in one position until the input voltage crosses a threshold; it then toggles and remains “stuck” in the new position until the input swings hard in the other direction. The result is a system relatively immune to noise.
Input Resistance
The test setup for measuring input resistance is shown in Figures 2 and 3. A 10 kΩ series resistor is added between the PLC input and the variable power supply as shown in Figure 3. This resistor can be seen installed in the terminal blocks to the right of the PLC in Figure 2.
Figure 2: Test setup used to measure a PLC’s logic level thresholds and input resistance.
Figure 3: Wiring diagram showing the PLC’s digital input resistance as approximately 11.7 kΩ. There will be a voltage drop across the test series resistance.
In a recent test, 19.3 VDC was injected into the series resistor as shown in Figure 3. There was a corresponding 8.94 VDC drop across the 10 kΩ series resistor implying a current of 894 µA calculated as:
I_{in} = \dfrac{8.94 \ V}{10 \ k\Omega} = 0.894 \ mA
A voltage of 10.44 VDC was then measured across the PLC. The resulting input resistance is calculated as:
R_{input} = \dfrac{10.44 \ V}{0.000894 \ A} \approx 11.7 \ k\Omega
Both the input voltage thresholds and the input resistance closely align with the Crouzet PLC’s datasheet.
Implications
Using the language of microcontrollers, the PLC behaves as if it has an 11.7 kΩ pull-down resistor.
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Unconnected PLC inputs are acceptable. They will be read as a logic 0.
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Switches and sensors must pull the input resistor up to the 24 VDC rail. In doing so, they must supply approximately 2 mA.
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Our previously used 5 and 3.3 VDC logic is not compatible with the PLC digital input. However, these small signals may be read using the PLC’s analog inputs – perhaps a topic for another article.
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PNP type sensors are preferred because they are designed to pull this input up to the 24 VDC rail. NPN devices that pull down to ground will not work unless a low value resistor is added. This external pull resistance must be sized so that the resulting voltage divider places the idle voltage well into the logic-1 area as shown in Figure 1. This PNP vs NPN distinction and solution is further explained in this previous article.
Please share your comments and suggestions in the space below.
Best Wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.