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What is the Difference Between Digital and Analog Inputs in PLCs?

PLC Inputs Overview  

PLC controlled systems have become the backbone of almost all industrial automation processes globally. A PLC collects data from field devices, processes it as pre-programmed, and passes it as output to another system. To accomplish this, PLCs are equipped with sophisticated electronic input modules (for collecting field data) and output modules (for providing important information to the field devices). The Input/Output modules form a fundamental part of the PLC system that connects the PLC’s CPU, to the real world, or rather the machines/devices being controlled.

The data collected from the field devices are considered as the input to the PLC controller. Essentially, PLC systems comprise a variety of input modules to monitor real-world (machine) parameters or to receive discrete and/or analog input command signals from human interface devices, central control computers, or other PLCs. In addition to detecting the status of the input signals, input modules are also required to connect the PLC system to the field input devices.

The multiple inputs used in PLCs are broadly categorized as Analog and Digital inputs. Hence, Analog and Digital Input modules make up a major subsystem in all PLC systems, as they provide an interface between the CPU and the Output modules, as shown in the diagram below. This article is meant to familiarize you with the two major categories of PLC inputs and the difference between them.

PLC Structure

1) Digital PLC Inputs

The most common types of PLC inputs are digital inputs, often referred to as discrete inputs. Digital PLC inputs are signals which are either ON or OFF, that are applied to PLC controllers from discrete field devices. The concept of digital signals is derived from the Binary number system, in which the only possible digits are 0 or 1, hence the name binary inputs. With 1 indicating a HIGH state and 0 representing a LOW state. Note, even though HIGH signals are represented as 1, in some PLCs these signals are not represented by 1V(volt). This is because PLCs are usually rated to operate on 24 VDC (volts DC). Meaning that such a PLC will only read HIGH at the input side when the output voltage of the connected field input device is at 24 VDC.

A PLC system uses binary inputs (1 or 0) to determine the status of a given field input device, whether it is OFF or ON. For example, if the operating voltage of a given PLC input module is 24 VDC, then 0VDC will act as OFF state (binary 0) while 24 VDC will act as ON state (Binary 1). Therefore, digital inputs are always either ON or OFF, and they can also be described as either 1 or 0, OPEN or CLOSED. PLCs can easily process digital input signals, as they are digital devices themselves. In summary, in a PLC system, the digital input module usually provides status in different forms such as:

  • General Status: High/ Low, 1/0 and True/False  
  • Load Condition: ON/OFF 
  • Switching Mechanism: Activated/Deactivated  
  • Switching Contact Status: Close/Open 

Note, digital inputs in PLCs have an LED indicator on the input module for setup, diagnostics, and troubleshooting. Simply, if the input module is ON then the field input device is also ON. But in cases where the LED is ON but the input device is required to be OFF, then you should check your PLC system wiring, or adjust the input sensor, and in some cases, the input device could possibly be defective. These checkouts also apply if the LED is OFF but it is expected to be ON, with the addition of inspecting the protection fuses of the input module. Thus, troubleshooting digital inputs to the PLC is much easier and faster.

Common Digital Input Devices in PLCs 

In most PLC applications, there are a number of input devices that you are most likely to come across. These input devices provide digital input signals to PLC systems. They include:

A) Push Buttons

Push buttons are usually the START or STOP buttons you find in a PLC control panel. These buttons function by either making or breaking contact. Hence, they are categorized as either Normally Open or Normally Closed.

Connecting a Normally Open (NO) push button to a PLC circuit creates an OPEN circuit. In such a case, there is no electrical continuity and current cannot flow through the push button switch. When Normally Closed (NC) pushbuttons are connected to the PLC circuit, they short the connected terminals thereby closing the circuit. This creates electrical continuity and current can readily flow through the circuit.

B) Selector Switch

Selector switches are manually operated by turning a lever or a rotary knob to select one of the two or more positions. Instead of just being Normally Open or Normally Closed, selector switches have more than two contacts to choose from. They are mainly used to make or break or change the connections in a PLC circuit, used to control electronic or mechanical, or electrical devices. The most common example of a selector switch application is in electric fans, where you turn the switch to select a number which then dictates the speed of the motor in the fan. What happens in such a process, is that by turning the switch you usually select a varying load for the fan’s motor thereby controlling its speed.

C) Proximity Sensor

In PLC automation, proximity sensors are often used to detect the presence or absence of nearby objects made of varying materials, without making contact. To achieve this, the proximity sensors use a beam of electromagnetic radiation (i.e., infrared) or they emit an electromagnetic field and determine the changes in the return signal or the field.

The object being located is known as the proximity sensor’s target. Different targets require different types of proximity sensors. For instance, inductive proximity sensors are often used in metal fabrication processes, as they always require a metal target. On the other hand, capacitive proximity sensors may be suitable for a plastic target. Sometimes, proximity sensors are called “proximity switches” as they provide binary output, HIGH or LOW—similar to a switch.

D) Photoelectric Sensors

These sensors use a light-emitting diode (LED) which functions as an emitter, and photodiodes or phototransistors at the receiver side. When the emitted light (normally Infrared light) from the LED hits the receiver, the state of the photoelectric sensor changes from LOW to HIGH. In PLC automation, photoelectric sensors can be used in creative ways as they have different modes of operation.

E) Limit Switches

As the name implies, these switches change state whenever the pre-determined or specified limit is reached. They are very useful in PLC-controlled systems as they can be used to set the limit where a specific process stops. There are different types of limit switches, which give you the flexibility to choose the physical quantity you would like to limit whenever you’re designing a PLC-based control system. They include:

  • Pressure Limit switch: This switch changes its state when the gas or liquid in a tank attains a high enough pressure. You can either have a Normally Closed or Normally Open pressure limit switch. These switches are commonly used in containers where pressure regulation of gases or liquids is crucial. 
  • Temperature Limit Switch: Also known as a Thermostat, this switch is used to detect when the specified maximum or minimum system temperatures are attained. Temperature limit switches can also be either Normally Closed or Normally Open, depending on the type you use.  
  • Level Limit Switch: Level switches or Level Sensors are used to control the height of liquid within a container, usually a tank. They are mostly used in liquid-level control systems in conjunction with inlet and outlet valves. 

Sourcing Versus Sinking in Digital PLC Inputs 

The concept of Sinking versus Sourcing in digital PLC Inputs is confusing to most people. But it is a fairly simple concept. The basic electrical theory states that to complete a circuit and for current to flow, Direct Current (DC) must flow from DC+ through a load to DC-. Therefore, sourcing and sinking have to do with which side of circuit polarity you’re completing when interfacing your PLC input module.

Usually, digital input devices have internal switching circuits which classify them as either Sourcing (PNP) sensors or Sinking (NPN) sensors. PNP sensors provide +24V as input, while the NPN sensors provide -24V as input. In that case, the sourcing PLC input is the input from the field device connected to the DC+ side of the circuit. While the sinking input is produced by the input device connected to the DC- side of the circuit.

2) Analog PLC Inputs

An analog PLC input is a continuous signal from a field input device to the PLC control system. Analog inputs are characterized by signals with a range of values much greater than just 0 or 1. The analog signals can either be an a13-bit or a 12-bit signal. Generally, in a PLC system, the analog input module measures current or voltage signals from an input device. These signals are usually measured in the range of 0-20 mA(milliampere), 4-20 mA, 1-5 VDC (volts DC), 0-10 VDC, etc. This means that the connected input device provides signals with any intermittent value between the initial to the final range (the two extreme limits), for the PLC’s analog input module.

The variations in the produced input signals are dependent on the field conditions. For example, if you’re using a PLC system to monitor the pressure of water flowing through a pipe, probably using a pressure transmitter calibrated between 0-10 Bar; then the pressure transmitter will produce a voltage signal varying between 0V to 10V proportional to the pressure variations in the pipe. Similarly, any other analog input to the PLC can decrease or increase in very small increments within the set extreme limits. Note, there are other types of analog signals that can be applied to a PLC, but the voltage and current signals are the most common types of analog PLC inputs.

Voltage-Mode Analog PLC Inputs are usually produced by analog sensors, in form of continuous voltage signals which vary depending on the changes experienced in the operating environment. They are most extensively used in distance sensing, pressure sensing, temperature sensing, and luminance sensing. While the voltage inputs are capable of almost all of the monitoring tasks in general applications of PLCs, current-mode signal transmission is much preferred for signals that need to be transmitted over longer distances. For this reason, a current-mode analog interface is used from the PLC end.

Examples of Analog Input Devices in PLCs 

A) Position/Displacement Sensors

Position sensors measure the distance between the location of the target object and a specified reference point. Often the resistor of a position sensor is inside the PLC module. These sensors are mostly used in fabrication processes where higher precision levels are required. Hence, they are undoubtedly analog input devices, as they produce a range of values. Examples of displacement sensors include:

  • Linear and Rotary Potentiometers: To show changes in displacement of target objects, these potentiometers operate on the principle of Ohm’s Law. Whereby, changes in their output voltages are used to represent distance variations of the target objects. In PLC automation, linear potentiometers are used when the target objects are being displaced in a straight line. While rotary potentiometers are used in PLC control systems when the movement of the target objects is circular, as they measure angular distance. 
  • Linear Variable Differential Transformer: This type of transformer is controlled by the displacement of a ferrous magnetic core between a primary winding and two secondary windings. Whenever the ferrous core is centered, the voltage values of the secondary windings will equalize. And when the windings are moved up or down by the core, the voltage differences between the two secondary windings can increase or decrease in small intervals. A Linear Variable Differential Transformer is used in PLC-controlled systems whenever a constant AC (Alternating Current) voltage is used to represent changes in distance. 

B) Thermocouple Input Devices

A thermocouple is a temperature sensor, that is used to measure temperature variations. It is made up of two different types of metal wires connected together to form a junction. Such that, whenever the junction is cooled or heated, a small voltage signal is produced in the electrical circuit of the thermocouple sensor. The generated voltage signals are usually small analog signals measured in the millivolt(mV) range.

The thermocouple input devices used in PLC systems are designed and built to be robust, with improved accuracy, linearity, and faster response time. They also comprise specially developed metal alloys, which enhance their capability to measure very wide temperature ranges. For example, a thermocouple sensor can be used to measure very high operating temperatures of up to 1600°C (white heat) in unique industrial applications.

D) Resistance Temperature Detector (RTD) Inputs

The RTDs are a class of sensors that change their resistance whenever the temperature of the medium in which they are inserted changes. The change in their resistance is linearly proportional to the temperature variations of the medium. This means that as the temperature of the medium increases, the resistance of the RTD sensor will also increase. Hence, by measuring the resistance of that particular RTD sensor, it is possible to determine the temperature of the medium.

RTD sensors are similar in functionality to thermocouple sensors, but they are generally more robust and accurate compared to the latter. They are therefore used in many industrial processes which require accurate and repeatable temperature measurements such as in chemical reactions, heating, and cooling, pasteurization processes, etc.


As explained in this article, digital and analog PLC inputs comprise the data collected from field input devices and applied to the PLC system. Digital inputs are binary in nature, and they can only be 1 or 0. In most PLC systems, the digital input is usually either 0V(volts) or 24V depending on the type of input module used. Some specific real-world examples of digital inputs to a PLC system include a conveyor belt position sensor, running or stopped generator, a water tank level sensor, and open or closed-circuit breakers.

On the other hand, analog inputs consist of continuous signals, that are usually measured in the range of 0-20 mA(milliampere), 4-20 mA, 1-5 VDC (volts DC), 0-10 VDC, etc. Some examples of real-world analog inputs to PLC systems used in an industrial setup would include weight scales, oil pressure sensors, and temperature sensors (thermocouples, RTDs, etc.)

In PLC automation, the knowledge of the different types of PLC inputs is a highly important prerequisite. If you have read up to this point, you are now familiar with the differences between digital and analog inputs applied to PLC control systems; along with a couple of examples of digital and analog input devices used in an industrial environment. This knowledge will assist you in expanding the number of control system designs that you create using PLCs. For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on October 18th, 2021 and is filed under PLC, Uncategorized. Both comments and pings are currently closed.

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