Discrete I/O Module reads

Discrete I/O Module reads
Discrete I/O Module reads

Discrete I/O Module reads;- The most common type of I/O interface module is the discrete type. This type of interface connects field input devices of the ON/OFF nature such as selector switches, pushbuttons, and limit switches. Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that require simple ON/OFF switching.

Discrete I/O Module reads
Discrete I/O Module reads

Discrete I/O vs. Analog I/O: What’s the Difference?

When it comes to learning the basics of PLC programming, it can be as easy as clicking an on and off switch. This is the main difference between discrete and analog I/O modules! Separate signals are turned on or off, while analog signals are different.

PLCs are responsible for collecting data from input signals and sending information through output signals to control devices in order to perform specific functions. Depending on the application or device, the PLC programmer must decide whether to use discrete or analog I/O.

When learning the basics of PLC programming, it is essential to have a basic understanding of inputs and outputs, and the main differences between discrete and analog input/output modules. Keep reading to gain a deeper understanding of these two mechanisms.

Discrete I/O vs. Analog I/O: What's the Difference?
Discrete I/O vs. Analog I/O: What’s the Difference?

Typical Discrete I/O Module Specifications

This discrete input module voltage value specifies the magnitude (e.g., 5 V, 24 V, 230 V) and type (AC or DC) of user-supplied voltage that a module is designed to accept.
Input modules are typically designed to operate correctly without damage within a range of plus or minus 10 percent of the input voltage rating. With DC input modules, the input voltage may also be expressed as an operating range (e.g., 24–60 volts DC) over which the module will operate.

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This discrete input module specification specifies two values: a minimum ON-state voltage that is the minimum voltage at which logic 1 is recognized as absolutely ON; and a maximum OFF-state voltage which is the voltage at which logic 0 is recognized as absolutely OFF.


This value specifies the minimum input current that the discrete input devices must be capable of driving to operate the input circuit. This input current value, in conjunction with the input voltage, functions as a threshold to protect against detecting noise or leakage currents as valid signals.


This value specifies the maximum temperature of the air surrounding the I/O modules should be for best-operating conditions.


Also known as response time, this value specifies the maximum time duration required by an input module’s circuitry to recognize that a field device has switched ON (input ON-delay) or switched OFF (input OFF- delay).
This delay is a result of filtering circuitry provided to protect against contact bounce and voltage transients. This input delay is typically in the 9 to 25-millisecond range.


This AC or DC value specifies the magnitude (e.g., 5 V, 115 V, 230 V) and type (AC or DC) of user-supplied voltage at which a discrete output module is designed to operate.
The output field device that the module interfaces to the PLC must be matched to this specification. Output modules are typically designed to operate within a range of plus or minus 10 percent of the nominal output voltage rating.


These values specify the maximum current that a single output and the module as a whole can safely carry under
load (at rated voltage). This rating is a function of the module’s components and heat dissipation characteristics. A device drawing more than the rated output current results in overloading, causing the output fuse to blow. As an example, the specification may give each output a current limit of 1 Amp. The overall rating of the module current will normally be less than the total of the individuals. The overall rating might be 6 Amps because each of the eight devices would not normally draw their 1 Amps at the same time. Other names for the output current rating are maximum continuous current and maximum load current.


An inrush current is a momentary surge of current that an AC or DC output circuit encounters when energizing inductive, capacitive, or filament loads. This value specifies the maximum inrush current and duration (e.g., 20 A for 0.1 s) for which an output circuit can exceed its maximum continuous current rating.


Short circuit protection is provided for AC and DC output modules by either fuses or some other current-limiting circuitry. This specification will designate whether the particular module’s design has individual protection for each circuit or if fuse protection is provided for groups (e.g., 4 or 8) of outputs.


This value specifies the amount of current still conducting through an output circuit even after the output has been
turned off. Leakage current is a characteristic exhibited by solid-state switching devices such as transistors and triacs and is normally less than 5 milliamperes. Leakage current is normally not large enough to falsely trigger an output device but must be taken into consideration when switching very low current sensitive devices.


Recall that I/O module circuitry is electrically isolated to protect the low-level internal circuitry of the PLC from high voltages that can be encountered from field device connections. The specification for electrical isolation, typically 1500 or 2500 volts AC, rates the module’s capacity for sustaining an excessive voltage at its input or output terminals. Although this isolation protects the logic side of the module from excessive input or output voltages or current, the power circuitry of the module may be damaged.


This specification defines the number of field inputs or outputs that can be connected to a single module. Most commonly, a discrete module will have 8, 16, or 32 circuits; however, low-end controllers may have only 2 or 4 circuits. Modules with 32 or 64 input or output bits are referred to as high-density modules. Some modules provide more than one common terminal, which allows the user to use different

Sources: Programmable Logic Controllers-Frank D Petruzella

Overview of Input and Output Communication

If you’re brand new to PLC training, I/O is a widely used term amongst technicians which stands for “Input/Output.” I/O is the communication between an information processing system and the outside world. Inputs are the signals, or data, received by the system and outputs are the signals sent from it.

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One of the simplest ways to think of I/O is by using your computer as an example. Your computer is an information processing system, and the user is “the outside world.” Computer inputs would be the keyboard and mouse which receive commands from the user, whereas the outputs would be the monitor and printer. When you think of it in terms of a PLC, you should note that PLC’s are composed of two basic sections: the Central Processing Unit (CPU) and the I/O interface system. The I/O is what connects the information processing system (the CPU) to the outside world, like the machines and Human Machine Interface (HMI). Input modules detect the status of input signals, such as push buttons, switches and sensors. The output modules control devices such as motors, relays, and lights.

Discrete Input/Output Module PLC

Now that we’ve covered the basic information with regards to input and output signals, let’s get into the differences between discrete and analog systems.

Also called “digital I/O,” discrete I/O modules in PLCs refer to data when it is in one of two states: on or off. The signals are also referred to as 1 and 0, or open and closed. Discrete I/Os are typically far more simple than analog I/Os when learning PLC programming. An example of a discrete I/O would be a push-button switch, which turns a motor off and on with the use of pressure. Proximity switches are also an example of discrete I/O, as they can detect the presence of an object without physical contact, turning off or on without the use of force. A PLC becomes aware of a discrete sensor’s state when it receives a signal from a discrete input channel. Inside a discrete input module are typically a set of light-emitting diodes (LEDs) which become energized when the device is turned on. A photosensitive device turns on when it senses the LED, which activates inside the PLC’s memory.

The PLC then sends this information to control devices through a discrete output channel. Similarly, the PLC sends an LED signal which activates a photosensitive device. When sent through the PLC’s computer circuitry, these electrical power loads allow the control devices, such as a motor starter or indicator lamp, to function.

Analog Input/Output Module PLC

Analog I/O refers to signals with a range of values greater than 1 or 0, and are not simply on/off or open/closed. It typically measures voltage or current from the input, and supplies it to the output. Because analog I/O refers to ranges rather than one of two states, it’s a bit more complicated to learn for PLC programmers in training, especially if you’re new.

Analog input examples include temperature sensors, oil pressure sensors, CO2 sensors, and weight scales. Outputs would be used to control the power, current, or voltage sent to these sensors. Unlike discrete input modules which only use binary numbers “0” and “1,” analog input modules usually measure analog inputs in one of the following forms: -10 to 10 VDC, 0 – 10 VDC, 1 to 5 VDC, 0 to 1 mA, or 4 – 20 mA. In order for the PLC to understand the analog signals, they must be converted through an A/D converter or an analog to digital converter.

The analog outputs work similarly to discrete outputs, with the PLC sending the signal to the control devices such as control valves or variable frequency drives controlled by the PLC. An analog signal that is most common in our homes, and is a great example for newer PLC programmers, would be the dimmer switch. The dimmer switch can increase and decrease in small increments, allowing light to filter through the bulb; making it more complex than a simple on and off switch.

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conclusion of discrete i/o

In this post, we looked at discrete I/O, analog I/O and industrial communication protocols. These three elements are responsible for carrying out all the wishes of the CPU PLC. The CPU looks at data from discrete input units, analog input units, and Modbus slaves (or other industrial communication devices). Then the CPU implements logic on that input data and activates/deactivates discrete output units, analog output units, or Modbus subunits. If you can put PLC I/O into these three categories in your head (discrete, analog, communications), you’ll have a great foundation for knowing how to get data in and out of a PLC.

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