PLC Discrete Input and Output Devices

Hi fellows, I hope all of you are fine. As you already know we have started a series of PLC tutorials. In last lecture, we discussed a detailed article on plc input and outputs that are already designed on the plc module. In today’s tutorial, I am going to explain to you about a detailed article on discrete plc input-output devices. In the previous article plc fixed inputs and outputs were discussed with detailed.

Plc is easy to use which provides a lot of features to users like fix inputs and outputs it also has discrete input and output capability. This makes it very useful because we can make connections of any instruments or components with it according to our uses and circuitry requirements. But the main thing we should also follow is the design of the connection of manufacturers of PLC. So let’s get started with a plc discrete input and output devices.

PLC Discrete Input and Output Devices

• In plc, a very common category of input and output interfacing modules is discrete.
• This category of interfacing links field input instruments of the ON or OFF landscape like selecting switches, push-buttons, and limiting switch.
• Similarly, the output controller is restricted to components like light, electrical relays, solenoids, and starter switches which need simply ON or OFF signal.

• The arrangement of separate inputs and output works on bit-adapted inputs and outputs.
• In this sort of input and output, every bit signifies a whole info in himself and delivers the rank of some exterior connection or directs the attendance or nonappearance of power in a procedure circuitry.

PLC Discrete Input and Output Devices Power Supply

• Every discrete input and output device gets power from separate supply systems.
• As these voltages can be of different ratings or categories, so input and output elements have ability to work on the different alternating current and direct current voltage levels.
• The components themselves take their voltage and current for appropriate process from the backplane of the frame into which they are introduced, it explained in a given diagram.
• Backplane current and voltage are delivered by the PLC unit power supply and are used to provide power the electronics circuits which exist in the input and output unit circuitry panel.
• Such output devices that consume higher power received voltage from a user-provided power source.

PLC Discrete AC Input Module

• In given diagram, we can see that there is one input of a distinctive (AC) discrete input unit.
• This input circuitry consists of two main parts first one is power portion and the other one is a logic portion.
• An optical separator is used to offer electrical separation amid the field cabling and the PLC backplane interior electrical structure (circuits).

• The input light-emitting diode blinking shows the condition of input components.
• Logic circuitry processes the digital signal to the CPU.
• Interior PLC controller circuitry usually works at 5V DC or a smaller amount volts.

PLC Single Input of a Discrete AC Input Module

• In a given diagram basic circuit of a single input of a distinct alternating current input unit is shown.
• The working of this circuit is explained below with a detailed description.
  • The noise filter at input consists of capacitor © and two resistors (R1), (R2), that eliminate the incorrect signal caused by bouncing of contact and electrical distortion.
  • When we closed the pushbutton, 120V AC is given to bridge rectification circuitry.
  • This circuit converts 120V AC into the less level dc voltage which is delivered to LED of an optical separator. The voltage of Zener is set at such a level which can be detected easily.
  • When light of led(light-emitting diode) collides with the photo-transistor it starts working and the position of the pushbutton is transferred in logical form to the CPU.
  • The optical separator not only isolates the complex alternating current (AC) input voltage from the logic circuitry but also stops harm to the CPU due to transitions exist in line voltage. This separation also reduces the electrical noise, which can source of the irregular process of the CPU.
  • For error analysis, an input light-emitting diode is on when the input push-button is pressed. This display can be connected with an optical separator at any side.
  • An AC or DC category of input unit is used for both alternating and direct current inputs as the input polarity has no effect.
  • A PLC input unit has capacity to isolate all inputs and have common connections for these inputs

PLC Discrete Input Modules Tasks

• discrete input module in plc do four important functions which are described below.
  • Sensing of signal when it is received by field instruments.
  • After sensing of signal Alter the signal into the precise voltage level for the specific PLC.
  • It also Separates the PLC from variations in the input signal voltage and current.
  • After all these processes it refers a signal to the CPU representing which sensor created the signal.

PLC Discrete AC Output Module

• The given diagram explains the circuit for one output of a distinctive separate output unit.
• Similar to the input unit, it also consists of 2 main portion, the first one is the power unit and the second one is the logic unit, these are joined with each other by separation circuitry.
• The output interfacing can be considered as an electric switch that controls the output instruments by changing their state from on to off, or off to on.

• Logic circuitry regulates the output position. An output light emitting diode shows the position of the output sign

PLC Single Input of a Discrete AC Output Module

• Basic figure for a single output of a distinct alternating current output unit is displayed below.
• The process of the circuitry is described here.
  • The digital logic circuitry of the CPU gives the outputs according to the feed program.
  • When the CPU sends commands to the energization of the load, then a voltage is provided to the light-emitting diode of the optical isolator.
  • The light-emitting diodes then start illuminating, and by this light photodiode start its operation.
  • Then this photodiode energizes the triac alternating semiconductor switch and current starts to flow to the output.
  • As the triac works in any way, the output to the load is ac.
  • The triac, instead of having ON and OFF position, in reality, has LOW and HIGH resistance points, correspondingly.
  • In it OFF conditions a lesser value leak current of some milliamperes still moving through the triac circuitry.
  • As with input circuitry, the output interfacing is normally connected with a light-emitting diode which specifies the rank of every output.
  • Fuses are usually essential for the output unit, and they are connected on a per circuitry base, it provides protection to each circuitry and works separately.
  • Several units also deliver pictorial displays for fuse position.
  • The triac circuit cannot be used to trigger a direct current load. For error analysis, the light-emitting diode output light is on when the PLC controller is sending commands for the on condition of the load.

Interposing Relay

• Single alternating currents outputs are normally restricted by the dimension of the triac to one ampere or two amperes. The extreme current load for one unit is also identified.
• For the protection of the output unit circuitry, the current value should be in the rating current limits.

• For monitoring of the higher loads, like high horsepower motors, a normal switch relay is linked to the output unit.
• The links of the relay can be used to switch a higher load that is shown in a diagram.
• Control relay used for this circuitry which is also shown in a given diagram is known as interposing relay.

PLC discrete control input and output devices

push-buttons light switches,  and proximity switches. discreet input devices and discrete output devices are relay lights and motor starters.

PLC discrete input module

Discrete input modules do 4 operations in the PLC control system that is signal sensing when getting from field devices Transform input signal to accurate voltage value for plc. Separate plc from variation in input signal voltage or current

PLC discrete input examples

pilot lights, solenoid valves, motor control relays, and valve controls.

I also have uploaded some related articles to plc you read them for further learning.

• Introduction to PLC
• Working of PLC
• Parts of PLC
• Input and Output Section of PLC

Faqs

What is discrete input and output in PLC?

• Discrete input and output are simpler than analog I/Os when learning PLC programming. The common discrete input and output are push-button switch, which turns a motor off and on with the use of pressure

That is the detailed tutorial on PLC Discrete Input and Output Devices if you have any queries about it ask in comments. See you in the next tutorial Analog Input and Output Devices of PLC.

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