Inputs and outputs from a PLC

PLC
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Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputs and outputs can be categorized into two basic types: logical or continuous.

Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to different levels, it is continuous.

Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control. As a result most controls applications (and PLCs) use logical inputs and outputs for most applications.

Hence, we will discuss logical I/O and leave continuous I/O for later. Outputs to actuators allow a PLC to cause something to happen in a process.

A short list of popular actuators is given below in order of relative popularity.

Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.

Lights - logical outputs that can often be powered directly from PLC output boards.

Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays.

Servo Motors - a continuous output from the PLC can command a variable speed or position.

Outputs from PLC

Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs.

Continuous outputs require special output cards with digital to analog converters. Inputs come from sensors that translate physical phenomena into electrical signals.

Typical examples of sensors are listed below in relative order of popularity.

Proximity Switches - use inductance, capacitance or light to detect an object logically.

Switches - mechanical mechanisms will open or close electrical contacts for a logical signal.

Potentiometer - measures angular positions continuously, using resistance.

LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling.

Inputs for a PLC

Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs. Sourcing and sinking inputs are also popular.

This output method dictates that a device does not supply any power. Instead, the device only switches current on or off, like a simple switch.

Sinking

When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied.

Sourcing

When active, current flows from a supply, through the output device and to ground. This method is best used when all devices use a single supply voltage.

This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular. This will be covered in detail in the chapter on sensors.

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A Programmable Logic Controller (PLC) is a specialized computerized device used in industrial automation to control and monitor machinery and processes. PLCs are equipped with various input and output modules to interface with the external world.

Let’s explore the inputs and outputs commonly found in a PLC system:

Inputs

PLC inputs are used to receive signals or data from external devices and sensors. These signals are typically used to monitor the status of equipment or collect data for control and decision-making.

Common types of PLC inputs include:

  • Digital Inputs (DI): Digital inputs receive binary signals, representing the presence or absence of voltage or current. They are commonly used to monitor the state of switches, push buttons, limit switches, sensors, and other digital devices.
  • Analog Inputs (AI): Analog inputs receive continuous signals, typically in the form of voltage or current, and are used to measure variables such as temperature, pressure, flow rate, or position. Analog inputs convert physical measurements into digital values for processing by the PLC.
  • Communication Inputs: Some PLCs also feature communication interfaces (e.g., Ethernet, serial ports) that allow them to receive data from external systems or devices, such as Human-Machine Interfaces (HMIs) or other PLCs.

Outputs

PLC outputs are used to control external devices and actuators based on the program’s logic and input data. They enable the PLC to activate or deactivate devices, such as motors, valves, solenoids, or indicator lights.

Common types of PLC outputs include:

  • Digital Outputs (DO): Digital outputs provide binary control signals to turn devices on or off. They are used to energize or de-energize coils, relays, or other devices that control motors, valves, or other digital equipment.
  • Analog Outputs (AO): Analog outputs generate continuous signals, typically in the form of voltage or current, to control variables such as motor speed, valve position, or process control loops.
  • Communication Outputs: PLCs with communication capabilities can also transmit data or control signals to external systems or devices, enabling data exchange or coordination with other equipment.

It’s important to note that the specific number and types of inputs and outputs available in a PLC depend on the model and configuration of the PLC hardware. PLC systems can be expanded with additional input/output modules to accommodate a larger number of signals or to support specific communication protocols and interfaces.

PLCs are programmed to monitor inputs, execute control logic, and generate appropriate outputs based on the desired automation tasks and processes.