Nowadays, Worldwide almost every industry is upgrading itself and shifting towards Automation technology to increase its production. PLC and DCS are the backbone of Automation technology. Analog IO (AI and AO) and Digital IO (DI and DO) are the most basic modules used in PLC, DCS, and microcontroller-based control systems.
Since in every system, Input and Output are very important. The system takes the input signal, processes it, and then produces the desired output signal. Both Analog IO and Digital IO are fundamental signal-processing circuits that have their own characteristics.
In this article, We will discuss different Analog I/O (AI and AO) and Digital I/O (DI and DO) modules, and the difference between DI, DO, AI, and AO. But before that let’s understand what is input-output (IO) modules?.
What are IO or I/O modules?
IO or I/O is pronounced as Input-Output modules that provide an interface between CPU and programmable device.
The module that connects with the input device to collect the input signal is known as the Input module. The module that connects with the output device to display the output signal is known as the Output module. Combinedly both are known as IO or I/O modules.
Types of IO or I/O modules
- Analog IO or Analog I/O
- Digital IO or Digital I/O
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- Analog input – AI
- Analog output – AO
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- Digital input – DI
- Digital output – DO
Analog IO or Analog I/O
It is a fundamental signal processing circuit mostly used in PLC and DCS. It is also called a continuous module. Analog IO refers to signals with a range of values ( like 0-10V ) rather than one or two states (binary state, 0 and 1). Whenever Analog I/O is used in PLC and DCS it requires an extra ADC (Analog to digital converter) circuit.
Analog IO modules are categorized as two types:
- Analog input – AI
- Analog output – AO
Analog input – AI
Analog input (AI) is a continuous time-varying input signal to the PLC/DCS. Whenever a system or device takes an input signal in analog format then it is known as Analog input – AI. In most of the standard automation systems, the Analog signal becomes generally continuous time-varying voltage and current signal having the range of values (0-10V), and (4-20mA) respectively.
Examples of Analog input (AI) include any switches and sensors that sense the physical data varying with respect to time such as The temperature sensor (provides analog signal and the voltage varies depending upon temperature), Water flow detector, Pressure detection switch, Position detector, etc. Transducers are also examples of Analog input – AI.
Analog output – AO
Analog output (AO) is a continuous time-varying output signal to the process control device (such as motor speed controller, valve actuator, etc.) A variable AO signal to a motor will control the speed of the motor where 0V = OFF and 1-10V = 10% to 100%. Similarly, a control valve actuator can have a variable position between opening and closing with a variable AO.
A variable frequency device (VFD) is a perfect example of Analog output – AO. If you have a variable frequency device and you want to give a speed reference signal to it then you can use Analog output (AO) of PLC/DCS.
Other examples of Analog output – AO includes: Temperature transmitter, Thermocouples, Pressure transmitter, Water flow transmitter, Level transmitter, etc.
Digital IO or Digital I/O
It is also the fundamental signal processing circuit mostly used in PLC and DCS. It is also called a discrete module. Digital IO refers to signals that have binary states (0 and 1) rather than a continuous range of values. Whenever a Digital IO module is used in PLC and DCS then it doesn’t require any extra convert circuit.
Digital I/O signal gives status in different forms like:
- High/Low, True/False, and 1/0 for general status
- ON/OFF for switching of load
- Close/Open for switch contact status
Digital I/O modules are categorized as two types:
- Digital input – DI
- Digital output – DO
Digital input – DI
Digital input – DI is basically a binary input signal (0 and 1) used in PLC/DCS. Whereas the magnitude of binary input is generally a constant step of 5V up to 230V depending upon the signal and types of the card used in PLC. DI will indicate the status of devices either ON or OFF.
In other words, Whenever any system takes an input signal in digital format then it is known as Digital input. In short digital input is abriviated as DI. It is a discrete signal.
Simply, We can understand it as Any push button and switch produce Digital input – DI signal to the automation system.
Digital inputs (DI) are used to check the status of any device whether it is ON or OFF. For any input module of 12V dc, 0V acts as OFF state (binary 0) and 12V acts as ON state (binary 1).
Examples of Digital input – DI signal includes:
- The keyboard in our computers and laptops is the best example of DI.
- Remote control and it’s receiver is another example of DI.
- A simple switch of a light bulb (only ON and OFF) is also an example of DI.
Digital output – DO
Digital output – DO is a binary output from PLC/DCS to the process control device. Whenever any system or device generates an Output signal in binary or digital format then it is known as Digital Output or in short DO. It is used to simply either ON or OFF any field equipment or devices. DO can also be used to operate relays, indicating lamps, solenoid valves, etc. remotely, or as a command to any other devices.
Examples of Digital Output (DO) signal includes:
- An LED display is an example of DO.
- H-bridge controller.
- Valve or Solenoid control using PLC.
- Relay control using PLC.
Difference between DI, DO, AI and AO
Since DI, DO, AI, and AO all are inputs and outputs of PLC/DCS. These are different from each other and are discussed below:
Difference between AI and DI
| Difference between AI and DI | |
| Analog input – AI | Digital input – DI |
| AI has a time-varying continuous input signal | DI has a constant discrete input signal |
| Generally, it is a continuous voltage and a current signal having a range of values (0-10V) and (4-20mA) | It has binary values (1 and 0) either ON or OFF state |
| Temperature detectors, Pressure detectors, Water flow detection switches, Position detectors, and Transducers are examples of AI | Push buttons, Selector switches, Rocker switches, Toggle switches, Limit switches are examples of DI |
Difference between AO and DO
| Difference between AO and DO | |
| Analog output-AO | Digital output-DO |
| AO has time time-varying continuous output signal | DO has a constant discrete output signal |
| Generally, It is continuous voltage, and a current signal having a range of values (0-10V) and (4-20mA) | It has binary values (1 and 0) either ON or OFF state |
| Temp. transmitters, Pressure transmitters, Position transmitters, and Thermocouple are examples of AO | Relays, Indicating lamps, and Solenoid valves are examples of DO |
Hope that the above content helps you to understand the Different Analog I/O and Digital I/O (DI, DO, AI, AO) and also the difference between DI, DO, AI, and AO.
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