DI and DO are Digital I/O whereas AI and AO are Analog I/O modules of PLC/DCS. Both are fundamental signal processing circuit of PLC and DCS.
Since in every system, Input and Output are very important. System takes input signal, process it, and then produced desired output signal. Both Analog IO and Digital IO are fundamental signal processing circuit have there own characteristics.
In this article, We will discuss different Analog I/O (AI and AO) and Digital I/O (DI and DO) modules, and difference between DI, DO, AI, and AO. But before that let's understand what is input-output (IO) modules?.
What is IO or I/O modules?
IO or I/O is pronounced as Input-Output modules that provide interface between CPU and programmable device.
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The module that connects with input device to collect input signal is known as Input module. Whereas The module that connects with output device to display output signal is known as 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
- Analog input - AI
- Analog output - AO
- Digital input - DI
- Digital output - DO
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Analog IO or Analog I/O
It is fundamental signal processing circuit mostly used in PLC and DCS. It is also called continuous module. Analog IO refers to signals with a range of values ( like as 0-10V ) rather than one or two states (binary state, 0 and 1). Whenever Analog I/O is used in PLC and DCS then it requires an extra ADC (Analog to digital converter) circuit.
Analog IO modules categorised 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 devices takes input signal in analog formate then it is known as Analog input - AI. In most of standard automaton system, the used 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 sence the physical data varying with respect to time such as: Temperature sensor (provide analog signal and thair voltage varies depending upon temperature), Water flow detector, Pressure detection switch, Position detector, etc. Transducers are also example 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 motor where 0V = OFF and 1-10V = 10% to 100%. Similarly a control valve actuator can have a variable position between open and close with a variable AO.
A variable frequency device (VFD) is 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 discrete module. Digital IO refers to signals have binary states (0 and 1) rather than continuous range of values. Whenever 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 form 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 categorised 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 magnitude of binary input is generally a constant step of 5V up to 230V depending upon the signal and types of card used in PLC. DI will indicate the status of devices either ON or OFF.
In other words, Whenever any system takes input signal in digital formate then it is known as Digital input. In short digital input is abriviated as DI. It is discrete signal.
Simply, We can understand it as: Any push button and switches produces Digital input - DI signal to 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:
- Keyboard in our computer and laptops is best example of DI.
- Remote control and it's reciver is another example of DI.
- A simple switch of light bulb (only ON and OFF) is also 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 devices generates Output signal in binary or digital formate 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 lamp, solenoid valve, etc. remotely, or as a command to any other devices.
So Examples of Digital Output (DO) signal includes:
- LED display is an example of DO.
- H-bridge controller.
- Valve or Solenoid controle using PLC.
- Relay controle 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 that are discussed below:
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Difference between AI and DI
| Difference between AI and DI | |
|---|---|
| Analog input - AI | Digital input - DI |
| AI has time varying continuous input signal | DI has constant discrete input signal |
| Generally it is continuous voltage and current signal having range of values (0-10V) and (4-20mA) | It has binary values (1 and 0) either ON or OFF state |
| Temperature detector, Pressure detector, Water flow detection switches, Position detector, and Transducers are examples of AI | Push button, Selector switch, Rocker switch, Toggle switch, 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 varying continuous output signal | DO has constant discrete output signal |
| Generally It is continuous voltage, and current signal having 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 transmitter, and Thermocouple are examples of AO | Relays, Indicating lamp and Solenoid valves are examples of DO |
Hope that above content helps you to understand the Different Analog I/O and Digital I/O (DI, DO, AI, AO) and also difference between DI, DO, AI and AO.
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