When we use devices such as PLCs, sensors, and servo motors, we often encounter some technical terms like AI, AO, DI, and DO, which many people find complex and difficult to understand. In fact, they are not as hard to grasp as we might think. Today, we will clarify what these terms mean to help you unravel the confusion surrounding them.
AI – Analog Input
First, what is AI? AI stands for “Analog Input,” which means the system receives analog signals from external devices. Typically, these signals come from sensors that can reflect changes in physical quantities in real-time, such as temperature, pressure, and flow.
For example, when you use a temperature sensor, it may only have two wires and a metal probe, yet it can feedback the current temperature data to your system. This is possible because the temperature sensor generates different current signals at different temperatures, such as 4~20mA or 0~10V. The system receives these current signals and converts them to ultimately derive the desired temperature value. This process is called Analog Input (AI).
AO – Analog Output
With AI providing input, there must also be output, which brings us to AO—Analog Output. AO is the opposite of AI; it is the process where the system outputs analog signals to external devices. Analog output allows signals to vary continuously within a certain range, unlike simple binary signals that can only represent on or off. For instance, we use analog output to control the state changes of certain devices, allowing them to transition smoothly within a range.
DI – Digital Input
Unlike the continuous signals of AI and AO, DI (Digital Input) deals with discrete, binary signals. Simply put, DI signals have only two states: on (1) or off (0). It is typically used to monitor the on/off status of devices.
For example, a limit switch is a typical DI device. When the limit switch is not engaged, the input value is 1; when the limit switch is engaged, the input value changes to 0, triggering an alarm or other actions.
DO – Digital Output
DO (Digital Output) is similar to DI in that it also represents binary signals, but it is the switch signal output from the system. It has two states: on (1) and off (0). For instance, when a PLC controls the start and stop of a motor, the output signal may be 0 or 1, indicating whether the motor is on.
Summary
In summary, AI and AO are primarily used for transmitting analog signals such as temperature, current, and pressure, suitable for situations requiring precise control or monitoring. In contrast, DI and DO transmit digital signals, suitable for monitoring or controlling switch states.
These four concepts are not as complex as many might think; their core idea revolves around the issue of “input and output.” You are either receiving or sending switch signals or analog signals. Once you understand these basic concepts, you will find that PLC control systems are quite logical.
With this foundational knowledge, you can easily understand how devices work together through different input and output signals.