The display principle of the seven-segment digital tube is as follows:

1. Common anode: In a conventional seven-segment digital tube, the common anode is connected to the positive power supply. When the common anode is powered on, it will illuminate with low level as "on" and high level as "off"
2. Segment selection: Each LED is called a segment, such as A, B, C, etc. By controlling the level of the segment select pin, some specific LEDs can be turned on or off. When the segment selection pin is high level, the corresponding LED lights up: when the segment selection pin is low level, the corresponding LED turns off
3. Common cathode and common anode: There are two types of digital tubes, one is common cathode and the other is common anode. In a common-cathode digital tube, the cathode is connected to the negative power supply. When one of the LEDs needs to be lit, the corresponding segment selection pin is set to low level, and the other segment selection pins are set to high level. At this time, the corresponding LED light will show a low level on, and other LED lights will show a high level off state. The opposite is true for the digital tubes with a common anode.
4. Coding table: In order to facilitate operation, each number and character has a corresponding coding table, indicating which LEDs need to be lit to display specific numbers or characters. For example, the number "0" is coded as (1,1,1,1,1,1,0), which means that the A~F pins should be set to low level and the G pin should be set to high level.
5. Multi-digit display: Usually, there is more than one seven-segment digital tube, and more digits or characters can be displayed by connecting multiple digital tubes in series. For example, a four-digit digital tube can display numbers from 0 to 9999.