Since engaging in the industrial control industry, I have been dealing with Siemens PLC almost every day, but due to the fixed PLC module used, I have been using a type of digital input and output module for many years, and I have not been in contact with other brands of PLC, so I rarely pay attention to the difference between source and drainage. In recent years, domestic PLC modules have risen rapidly, and their share in small and medium-sized projects is getting higher and higher. The company also intends to replace PLC localization, so it will learn in advance the two types of PLC input and output modules: source type and leak type. The concept of source type and drain type is generally for transistor type circuits, and it can be understood that the source type is the source type when the IO circuit provides outflow current, and the IO circuit absorbs the incoming current from the outside is called the sink type. For the digital input module DO, the output of the PNP transistor is generally the source type, and the power supply has been connected to the output module, and the current flows out through the output terminal, and the load can be directly driven without any external power supply. The NPN transistor output is drain type, which requires an external 24V power supply, and the current flows from the load to the module output port. Note: Siemens 300 400PLC (European PLC) is accustomed to using PNP source output;Japanese, Siemens 200 series and most domestic PLCs generally use NPN drain output. Similarly, for digital input modules, there is a distinction between source type and drain type according to whether the current from the signal port is flowing out or flowing in. Generally speaking, the common termination is 24V, the input is active at 0V, and the current flows out of the terminal, which is the source type;The common termination is 0V, the input is active at 24V, and the current flows in from the terminal, which is a drain type. As shown below:
For different types of input and output modules, the corresponding external wiring is not the same, the following is the external wiring of two different types of input and output modules: drain type and source type. 
The drain output load is located between the DC24V and the module
The source output load is located between the module and ground.
The drain input switch is located between the DC24V and the module
The source input switch is located between the module and ground