Nowadays, PLC can receive encoder and sensor signals without amplifier, so what's the fun of "source" and "drain"? Only the output of PLC will have the concepts of "source" and "leakage".

In fact, except for Japanese PLC, it is not necessary for other PLCs to use abbreviations such as "source" and "drain", but PNP and NPN can be used directly.

PNP is a low-voltage public line, and NPN is a high-voltage public line. As long as this connection is correct, it seems that it doesn't matter if the concepts of "source" and "drain" are reversed. However, another concept "positive logic and negative logic" is also wrong, which will cause the signal to be inverted.

Positive logic and negative logic

In PNP-type circuits, the common terminal of power supply and signal is at a low level of 0V. When the input is at a low level, there is no voltage difference with the common terminal, so there is no amplified switching signal output (digital quantity is 0). Only when the input is at a high level, there is a voltage difference with the common terminal and an amplified switching signal is output at a high level. This digital mode (digital quantity is 1) which is effective when the signal is at a high level is called PNP positive logic.

In the NPN circuit, the common terminal of the power supply and the signal is at a high level (the characteristic of this amplifier circuit). When the input is at a high level, there is no voltage difference between the input and the output, so there is no switching signal output (the digital quantity is 0). Only when the input is at a low level (forming a funnel), there is a switching signal output, and this signal is an effective (digital quantity is 1) digital mode at a low level, which is called NPN negative logic.