whats the difference between npn plc and pnp plc?

Answer 1

i have my difficulties to determine what you mean with PLC but in case you talk about chip technology or the standard transistor in semiconductors the answer could be:
there is semiconducting material which is mixed with specific conducting material.
out of this process you can have material that prefers negative charging or positive charging, N or P material.
now the function of a transistor is that you put 3 layers of such material together in a NPN or PNP configuration, connect the base of the transistor to the material in the middle, and two other connections to that material to both 'ends'.
now if you put some voltage to the outer connections nothing much will happen except you start filling up the middle section with a negative or in other configuration a positive charge. In this case the whole setup becomes 'conducting' and you can observe current running from one side to the other.
this can be used as a switch, or a amplifier in electronics
now the difference is which kind of charge you prefer to make the transistor switch.

does this solve your question ?

Answer 2

By PLC, you mean Programmable Logic Controller.

PNP and NPN are also called "sourcing" and "sinking".

PNP = sourcing
NPN = sinking

Using sourcing and sinking always make it easier for me to visualize.

Think of a sourcing device (PNP) as a device that is a source of power. That is, in the case of a PLC input, the device sends a positive signal, such as +24 VDC to a PLC that sees this +24 volts, and recognizes the signal as being on. In this case the device is sourcing, and the PLC input is sinking.

A sinking device (NPN) is just the opposite. Instead of sending a positive signal, it uses a positive signal, again such as +24 VDC. In this case, the PLC input point provides a 24 VDC signal that is taken (drawn down) by the device, and the PLC input detects the power being taken. In this case the PLC is sourcing and the device is sinking.

When I design PLC systems, I always select sourcing (PNP) devices to wire to my PLC inputs, only because this seems the most logical direction of signal flow to me.