Why NPN system is preferred over PNP?

Hello;

I have noticed one thing more that some time OEM selected NPN PLC I/Os instead of PNP while with PNP the trouble shooting is easy. This happens especially at those machines which have some middle or high speed inputs. What could be the reason of this?
I do prefer PNP system because of ease of testing and trouble shooting.

NPN transistors have a faster response time than PNP because of the mobility of the current carriers, so often used for high speed inputs, however, over the years I believe that modern types although there is still a difference the speed is high enough in NPN for most applications. There is an old theory that sinking is less prone to noise than sourcing, this is probably based on some fact about with respect to ground, most noise tends to be positive. I think this theory is wrong as there is a known difference that NPN causes more noise than the other but the noise level is due to internal generated noise not external influences and is the wrong way round for the noise theory above.
Bear in mind, the above is based on my years in electronics and this was a long time ago so I could have lost a few brain cells since then.

I agree with parky, you will very rarely see any NPN in the states unless it comes from overseas. Another reason is that lots of controls components come out of japan. Japan is still very much engrained with NPN. With that in mind, Offerings from those manufacturers are typically much more readily available in NPN. This has lessened over the years as the tide shifts from NPN to PNP and is and Europe based manufacturers became more dominate.

Example of high speed inputs in PNP is the Mitsubishi FX range, the FX3U is capable of 40Khz and if higher speeds are needed then use a high speed counter card. On the Mitsubishi the first 7 inputs are hard wired to hardware high speed counters as well as the I/O memory map in the PLC. just enabling the dedicated counters in code allows the counters to use these inputs, however, in this mode you cannot use the inputs for other functions (obviously). The interesting thing is the actual inputs are not called anywhere in the program. These days the only reason you need to use NPN is if the sensors/equipment connected to it requires it.

OT: this friday I was about to change a sensor on a machine. It took me a while to figure out why my new PNP sensor didnt work.. NPN inputs, machine from Italy.

In the early days, there was a significant cost savings when using NPN. I am not sure if that is still the case or as important.

Almost every other answer says NPN transistors are preferred because of faster mobility of electrons. However for circuit designer like me, NPN transistor is preferred because it is most suitable for negative ground system. Common emitter configur... ... However for circuit designer like me, NPN transistor is preferred because it is most suitable for negative ground system. Common emitter configuration is most widely configuration. With PNP transistor, +ve supply line becomes common point (circuit ground) of input and output signal, which is not so convenient for design, testing and maintenance.

the best reason for NPN over PNP is safety
assuming supply NEG is grounded as most are.
with NPN the input signal is + POS if it get shorted to ground the input is 0 or False
with PNP the input signal is - NEG if it gets shorted to ground the input is 1 or True
so a grounded input would show in program as True when it's false that's less likely on a NPN input

I was going to start a separate thread for this but Gary forced my hand. The first post shows PNP connection and the next will show NPN. The most common shorting fault is to earth so the PNP is better in a negative grounded system. Not drawn but the NPN is better is a positive grounded system.

I was always taught not to switch on the Neutral or Negative side, so would always use PNP in a negatively grounded system. Is that still taught?

In reference to the speed and cost saving comments - When you are switching a 24V DC "PNP" output from a TTL circuit (uP, uC or FPGA, for example), you require additional components. Generally the Base of BJT or Gate of FET type transistors used in High-Side switch applications would be driven using another transistor based circuit. There are ways that avoid this requirement, but they also do things like invert the signals between the controller and the output.