![]() The outside 4 transistors are usually small power (typical 2n39) that just take the logic from your controller, while the inner four are higher powered to be able to apply the appropriate power to your device. But, if I need to drive a device that's bi-polar (needs current one way to operate forward, other way to operate reverse), I use both NPN and PNP, kind of like in this schematic (essentially an h-bridge). When I need a transistor to drive a device (e.g., motor, sensor, etc), I usually use an NPN. These small differences really don't enter into this kind of situation at all. PLC applications tend to have multi-decade legacy installed base that some parts of electronics/EE don't have experience with. There are minor performance issues (NPN is a bit more efficient for tedious semiconductor physics reasons) but much of this can be designed out and likely has been in a commercial PLC system. So now because of that variety, you have to have the flexibility for newer components because both types are floating around.
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