The obvious or trite answer is; it depends. It depends on the needs of the application such as speed, timing, number of I/O, etc.
But there’s also the notion that what PLC you happen to be using will dictate the choice of network. So, a Rockwell PLC would be Ethernet/IP or DeviceNET, Siemens would be Profibus, and so on.
Not to be overlooked is the power of the installed base. The numbers indicate that fieldbuses are still the dominant networking choice for most PLC applications. In fact, they are substantially more than 50% of all PLC networks. However, Ethernet and Ethernet-based networks have been steadily gaining ground over the last few decades and will only continue to do so in the near future.
So why are fieldbus networks still so common? One simple answer may just be their legacy, the existing installed base, which provides little incentive to go with a completely new network setup. Switching to a new industrial network is a major undertaking in terms of engineering costs, time and effort. And if there isn’t a significant advantage or payoff from switching networks, it likely won’t be done.
A key differentiator is determinism, with fieldbus networks claiming guaranteed response times that Ethernet can’t deliver. Distance is another factor. Generally speaking, fieldbuses can handle longer distances than Ethernet.
Another advantage of fieldbus networks is their simplicity. Connecting typical control components such as encoders, motors, or prox switches to control systems is easier and less expensive using fieldbuses. In other words, in some cases Ethernet may be overkill.
Of course, Ethernet offers advantages as well, particularly the ease with which it allows factory floor systems to be connected to larger enterprise-wide networks. It also offers advanced I/O capabilities and can readily handle large amount of data, an important feature given the interest and potential in the Industrial Internet of Things (IIoT).