The big difference between PCs and PLCs is the scope of work. Both have limitations in their scope, but PCs have a much wider scope. For the purpose of this discussion, I will use the term PC to represent non-PLC based computers and their operating systems, so don’t get hung up on that term.
A PC is designed for the masses, allowing functions that the masses want: web surfing, communications, gaming, multimedia, and office software to name just a few. Each of those functions are wide-ranged and because of that wide range of needs, creates a situation where the PC becomes the ‘jack of all trades’, or good at most things, but an expert at nothing. The wide range of applications and speed to market of PCs create a situation where the hardware, firmware and software that make them run is constantly updated. A person who installs only office software and minimal other software will not necessarily see the “bugs” of someone who is doing 3D modeling, gaming, communications and multimedia. Open communications and networking alone create huge variables and potential for updates in a PC. In addition to the broad range of software that can be installed, many different manufacturers and models of hardware can be installed in a PC (memory, storage media, graphics and comm cards).
A PLC is designed for specific purposes. Yes, the purposes are broad, but nowhere near the scope of a PC. This allows the hardware and firmware to be fine-tuned for the limited software (application) that will be loaded to it. The hardware that can be installed in a PLC is limited. If the hardware is not made by the manufacturer of the PLC, then it has to be vetted by the manufacturer to ensure it works properly in that environment. Because of this tighter scope, the firmware and software updates are less frequent than that of a PC.
I like to compare a PLC vs PC in terms of using the right tool for the job. I carry a multitool on my belt and use it regularly. I use the screwdrivers, file, saw, scissors and pliers regularly, but only for quick, non-critical jobs. If I were installing a deck, I would not use the screwdriver on my multitool. I would use a screwdriver (powered, if possible). I could use my pliers to remove the nuts holding a subplate in, but a better choice would be a ratchet and socket. If I use my multitool beyond the scope it was developed for, I will need to repair it or buy a new one more frequently than a dedicated tool.
I have heard the death knell of the PLC since I got into this business. That was many years ago. I don’t see that happening any time soon unless the computers become more stable, are guaranteed to work for every hardware option and do not require frequent updates.
Sorry for the long post.