It's funny -- PCs used to be good for real-time applications, because they were essentially real-time. People still want the convenience of a user-friendly (or at least, accessible) operating system -- not to mention the ready availability and price of consumer electronics, but current technology is so many orders of magnitude beyond what is appropriate for those applications, people try to force it anyway and end up with crappy solutions.
What's really needed is a cheap, small, low power, high I/O, not-very-brainy platform. Arduino is close, but probably not brainy enough for most purposes -- hardly enough processing power, let alone memory, to deal with something like an OS. RPi is exactly the opposite: just a little too powerful for its own good. It has the accessibility (runs Linux, common HID peripherals) and some IO (SPI, GPIO), but lacks the real time aspect. It's a stripped down PC, not an old PC.
I guess on the plus side, Pentiums (or 486, or..) are cheap and plentiful? Runs Win95, a modern enough user experience (can even be connected to the internet -- though that's probably not recommended; that said, are bots even targeting anything as archaic as Win95?), the bus is generally real time (a Pentium has cache, and may have a PCI bridge, but you'll still get data out within microseconds of when the processor says so), it's powerful enough to do anything you could ever want with motion -- maybe not image processing, but come on, it ran Quake; even without the optimizing talents of Carmack and Abrash, you've still got a lot of capability.
Or if you don't mind DOS, a 386 or below (little or no cache -- what you program is what's on the bus) only gets easier to interface; but networking gets proportionally harder, until you're practically writing a "this is what we do and when" communication / translation / scripting / sequencing program, at which point you don't have much advantage over something like an Arduino, where you're probably writing the software fresh for every slightly different application.
I wouldn't recommend anything PII or above; the Northbridge and all the bus bridges add latency already, even if it's just PCI (which, in and of itself, isn't a nightmare to interface -- FPGA-on-PCI cards are probably still available?).
I suppose the other ideal would be -- well let's get a computer that's new but the same power. A Cortex M0 or M4 would be pretty kickin' against a 486 or something like that, and they usually come with enough memory to at least run something like DOS, and enough peripherals to build more than a basic computer-as-we-know-it (graphical display, keyboard, optional mouse, disk IO -- say, SD card in this case). Dev kits with this are available, but has anyone really went to the extent of making one with all the hardware, plus a good OS for it? And is it cheap enough to be worthwhile?
That should really be the direction the CNC community should look at. No clue if there's enough interest from other fields to really get things going like the Arduinos and stuff. It would also be enough for, well, an older cellphone -- enough to be functional, but no apps, no internet. Who would want that? Likewise, modern phones are too advanced, too locked down and too specialized (no GPIO or ports, beyond inappropriate standards like USB or BlueTooth).
Tim
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