[klick0]

A lot of arguing going on here...

I think it would be great to see the RPI run LinuxCNC, although even if the LinuxCNC team decided to get it running on there, the support would probably be not much. How much of their limited resources are they going to put into a platform that is used by 0.01% of the userbase? The only hope would be that one of the programmers there goes crazy over RPI and decides it's his life mission to get it working on it. RTAI does have some work done on it if you search for it, here's from the website:
ARM (StrongARM; ARM7: clps711x-family, Cirrus Logic EP7xxx, CS89712, PXA25x)

I'm not sure if RPI falls into that, I don't think it does, but i'm not expert on arm. Also I think some versions of LinuxCNC used to use rtlinux, or i'm not sure what it's called, but one of the other realtime apis, but i'm pretty sure they use RTAI exclusivily now.

If cost is your primary purpose for attempting to use RPI, you may look at intel atom motherboards. I have used over 5 of them, and they all work fine, and they come with parallel ports (well some of them). I recently got 2 motherboards for $100 off ebay, new, but a reliable source like newegg has them for $75, you only need to add $10 worth of memory. They are not very power hungry either, although they will use more then RPI i'd imagine. So in my case, $50 motherboard, with $10 ram, that's $60 vs $35 for the RPI, and LinuxCNC works out of the box. I use a $15 USB 3.0 flash drive (not that the board even has usb 3.0, but the 3.0 flash drives are usually a bit faster), plus $15 power supply, and I made my own case out of 1/2" pvc sheet, $10. So i'm at $100 for a PC that runs some of my machines. I actually only made my own case for one, i had a bunch of old cases I threw the others into.

Not that my intel atom has anything to do with RPI, i just thought i'd mention there are low cost alternatives that are out today. Now if your main reason for wanting to use it is size, then the RPI destroys the mini-itx intel atom boards.

I do plan to have a RPI connect to a arduino/grbl setup.

[mpack]

How much of their limited resources are they going to put into a platform that is used by 0.01% of the userbase?

It's not as bad as all that.

First off, the CPU is an ARM (at least the CPU of concern to us is an ARM), which is already a very important platform for Linux. True, a set of drivers needs to be written for the RPi peripherals, but that's not really a big job - we don't even need a complete set, we only start with the stuff actually used by EMC.

Of course EMC doesn't run on any old Linux, it only supports a particular few variants with (their own variant?) real time kernel. We would need to port that kernel to ARM, if it hasn't been done already.

And I remind everyone that Linux is not a necessity. An OS is a convenience if you need a fancy shell, fancy filesystem, ability to choose any of several programs to run, run several apps at once etc... if you need none of those things then you can dispense with the OS.

I had a look at the EMC2 sources. It does indeed have what it calls a HAL (hardware abstraction layer - which hopefully also abstracts out the OS) - but whether this is intended to do what I hope it does is too soon to tell.