Quote Originally Posted by DavidS View Post
Well I would imagine that we all have a computer. If your computer happens to be a Raspberry Pi (like mine), then you are saving money, as no secondary device needed as a controller.

If your computer is something else, you can use GPIO directly on the other computer (on the PC's use the parallel ports, on Macintoshes use the SCSI interface with $3 worth of 74HCxxx IC's to decode the SCSI, on others use what ever equivalent).

So if you already have a computer you do not need an Arduino (or any other MCU), and I think that is the point.
Except that without real time control of the IO you cannot run a 3d printer. That is why people use an Arduino and generally not a Raspi. You can use a Raspi, it will work fine but it will not work whlie you are running Linux in the background, it does not give enough timing accuracy to control any CNC machine that is working at a reasonable speed. To use a Raspi with enough accuracy for the IO you need to write a new OS specifically for the printer.