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sakunamary
GOOOOOOD JOB！！！cdsteinkuehler ， but I do not know about the BBB...... raspberry , PCduino .BBB ,i can't tell which is better....
Well, for machine control I would say the BBB is best:
The Raspberry Pi is a nice system, but it's CPU is basically designed to be a set-top box or Blu-Ray disk player. It's great if you want to play videos, but lacks I/O and hard-real-time for machine control.
The PCDuino is better, but still has issues doing real-time control. All ARM Cortex-A parts don't have great interrupt latency (this includes the Pi and BBB) and are not optimized for rapidly toggling I/O pins (ie: generating step pulses for servo motors).
The BeagleBone Black has lots of I/O pins, and while the ARM Cortex-A doesn't have great IRQ latency, there are dual 200 MHz 32-bit PRU micro-controllers you can use to generate timing critical signals like the step/dir pulses. It's also already got LinuxCNC running on it, as well as at leats two other motion control frameworks (one from Elias, maker of the Replicape, and one from Bas, maker of the BeBoPr).
...but I could be biased. :)
The Cortex-M parts (like on the Due) DO have good interrupt latency and are great parts for "twiddling bits" like driving stepper motors. The biggest issue vs. the above parts is the Cortex-M won't run Linux, so you get to do custom firmware instead of running a regular OS.