GRBL or LinuxCNC Components - smartroad/GRBL-LinuxCNC-Controller GitHub Wiki
Depending on which mode you want to use the board in, you can omit certain components.
GRBL Arduino Shield
Omit:
R8-10 - Pullup resistors
X1 - 25 D Socket
IC1 - 5V Regulator
C4 - Regulator output smoothing capacitor
##LinuxCNC Parallel Interface Omit:
S1 - Reset Switch
SV9-12 - Arduino Header Pins