GrblShield v1.0 uses the Arduino’s 3.3v supply for the digital logic, so it only requires motor power to be provided.
This also means really old (serial) Arduinos that do not provide 3.3v will not work.
This is what was confusing me,now you say 5 volt logic. The RBBB board uses a 328.The BUB USB to serial board can be set to 3.3 or 5 volt logic. Sorry for being a pest.