dulk:
Thanks for your help, didnt know that we could not use a 9V battery to power the motor driver. Is it necessary to connect the motor driver ground and the Arduino ground? Our Arduino has a seperate power source so I did not think it mattered.
If the arduino needs to apply a signal of say 5 volt DC to a particular input port of the motor driver, then that 5 volt DC will certainly be relative to the arduino's ground.
So if the local 'ground' of the motor driver is at some 'other' voltage that is not the same as the arduino's ground voltage ........ then you can think about whether that matters or not. Also, a circuital loop is required to complete a circuit. So if the local 'gnd' of the arduino is not connected to the local 'gnd' of the motor driver, then connecting arduino 5V output (relative to arduino gnd) to the motor driver isn't going to do much (or anything).