It’s been a while & I’m rusty so be nice.

I’m installing the driver pictured below on my mill’s Z-axis and I’m confusing myself.  I have +5V from my PSU connected to the PUL+ & DIR+ terminals and Pin 6 & 7 from my BOB connected to the PUL- & DIR-. 

Question: Should I connect the 5V PSU’s common to the GND terminal next to the +V terminal or should it go the ground terminals on the BOB?  It’s just a simple PP BOB with no fanciness.

There are two ways to connect the driver to the BOB. For the common anode method you connect +5 to the positive terminal of the input and the BOB output to the negative side. The BOB outputs will be active low. This is the typical connection. Change the DIR setting if the motor goes the wrong way. This assumes you have a BOB with 5 volt outputs.

The manual says the enable function will be on when the input is left disconnected. You would only need to disable the drive if you are concerned about power usage and leave the system idle for long periods.

https://www.stepper-store.com/wp-content/uploads/2019/02/DM542T.pdf

Question: Should I connect the 5V PSU’s common to the GND terminal next to the +V terminal or should it go the ground terminals on the BOB?  It’s just a simple PP BOB with no fanciness.

No, that would break optical isolation from input (PUL/DIR) and output stage of your drive.

Thanks for your input fellows!  My BOB is an old straight through type from Winford Engineering with no opto isolation.  I think now's a good time to upgrade it.

The opto's in the drivers isolate the outputs of the BOB. If all you connect to the inputs are switches the there will be no connection of the PC ground and 5V supply ground to the drive supply ground or other machine grounds. The spindle PWM to analog output is another issue should you need it.