So I haven't been able to find anybody with this issue through search. Forgive me if this has already been answered.
The high voltage input on the CNC shield seems to be backfeeding to the 5V pin on the UNO/shield, and therefore the USB connection. This results in the Uno's USB to fry and my computer to immediately shut down as protection when I plug the USB connection in.
More info:
I have the CNC shield ( https://www.digitaldroid.co.za/blogs/tutorials/77051265-quick-start-cnc-shield-v3-for-arduino ) and an Arduino Uno. GRBL has been installed on the Arduino and is operating normally.
I have stepper motor drivers (A4988 compatible) installed correctly, although this problem persists whether they are installed or not.
The shield is installed correctly on the Uno.
If I have the Uno connected via USB things seem to work well, with the exception of the stepper motor not responding. I know that the stepper motor can't be powered from the Uno's USB so when I connect the blue "12-36V" connector to 12V the 5V pin on the shield and Uno both read 12V instead of 5V and things get messy from there.
Obviously I need to have this connected to the computer in order to send GCODE commands and I don't want to destroy anything.
What am I missing?
Place an insulating material (electrical tape?) over the part circled in red on your Uno. There are parts of the CNC shield that short out on it. Hopefully, that is all that is wrong in your scenario.
gilshultz:
A lot! Why not start by defining the problem with specifications including a flow chart, schematic, power requirements, and EMI requirements if any. Define what the expected outcome needs to be. Purchase the Arduino cookbook and read it, this will give you some basics. also use the online tutorials and videos available, there are many good ones on this web site. At this point you will be able to define the problem and may have already solved it.
My description was just fine. I supplied the expected behavior as well as the aberrant behavior (5V should be 5V and not 12V, overloading the USB port with backfeed). I supplied part numbers and a link to the product in question. What the heck does EMI have to do with this? EMI would not cause a 5V pin to magically become a 12, 24 or 36V pin.
With all due respect, I am not an Android newbie, merely a newbie to this fine forum. I've been doing this for many, many years.
This is NOT normal behavior for a shield. Any shield. In addition I have followed the documentation for this shield to a T. I am trying to determine if the shield is bad or something else is going on (such as the insulating issue mentioned in another reply).
While I appreciate you trying to put a noob in his place, I'm not that noob.
junipllc:
My description was just fine. I supplied the expected behavior as well as the aberrant behavior (5V should be 5V and not 12V, overloading the USB port with backfeed). I supplied part numbers and a link to the product in question. What the heck does EMI have to do with this? EMI would not cause a 5V pin to magically become a 12, 24 or 36V pin.
With all due respect, I am not an Android newbie, merely a newbie to this fine forum. I've been doing this for many, many years.
This is NOT normal behavior for a shield. Any shield. In addition I have followed the documentation for this shield to a T. I am trying to determine if the shield is bad or something else is going on (such as the insulating issue mentioned in another reply).
While I appreciate you trying to put a noob in his place, I'm not that noob.
That seems to be his "cut-n-paste" reply, however impractical it may be. Just take it with a grain of salt and move on.
[That seems to be his "cut-n-paste" reply, however impractical it may be. Just take it with a grain of salt and move on.]
Thank you for that. Salt taken.
I was able to isolate and fix the issue. As it turns out, the board was, in fact, faulty. See attached image.
The high voltage connector traces to the clear-insulated wire, which seems to bridge a gap of some sort. Looks like a bodge wire but I digress. You can see the connection via the dotted cyan line.
The problem was that during manufacturing a small, tiny little piece of this stranded wire (in the red circle) had "escaped" and was pushed over to the pin indicated by the red arrow. This pin is connected directly to the +5V rail. I pushed the offending wire out of the way and everything was right with the world again.
So basically the power used strictly for moving the stepper motors was being pumped directly into the logic circuits, and subsequently the Uno, triggering the protection circuit on my USB port on my computer.
I'm glad it only fried the Uno USB interface and not my laptops.
Thank you for your help. I wanted to include the explanation for future reference for people who may experience the same issue.
junipllc:
[That seems to be his "cut-n-paste" reply, however impractical it may be. Just take it with a grain of salt and move on.]
Thank you for that. Salt taken.
I was able to isolate and fix the issue. As it turns out, the board was, in fact, faulty. See attached image.
The high voltage connector traces to the clear-insulated wire, which seems to bridge a gap of some sort. Looks like a bodge wire but I digress. You can see the connection via the dotted cyan line.
The problem was that during manufacturing a small, tiny little piece of this stranded wire (in the red circle) had "escaped" and was pushed over to the pin indicated by the red arrow. This pin is connected directly to the +5V rail. I pushed the offending wire out of the way and everything was right with the world again.
So basically the power used strictly for moving the stepper motors was being pumped directly into the logic circuits, and subsequently the Uno, triggering the protection circuit on my USB port on my computer.
I'm glad it only fried the Uno USB interface and not my laptops.
Thank you for your help. I wanted to include the explanation for future reference for people who may experience the same issue.
Good deal!
But still cover that area. It's been the ruin of many a young man.... err.. many a board. 
Not all shields suffer from this issue.
Mainly affects the cheaper Chinese ones.
There are some higher quality versions that have slightly longer pins to avoid this issue.
It can affect a few other shields too not just the CNC.
Ethernet shields are especially prone.
Bob.
