Pololu Ground problem on Rev 4 boards
There is a problem with (all?) Rev4 boards: GND on three of the 4 Pololu's is not connected to the main GND of the board. This is a severe problem and it can blow up your MBED or Pololu!
Still need to find out where in the production process this went wrong and if it applies to all Rev4 boards.
To check if your board has this problem, chek if there is connection between GND of the Pololu's X,Y and E and the GND of the power connector. If not, connect it, for example by making a bridge on the backside between the Pololu GND of the E-Axis and the GND pin of capacitor C3 (near the power connection).
#1 Updated by jaap over 4 years ago
The bug is in all Rev4 boards. There is missing connection between the lowest Pololu (Extruder) GND and the GND surface of the GND solder pad next to it (J35).
The missing connection is the only connection between power GND and GND for the lower three Pololu's! It does not only affect the extruder Pololu, but all three.
Since the extruder Pololu is not needed for (most) lasers, you can simply solve the problem by connecting a small wire from the Pololu GND hole to the solder pad.
I hope the attached picture clarifies what you need to do.