Gizmo Motor Port 1,2 Issue

[plurt97]

Does anyone know what happens to the Gizmo if one plugs the motor wire in backwards in Motor Ports 1 or 2? We were warned last year that it would fry the student processor (bye-bye magic smoke :) just like plugging in the cable to the left-most of the 4 pins in the motor port. (Sadly, I accidentally tested the latter, but not the former.)

Reason asking is that a couple of schools I've been working with to help with their connectivity and software issues both have the same problem of motor ports 1 and 2 not working. Now, they are both using the Python default code and recently were able to get their gizmo and Driver Station bound (including one having to re-flash the gizmo), but I find it odd that both have that exact same problem. I figure if it's not a coincidental hardware issue, it's either firmware or python. However, one school did have the cable in motor port 1 plugged in backwards. Not sure of the second school.

Any ideas? (Please don't test this unless you have a few spare raspberry pi or gizmos lying around. :-) Thanks in advance!

[the-maldridge]

Assuming that you are affiliated with BEST and are using an MC-29 as the controller, plugging in an MC-29 backwards can under the right circumstances lead to VMOT being connected directly to the signal line as in the floating state, the PWM control pin on the motor controller is tied to ground, which if plugged in backwards would be VMOT potential, anywhere between 8-12 volts.

[plurt97]

Wow, I appreciate all the big words. Really. However, it would be more helpful if you could just tell me a more straightforward answer. Will plugging in a motor backwards on a gizmo (where the white cable is to the outside) cause the student processor to burn out, cause the motor ports to be burn out (and thus not work), or be perfectly fine. I've got two other teams that are asking help to get their robots moving and right now they both have motor ports 1 and 2 that are not working. If you don't know, that's ok. I appreciate your time on this. It's just that we need to try and figure out if they broke the gizmo or if it's a configuration/coding problem. (I have functioning code, but it's in Arduino/C and they are far away, so it's too long of a process to get them the code, configure everything for Arduino for the pi, and test.)

[plurt97]

Ok, so I did a little more research and, yes, we are using the MC-29 motor controller, the standard one from the BEST robotics returnables kit. The VMOT (motor voltage) is something that was definitely going through the MC-29 as we had the Gizmo turned on. As far as the Pulse Width Modulation, I can't tell how much power was being sent since the teams probably had their 12.4 V battery plugged in that came with the kit, but I don't know how fully charged the battery was. I think that fills in all the blanks, but still leaves us in a quandary to determine if the amount of motor voltage power pulsing through the control pin would be enough to burn out the motor 1 and motor 2 inputs. Again, appreciate the time on this.

[plurt97]

For completeness: here's what we determined: Plugging in the motor cable backwards will not burn out the motor ports, but instead burn out "part of" the student processor. Replacing the student processor solved the problem. So, plugging in the motor port to the left most pin (when 4 pins are shown) will completely burn out the student processor. Plugging int he motor backwards on the three pins will burn out part of the student processor that uses those motor ports.

[the-maldridge]

Some local testing suggests that what's happening is just enough voltage is being passed to fuse one of the configurable elements on the port. You'll get lucky most of the time and only burn out that one specific port, but not always. I went through my collection of MC-29s and some of the older stock pass the full 12v through in the event they're hooked up backwards, which would surely burn out the entire module, not just the one pin. On the other hand, my newer MC-29s only pass about 20% through, which at 12v is still more than enough to cook the pins.