Hello! This is my first post on AAC and I'm hoping I got to the right place.

I have a USB to RS485 cable connected between a laptop and the device I need to communicate with. I believe there is an issue with the device causing a ground loop I don't fully understand.

When the laptop is "floating" on battery power everything seems to work fine. However, when I connect the laptop to a power adapter it will intermittently fail to communicate properly.

What I would like to do is induce the failure mode on purpose so I can take some measurements and have something I can compare a solution against. I was able to observe the failure mode once but haven't been able to produce it again.

 

I have seen this happen if you do not have a signal ground between the laptop and the other device due to common mode voltage limits being exceeded as the isolated laptop's 'ground' drifts around with respect to the other unit.

 

So...run a ground wire from some place thought to be ground on your laptop to the ground point on your circuit. I chose to earth the ground pin on one of the USB ports on my desktop computer which is otherwise not grounded.

 

So...run a ground wire from some place thought to be ground on your laptop to the ground point on your circuit. I chose to earth the ground pin on one of the USB ports on my desktop computer which is otherwise not grounded.

I'm currently attempting to do that by shorting the ground of the USB to RS485 Cable to the ground of my laptop's DC input. However, that's still not inducing a failure. Should I be shorting the cable's ground to earth ground instead?

Excuse me if these questions sound like they're coming from an amateur. That's because they are.

 

I did not do a good job of explaining the purpose of the ground connection, resulting in a minor misunderstanding.

The purpose of connecting the ground of your computer to the ground of the other end of the serial connection is to prevent failures rather than to make them happen.

 

I did not do a good job of explaining the purpose of the ground connection, resulting in a minor misunderstanding.

The purpose of connecting the ground of your computer to the ground of the other end of the serial connection is to prevent failures rather than to make them happen.

Thanks, that makes more sense. My goal at this point is to make the failure repeatable for testing purposes.

My colleagues assure me that noise caused by a ground loop is causing communication issues. I was able to observe it once but have not been successful in making the problem repeatable.