No internet through ethernet when another device connected to second ethernet port

I have a Dell Vostro 3900 with two ethernet ports. I use one to acquire data from a scientific equipment, and I’d like to use the second one for internet access. When the equipment is disconnected the internet works well, but when I plug it in there is no internet.
Windows doesn’t see any problem.

Any hints on that?

I am using Windows 7 Professional. Upgrading Windows unfortunately is not an option.