A company can have multiple pieces of equipment from different manufacturers. And each manufacturer has several different models of equipment. Manufacturer table will hold manufacturer company name, address, etc. While the equipment will have model name, description, etc. Does that logic make sense? I've had to tweak my db a few times, so by no means is it the end all be all.

Then no it isn't dependant on anything and should be populated on page_load, or onselectedindexchanged of dropdownlist1