Solved: Optiplex 755 DT -USB 3.0 add-on card, riser, front panel, etc. - Dell Community
Optiplex 755: SM Bus Controller and PCI Serial Port not recognized
Dell OptiPlex - Wikipedia
Used Dell Black/Silver 755 Desktop PC with Intel Core 2 Quad Processor, 4GB Memory, 160GB Hard Drive and Window 10 Pro (Monitor Not Included) - Walmart.com
Solved: Optiplex 755 - Dell Community
Solved: PCI serial port driver download issues - Dell Community