Motherboard or modem?

New motherboard refusing to go through POST. Take out modem (GVC Model F-115I/R2F Conexant RS56-PCI chipset based) and mobo and computer work perfectly. Put modem in, mobo doesn’t POST. PS is known good. Given everything equal in the two scenarios, how (electrically speaking) can a PCI card prevent a mobo from going through POST. Is it something of a short circuit? Need to know because I don’t want to diagnose bad mobos incorrectly.