A modular gaming machine system has a first gaming box with a main processor configured to control a game of chance played on the gaming machine by executing game code that uses gaming instructions received from a PCI-e controller. At least one interface is coupled to the first gaming box to receive a PCI Express (PCI-e) bus to communicatively link the PCI-e controller and the main processor. A second gaming box contains the PCI-e controller for communicating with at least one peripheral device on the gaming machine and at least one critical gaming device in communication with the PCI-e controller. There is at least one access port coupled to the second gaming box to receive a free end of the PCI-e bus, wherein the PCI-e bus extends outside the first and second gaming box and is secured by one or more doors, locks, sensors, evidence tape, and/or combinations thereof.