The Universal Serial Bus (“USB”) 2.0 Specification defines three speeds of communication for its bus, and each has its own signaling characteristics. Due to the uniqueness of each speed, PHYs must be placed in a separate mode for each signaling rate. Although USB devices may know its communication speed, a general purpose USB analyzer must be able to analyze all USB communications. Rather than force the user to manually set the operating mode of the analyzer, this invention describes circuits for automatically and reliably determining the monitored USB communication speed.