The POST (Power On Self Test) Cards display codes that indicate what the system BIOS (Basic I/O System) is doing prior to loading the operating system. The last POST code from the BIOS is shown on the 2-digit LED display if the BIOS fails to load the operating system. In addition, the POST Cards display voltage levels for the system and the clock speed of the PCI/miniPCI bus. The smaller three digit LED display shows the selected measurement value. The current selection is indicated by a small yellow LED. Pressing the button briefly will change the current selection.