POST card
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In computing, a POST card is a card that reports error codes produced by a POST. They are easy to use, by simply inserting one into an available expansion slot. Postcards are available in ISA, parallel port, PCI and AGP variants. The card reports either a number, consisting of two hexadecimal digits or an equivalent pattern of LEDs.
The code depends on the manufacturer of the BIOS of the motherboard. A book of reference tables usually accompany the card and can tell what is wrong. These can cover a large range of motherboard manufacturers, as different BIOSes use different codes. Postcards are often used by technicians to troubleshoot computers that refuse to boot. Some motherboards come with a built in display to diagnose hardware problems.