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In computing, a POST card is a plug-in interface card that displays progress and error codes generated during power-on self-test (POST) of a computer. It is used to troubleshoot computers that do not start up.
At a minimum, if the CPU, BIOS, and the I/O interface upon which the POST card relies are all working, a POST card can be used to monitor the system's Power-On Self Test (POST), or to diagnose problems with it. The system sends two-hexadecimal-digit codes to a specified I/O port (usually 80 hex) during startup, some indicating a stage in the startup procedure, others identifying errors. The description for each code must be looked up in a table for the particular BIOS. For example, for the 1984 IBM PC/AT code 1D is issued when about to Determine Memory Size Above 1024K, and code 2D in the event of 8042 Keyboard Controller Failure, 105 System Error. If startup does not complete successfully, either an error code, or the code of the last operation, is available.
POST cards provide information even when a standard display is not available, either because connecting a monitor is impractical, or because the failure occurs before the video subsystem is operational.
POST cards are inserted into an expansion slot, and are available in ISA (also supporting EISA), PCI, parallel port, and other variants (Mini PCIe, for laptop computers, is supported by some cards, but with restrictions). A typical card for desktop computers has both an ISA interface on one edge, and PCI on another; a card for laptop computers has both a miniPCI and a parallel port connector (plus USB to supply power).
Information on the meaning of POST codes for different BIOSes is needed to interpret the codes. This is supplied with cards, but becomes dated as later BIOSes are issued; up-to-date information is available on manufacturers' and independent websites.
In addition to displaying numeric codes, many cards monitor power supply voltages, clock and oscillator signals, reset signal, and other parameters.
POST cards for PCs, while originally high-priced, cost from just a few US dollars upwards in the 21st century.
Some motherboards have a built-in display to diagnose hardware problems. Most also report POST errors with audible beeps, if a PC speaker is attached. Such motherboards make POST cards less necessary.
When these diagnostic cards were first introduced motherboards were expensive and well worth troubleshooting and repairing. By the late twentieth century large scale integration, mass production made motherboards inexpensive components. Motherboards were rarely repaired, but replaced; the main purpose of a POST card is to determine that parts mounted on the motherboard itself, rather than plugged-in video cards, RAM, etc. are at fault.