POST card

Lore & Background

POST cards were introduced for IBM PC computers, which send 8-bit byte codes to a specified I/O port during startup. Each code indicates a stage in the startup procedure or identifies an error, and 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 an 8042 keyboard controller failure. If startup does not complete successfully, either an error code or the code of the last operation performed is available.

POST cards are inserted into an expansion slot and are available with connectors for various buses. A typical card for desktop computers has a different bus interface on each edge; a card for laptop computers may have both a miniPCI and a parallel port connector, plus USB to supply power. Modern motherboards often do not broadcast POST codes to their PCI Express slots, so the Low Pin Count (LPC) bus may be the only bus where POST messages can still be seen. LPC connectors are not standardized, with between 9 and 19 pins and both 2.54 mm and 2 mm pin headers commonly used, so an LPC POST card may have to auto-detect the pin assignment.

Another option is USB POST cards such as AMI Debug Rx, which use the debug port common on USB 2.0 EHCI controllers. Information on the meaning of POST codes for different BIOSes is needed to interpret the codes; this may be supplied with cards but becomes dated as later BIOSes are issued. In addition to displaying numeric codes, many cards monitor power supply voltages, clock and oscillator signals, reset signal, and other parameters.

Reader's Guide

POST cards were originally high-priced but cost from just a few US dollars upwards in the 21st century. When first introduced, motherboards were expensive and well worth troubleshooting and repairing. By the late twentieth century, large-scale integration and mass production made motherboards inexpensive components; motherboards were rarely repaired but replaced. The main purpose of a POST card became to determine that parts mounted on the motherboard itself, rather than plugged-in video cards, RAM, etc., are at fault. Diagnostic cards are today mainly used by designers of motherboards and extension cards, along with logic analyzers and other debug tools and interfaces. They are less commonly used in the 21st century for computer repair and by system integrators, but remain available. Some motherboards have a built-in display to diagnose hardware problems, and most also report POST errors with audible beeps if a PC speaker is attached, making POST cards less necessary.

Did You Know?

• POST cards display 8-bit byte codes, usually as two hexadecimal digits, sent to I/O port 80 hex.
• For the 1984 IBM PC/AT, code 1D indicates 'Determine Memory Size Above 1024K' and code 2D indicates '8042 Keyboard Controller Failure'.
• Modern motherboards often do not broadcast POST codes to PCI Express slots; the LPC bus may be the only bus where they can still be seen.
• Some motherboards have a built-in display for POST codes and also report errors with audible beeps if a PC speaker is attached.