power-on reset


(POR) The processes that take place when a hardware device is turned on. This may include running power-on self-test or reloading software from non-volatile storage. The term implies that the device has some reasonably complex internal state that will be set back to a "normal" initial condition. This state may include the physical state of the device (e.g. a printer) as well as data in the memory of an embedded system.

If a device has no reset button, and sometimes even if it does, turning it off and on again (power cycling) may be the only way to clear a fault.

Last updated: 2012-02-09

power-on self-test

<hardware, testing>

(POST) A sequence of diagnostic tests that are run automatically by a device when the power is turned on.

In a personal computer a typical POST sequence does the following:

- checks that the system board is working

- checks that the memory is working

- compares the current system configuration with that recorded by the PC's configuration program to see if anything has been added or removed or broken

- starts the video operation

- checks that the diskette drive, hard disk drive, CD-ROM drive, and any other drives that may be installed are working.

When POST is finished, typically it will beep, and then let your operating system start to boot. If POST finds an error, it may beep more than once (or possibly not at all if it is your PC speaker that is broken) and display a POST error message. These messages are often nothing more than a single ominous number. Some common numbers and their meanings are:

161 Dead battery (get a new battery for the system board)

162 Configuration changed (you added some memory or a new card to the PC)

301 Keyboard error (take the book off the corner of the keyboard)

Because a successful POST indicates that the system is restored to known state, turning the power off and on is a standard way to reset a system whose software has hung. Compare 120 reset, Big Red Switch, power cycle.

Last updated: 2001-03-30

Nearby terms:

Power Macintoshpower-on resetpower-on self-testPowerOpen

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