11.2 Troubleshooting the Hardware Box
11.2.2 POST errors
Every time the computer is turned on, it runs through a Power-On Self-Test (POST). The POST is a series of self-diagnostic tests that the computer runs through to test the major hardware. It is the first task run by the computer BIOS. The POST performs basic test routines on the motherboard and major hardware devices. It does not perform in-depth testing on the computer system. It can only detect major failures that will prohibit the boot-up process.

The POST is stored in the computer ROM BIOS.  As the computer is turned on, the POST function is passed to the first bank of RAM. For example, the computer checks for a properly functioning system timer, CPU, video card, memory, and keyboard during the POST. If an error occurs, the BIOS has predefined error codes that will be reported to the users. These errors can be reported visually or through a series of beep codes.

The beep code error messages are useful troubleshooting aides. If the BIOS encounters any hardware errors while running the POST routine, it will issue a series of beeps. These beep codes indicate or confirm that there is a problem with the computer hardware. Beep codes are produced with a combination of short and long beeps. Beep codes for a computer with AMI BIOS are described in Figure .

Note: The error reports generated by the POST will vary slightly depending on the BIOS that is installed on the computer. For specific information about POST errors, please refer to the BIOS documentation or the manufacturer website. Please note that the following error messages apply only to the referenced BIOS.

AMI BIOS beep codes are demonstrated in Figure and common visual display errors are illustrated in Figure . Note that these error codes or messages are BIOS-dependent. They depend on the specific BIOS that the system is using.

POST Card Error Indicators
A POST card is a device that helps troubleshoot computer problems that occur before the BIOS can report an error. There will be times when a computer fails before a BIOS error can be reported. In case of these events, a POST card is useful because it provides the technician with a digital readout of any POST errors. POST cards are useful for troubleshooting power supply voltages, IRQ/DMA conflicts, and motherboard timers. Also, POST cards are compatible with either ISA or PCI slots.

For example, a computer crashes before the video card can be initialized. A POST card has the capability of deciphering the POST error and will report the error in hexadecimal code. Each code represents a particular error message. POST card error messages are displayed on the card itself. No video display is necessary to read the error message.
 

Lab Activity  (PDF, 11 KB)
  In this lab, students will identify common POST errors. The ability to identify POST errors is essential for troubleshooting computers.