Whenever you power on your computer, the ROM BIOS runs a diagnostic test of the hardware, called the Power On Self Test (POST). Each stage of testing generates a two-digit hex code. By tracking these codes, you can view a progress report of the boot-up process and diagnose computer problems.

By monitoring POST codes using a POST diagnostic card, which plugs into a machine’s ISA or PCI port, you can troubleshoot a computer right on the bench, without needing to hook up a monitor, keyboard, or even a disk drive. These cards allow you to make quick work of fixing components and avoid the usual time-consuming method of swapping them out one at a time and retesting.

In this Daily Feature, I’ll examine how you can use POST diagnostic cards to quickly gather information when troubleshooting a system failure.

POST codes vs. BIOS beep codes
You may already be familiar with BIOS beep codes (if not, see my previous article “Deciphering BIOS beep codes”). Beep codes use audible signals to alert users to computer problems—an especially useful feature if a hardware failure occurs before the video card is initialized.

While some sets of beep codes, such as those generated by the latest Phoenix BIOSs, can provide extensive troubleshooting information, others are less revealing. There may be only 10 beep codes—a tiny subset of the up to 255 two-digit POST codes that can be displayed right on a POST diagnostic card, via digital readouts (Figure A).

Figure A
This POST diagnostic card, called POSTmortem, plugs into an ISA port. In addition to digital readouts, the POSTmortem also includes four LEDs to diagnose power-supply voltage problems.

POST card quick facts
POST codes are most often sent to port 80h, although some manufacturers use different ports. Compaq, for example, uses port 84h. POST cards either automatically detect the port or contain jumpers for setting it manually.

In addition to providing digital readouts of POST codes, these cards typically include LEDs for troubleshooting power-supply voltages. For example, the POSTmortem ISA card pictured above contains four LEDs for testing +5V, +12V, -12V, and -5V power supplies, as well as five contacts for testing these voltages with a probe.

For a more complete troubleshooting solution, get an ISA card if possible. (Your motherboard will likely have an ISA legacy port.) Sometimes PCI ports do not show all the available codes, for two reasons. First, ISA busses are initialized before PCIs—the PCI bus will only receive the codes that are generated after it is initialized. Second, ISA busses connect to the South Bridge chip; motherboards with damaged North Bridge chips may not be able to send codes to the PCI bus at all.

POST card makers and code sources
You can obtain ISA or PCI cards from these manufacturers: Xetal Systems Inc., maker of POSTmortem; Microsystems Development Technologies Inc., maker of Post Code Master; and PC Certify Inc., maker of several models of diagnostic cards. You can find more manufacturers by conducting a Web search.

Before using a POST diagnostic card, you’ll need a code manual. A good source for this is The BIOS Companion, published by Electrocution Technical Publishers. This comprehensive guide presents a catalog of POST codes of many models. It also includes a manual of many current and legacy BIOS beep codes and a vast description of BIOS settings. It can be purchased online (as a PDF file) for $15 or printed for $45, or as part of a larger set, called The PC Engineer’s Reference Book. The Companion is especially useful if your enterprise stocks many brands of recent and older PCs. If you prefer to get your information on the Web, Mr. BIOS (http://www.mrbios.com/techsupport/award/postcodes.htm) is a useful online source. Note, however, that computer makers constantly tinker with their BIOSs, so check with your manufacturer for the latest specs to be sure your POST codes are accurate.

POST card usage
As simple as POST cards are to use, they do require some care. Not all models, the POSTmortem included, have protection circuits built in. Therefore, be careful not to install these cards in reverse or upside down. Doing so could burn out the card. A marker shows which way to orient the card. After you plug in the card, power up the machine and follow the progress of the POST test (Figure B).

Figure B
The POSTmortem card is in place among the components of a legacy Compaq Presario tower.

The code you’ll see is sent to the card just prior to the start of each test. In Figure B, the code displayed (53) indicates the start of a video display ROM test. The four lit LEDs indicate normal power from the power supply.

Should the computer fail during the POST, the failing test number will be the last one displayed. Consider the failed test as only a possible indicator of the real problem at hand—another component may, in fact, be the culprit. For example, a failed memory test could indicate a badly seated chip or a motherboard problem.

What POST codes tell you
Table A illustrates the type of information you can derive from POST codes. The listing is a sampling of some diagnostic tests run by current (post-1995) American Megatrends Inc. (AMI)BIOSs. These BIOSs are used by many computer makers and are included in off-the-shelf motherboards. Click here for a complete manual of AMIBIOS POST codes.

Table A: AMIBIOS POST codes (sample selection)

POST time
As Table A suggests, POST codes can give detailed information useful for diagnosing computer problems. More informative than many sets of BIOS beep codes, POST codes are a solid diagnostic tool that can save the technician time in comparison to the standard method of swapping out suspected faulty parts.