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POST Probe How-To

When it comes to technicians, nothing will distinguish an old pro from a novice faster than a field service call for a PC that won't boot up, even to a repair disk or an OS-independent diagnostic like Micro-Scope. The power light comes on, the fan can be heard and maybe some strange beeps from inside the case, but the screen is black as night and the customer is looking at you expectantly to see what magic you have up your sleeve.

In this situation, there are three things you know for sure.

1) The system is getting power.
2) It's almost certainly a hardware problem rather than software, but beyond that distinction it could be just about anything.
3) The customer will either be impressed or suspicious, based on what happens in the next few minutes.

The beeps may provide a clue, but every BIOS has its own set of beep codes, and you won't know what BIOS version you have until you take off the case. No matter which BIOS it is though, the number of beep codes will invariably be quite small compared to the number of things that could be wrong, so getting help from that quarter is somewhat a matter of luck. You can also try swapping parts, but even IF it happens to be a field-replaceable part AND you happen to have the right one with you, it's hard to justify charging professional rates when the customer can see that you are really just guessing.

The right tool for this situation, the professional tool, is a POST reader card like the Micro 2000 POST-Probe. This card will read and display the codes put out by the POST or Power-On Self Test that the BIOS performs during boot-up. Within seconds you can determine which part of the machine is failing, and even if you have to take it into the shop or make a second trip with the right part, the customer will have confidence in you. And to make sure you come across at your very best, here's a quick rundown on using the POST-Probe most efficiently.

1. Open the case and check all of the cables, the expansion cards, RAM modules etc. to make sure everything is properly plugged in and connected. Locate the BIOS chip, near the lithium battery, and make a note of the manufacturer and the version number.

2. Take the POST-Probe out of its case, with a flourish. Near one corner of the card you will see a set of 4 DIP switches. These are used to set the card to the same port that the BIOS uses to output the POST codes. Most BIOS versions use Port 80, which requires all of the DIP switches to be On (toward the outside of the card). A few will use other ports. For instance, most Compaq systems use Port 84. This info is found on page 11 in the most recent manual.

3. Remove all of the expansion cards: the modem, NIC, sound card and whatever else you have. The video card can come out too, because it's not needed for the POST. Plug the POST-Probe into one of the now-empty bus slots, either ISA or PCI. If you haven't used the card before, take a moment to locate the row of yellow and green LEDs along one edge, and the two yellow and one red LED somewhat in from one edge. You will also see, on each side of the card, a 2-digit alphanumeric readout.

4. Turn on the power. While the system tries to get through the boot routine, watch the yellow lights (each of which monitors an important bus signal) to see that they all have signs of life. Then check the green lights to see that all the motherboard voltages are within tolerance. The one red light indicates the Reset line, and it should only blink once at the very beginning.

5. It's possible that the system will get through the boot process now, which means that one of the cards you removed was shorting out the bus. If a close visual inspection of the connectors and connector slots doesn't reveal any damage, it's a process of elimination by replacing cards until the problem recurs.

6. Most likely the system will still fail to boot, and the point where it stops is a vital piece of information revealed by the 2-digit hexadecimal code displayed by the POST-Probe. In the manual, find the code table for the BIOS in question. There are over 40 BIOS versions listed, so yours will almost certainly be there. In the left-hand column locate the code being displayed, and the Description column to the right will tell you what test the BIOS was unable to complete. (Obviously these codes get updated fairly often, so if it's a new code, and it's not in the manual, you can go to our Tech Support pages where all the latest codes are listed).

7. That information alone is usually sufficient to nail the culprit, but if you want more data about it, the middle column will refer you to the appropriate section of Chapter 2 in the manual, where you will find a write-up about the particular part or function that failed.

Disclaimer - The Micro 2000 Tech Tip is a free service providing information only. While we use reasonable care to see that this information is correct, we do not guarantee it for accuracy, completeness or fitness for a particular purpose. Micro 2000, Inc. shall not be liable for damages of any kind in connection with the use or misuse of this information.