The LED display, the code tables for your type of BIOS (as can be seen in the Post Probe manual), and the diagnostic procedures (also in the manual) will allow you to diagnose the failure, in many cases to the exact component that needs to be replaced. Together with Micro 2000’s Micro-Scope diagnostic software, you will have the arsenal to conquer virtually any PC computer problem you may encounter.
Theory of Operation
When a computer boots up, it starts by executing hard-wired instructions stored in the BIOS. These instructions do a check of the various essential machine functions, and then turn control of the system over to the boot loader routine stored on the hard drive. The series of instructions the BIOS does are called the Power-On Self Test, abbreviated POST.
At each step of the POST, the BIOS puts a code out on the bus. The original purpose of these codes was to allow the programmers to debug the BIOS program, but the codes are still present in the completed computer.
A POST-code reader monitors these codes and displays them on a 2-digit readout that is called a hex display because it changes an electronic hexadecimal signal into a visual decimal number. Normally the codes change so fast they are just a blur, but if the BIOS fails to get past a particular point in the POST, the boot process will stop and the reader will display the code for the last step that was being attempted. That code will tell us what step the BIOS was unable to do.
If all BIOS manufacturers had used the same codes, it might be possible to memorize them. Unfortunately, not only do the codes differ from one maker to the next, but even from one version to the next for the same manufacturer. Therefore it is necessary to refer to a table listing the codes and their meaning for each BIOS being used.
The Post-Probe
The description given above applies to all post-reader cards, of which there are several. The Micro 2000 Post-Probe though is unique in several ways.
- Both ISA and PCI connectors are available on the same card. All other post readers can be used in either the ISA bus or the PCI bus. This requires the field technician to carry two cards, or take a gamble on being able to use the right bus.
- The codes are displayed on both sides of the card. Some computers have the bus slots on a riser card rather than directly on the motherboard, which means the post reader could be upside down with the display hidden. Having a second display on the back side eliminates this problem.
- There is a stepping switch to retrieve the sequence of codes. If the BIOS is corrupt it can execute the instructions out of sequence, and the boot-up could fail for this reason rather than failure of the hardware indicated by the last code. This is nearly impossible to detect without the Post-Probe's stepping switch.
- A separate set of LEDs give a direct indication of several of the key bus signals and whether the power supply voltages are within tolerance, making it possible to diagnose many problems at a glance.
The Manual
The Post-Probe comes with a 265-page spiral-bound manual, which in itself is part of the technician's troubleshooting arsenal. It has tables listing the post codes for over 60 BIOS versions, a list that includes every common BIOS and a few that most technicians will probably never see in their lifetime.
Each code also has a reference to the appropriate section of a diagnostic chapter, which explains the function in question, both in theory and in practical data on repair.
A separate chapter also has tables for the beep codes. These are audible codes that a number of BIOS versions use in addition to the post codes, with a unique pattern of beeps for different types of post failure. At the back of the manual is some reference material such as the standard IRQ assignments, standard adapter ROM memory allocations, and the bus pin-outs for both ISA and PCI.
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