Appendix B: LED Decoder
During the system boot process, the BIOS executes a number of platform configuration
processes, each of which is assigned a specific hex POST code number. As each
configuration routine is started, the BIOS displays the POST code to the POST Code
Diagnostic LEDs on the back edge of the server board. To assist in troubleshooting a
system hang during the POST process, you can use the Diagnostic LEDs to identify the
last POST process that was executed.
Each POST code is represented by the Eight amber Diagnostic LEDs. The POST codes
are divided into two nibbles, an upper nibble and a lower nibble. The upper nibble bits are
represented by Diagnostic LEDs #4, #5, #6, and #7. The lower nibble bits are represented
by Diagnostics LEDs #0, #1, #2, and #3. Given the bit is set in the upper and lower
nibbles, then corresponding LED is lit. If the bit is clear, corresponding LED is off.
The Diagnostic LED #7 is labeled as “MSB”, and the Diagnostic LED #0 is labeled with
“LSB”.
Figure 21. Diagnostic LED Placement Diagram
In the following example, the BIOS sends a value of ACh to the diagnostic LED decoder.
The LEDs are decoded as follows.
Table 6. Diagnostic LED Placement Diagram
A. ID LED B. Status LED
C. Diagnostic LED #7 (MSB LED) D. Diagnostic LED #6
E. Diagnostic LED #5 F. Diagnostic LED #4
G. Diagnostic LED #3 H. Diagnostic LED #2
I. Diagnostic LED #1 J. Diagnostic LED #0 (LSB LED)
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