Intel SERVER SYSTEM SR2500AL - Manual (Page 149)

Appendix D: LED Decoder
During the system boot process, 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, BIOS will display the given POST code to the POST
Code Diagnostic LEDs found on the back edge of the server board. To assist in
troubleshooting a system hang during the POST process, the Diagnostic LEDs can be used
to identify the last POST process to be executed.
Each POST code will be represented by a combination of colors from the four LEDs. The
LEDs are capable of displaying three colors: green, red, and amber. The POST codes are
divided into two nibbles, an upper nibble and a lower nibble. Each bit in the upper nibble
is represented by a red LED and each bit in the lower nibble is represented by a green
LED. If both bits are set in the upper and lower nibbles then both red and green LEDs are
lit, resulting in an amber color. If both bits are clear, then the LED is off.
In the below example, BIOS sends a value of ACh to the diagnostic LED decoder. The
LEDs are decoded as follows:
• red bits = 1010b = Ah
• green bits = 1100b = Ch
Since the red bits correspond to the upper nibble and the green bits correspond to the
lower nibble, the two are concatenated to be ACh.
Table 11. POST Progress Code LED Example
8h 4h 2h 1h
LEDs Red Green Red Green Red Green Red Green
ACh 1 1 0 1 1 0 0 0
Result Amber Green Red Off
MSB LSB