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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, the
Diagnostic LEDs can be used to identify the last POST process that was executed.
Each POST code is represented by a sequence of 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. If the bit is set in the upper and lower nibbles, then the
corresponding LED is lit. If the bit is clear, then the corresponding LED is off.
The diagnostic LED #7 is labeled as "MSB", and the diagnostic LED #0 is labeled as "LSB".
In the following example, the BIOS sends a value of ACh to the diagnostic LED decoder. The
LEDs are decoded as follows:
Upper Nibble AMBER LEDs
MSB
LEDs
LED #7
8h
Status
ON
1
Results
Ah
Upper nibble bits = 1010b = Ah; Lower nibble bits = 1100b = Ch; the two are concatenated as ACh
®
Intel
Compute Module MFS2600KI User Guide
Figure 50. POST Code Diagnostic LED Decoder
Table 7. POST Progress Code LED Example
LED #6
LED #5
4h
2h
OFF
ON
0
1
Lower Nibble GREEN LEDs
LED #4
LED #3
1h
8h
OFF
ON
0
1
Ch
Appendix B: LED Decoder
LED #2
LED #1
4h
2h
ON
OFF
1
0
LSB
LED #0
1h
OFF
0
95
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