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Intel® Server System M20MYP1UR System Integration and Service Guide
Appendix D. POST Code Diagnostic LED Decoder Table
As an aid in troubleshooting a system hang that occurs during a system POST process, the server board
includes a bank of eight POST code diagnostic LEDs on the back edge of the server board. During the system
boot process, Memory Reference Code (MRC) and system BIOS execute a number of memory initialization
and platform configuration processes, each of which is assigned a hexadecimal POST code number. As each
routine is started, the assigned hexadecimal POST code ID is displayed in binary to the bank of eight POST
code diagnostic LEDs on the back edge of the server board.
During a POST system hang, the displayed post code can be used to identify the last POST routine that was
run prior to the error occurring, helping to isolate the possible cause of the hang condition.
Each POST code is represented by eight LEDs, four green and four amber. The POST codes are divided into
two nibbles, an upper nibble and a lower nibble. The upper nibble bits are represented by amber LEDs and
the lower nibble bits are represented by green LEDs. For each set of nibble bits, LED 0 represents the least
significant bit (LSB) and LED 3 represents the most significant bit (MSB) as shown in the following figure.
Note: Diagnostic LEDs are best read and decoded when viewing the LEDs from the back of the system.
In the following example, the BIOS sends a hexadecimal value of AC to the diagnostic LED decoder. The LEDs
are decoded as shown in Table 3, where the upper nibble bits represented by the amber LEDs equal 1010b
or Ah and the lower nibble bits represented by the green LEDs equal 1100b or Ch. The two are concatenated
as ACh.
Nibble
8h (MSB)
Upper
Lower
74
Figure 67. POST diagnostic LED location and definition
Table 3. POST progress code LED example
4h
ON
off
ON
ON
2h
1h (LSB)
ON
off
off
off
Binary Code
Hexadecimal Code
1010
1100
A
C
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