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Appendix E. POST Code Diagnostic LED Decoder
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 several memory
initialization and platform configuration routines, each of which is assigned a hexadecimal POST code
number.
As each routine is started, the given POST code number is displayed to the 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 before the error occurring, helping to isolate the possible cause of the hang condition.
Each POST code is represented by eight LEDs, four green LEDs and four amber LEDs. The POST codes are
divided into two nibbles, an upper nibble and a lower nibble. The upper nibble bits are represented by
amber diagnostic LEDs and the lower nibble bits are represented by green diagnostics. If the bit is set, the
corresponding LED is lit. If the bit is clear, the corresponding LED is off. For each set of nibble bits, LED 0
represents the least significant bit (LSB) and LED 3 represents the most significant bit (MSB).
In the following example, the BIOS sends a value of AC to the diagnostic LED decoder. The LEDs are decoded
as shown in the following table.
LEDs
Status
Binary
Read
Value
Hexadecimal
Result
Upper nibble bits = 1010b = Ah; Lower nibble bits = 1100b = Ch; the two Hex Nibble values are combined to
create a single ACh POST Progress Code.
Intel® Server D50TNP Family Integration and Service Guide
Figure 292. Onboard POST Diagnostic LEDs
Table 8. POST Progress Code LED Example
Upper Nibble AMBER LEDs
MSB
LED #7
LED #6
LED #5
8h
4h
ON
OFF
1
0
Ah
LED #4
LED #3
2h
1h
8h
ON
OFF
ON
1
0
ACh
Lower Nibble GREEN LEDs
LED #2
LED #1
4h
2h
ON
OFF
1
1
0
Ch
LSB
LED #0
1h
OFF
0
207
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