During power-up self test (POST), most system BIOSes write status codes to I/O port 80h or 84h.
A POST code display shows the last value written to this port as a hexadecimal number (e.g. 1B) on a LED display.
If the POST code remains blank, the system BIOS is not coming up at all. Something is seriously wrong with the system board.
The meaning of individual POST codes is specific to each BIOS and system vendor, and should be obtained from them.
 

Product Links:
Schematic

BIOS Central has a very complete collection of POST code tables.

• MiniPCI form factor
• Port 80h/84h auto switch (whichever is accessed first)
• Test pins to connect logic analyzer to PCI bus
  J1: JTAG for PLD programming, manufacturing only
  J2: RST, CLK, FRAME2# = FRAME# delayed by one cycle
       (filter assistance for logic analyzer), INTA#, INTB#, 3.3V, 5V, GND
  J3: CBE3, GND, AD24..AD31
  J4: CBE2, GND, AD16..AD23
  J5: GND, PERR, DEVSEL, SERR, STOP, FRAME, TRDY, PAR, IRDY
  J6: CBE1, GND, AD15..AD8
  J7: CBE0, GND, AD7..AD0