All the LEDs are on the front of the board.
These LEDs are normally fitted:
- a column of 8 yellow ones at the top left.
- a red one by the reset switch
- a green one nearby (by the serial/JTAG connector)
- a row of three - one green, one yellow, one green on the bottom right edge (below the Smartmedia socket/NAND chips)
1 - This column D2 - D9 is the bottom 8 bits of the LCD data.
2 - The red LED D1 by the reset switch is on when the board is in reset (i.e. the 3.3V line has dropped too low and the reset chip has tripped). If this happens repeatedly then your power supply is probably not up to the job or the CF card/add-on boards are drawing too much current for the 5V->3.3V regulator to supply (> 500mA).
3 - This green one D18 indicates 'power on' - strictly '3.3V power on'.
4 - These show device access - D20 the left (green) one is NOR flash access, D25 the middle (yellow) one is NAND flash access, D24 the right (green) one is ??