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There are 5 status LEDs on the Raspberry Pi board. While I can guess what the POWER LED signifies (the power being attached), I am not sure about other four.

So, what do OK, FDX, LNK and 10M signify?

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Explained here. – ACarter Jul 1 '12 at 15:48
possible duplicate of How do I control the system LEDs using my software? – Tibor Jul 1 '12 at 17:32
Yes it's duplicate answer - but the question is asked so differently that it's worth keeping. – Maria Zverina Jul 1 '12 at 20:23
No, it is not a duplicate. The linked question asks how to control the LEDs, while this asks what do they mean. Very closely related, but no duplicate. – That Brazilian Guy Feb 27 '14 at 2:04
up vote 19 down vote accepted

According to http://elinux.org/RPi_Hardware

Rev. 1.9 Boards

  • D5(Green) - OK - SDCard Access (via GPIO16)
  • D6(Red) - PWR - 3.3 V Power
  • D7(Green) - FDX - Full Duplex (LAN) (Model B)
  • D8(Green) - LNK - Link/Activity (LAN) (Model B)
  • D9(Yellow) - 10M - 10/100Mbit (LAN) (Model B)

Rev.2.0 Boards label the LEDS as

  • ACT
  • PWR
  • FDX
  • LNK
  • 100
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Update: according to that page, on Rev2.0 boards, the first led is "labelled as 'OK' on Rev1.0 boards and 'ACT' on Rev2.0 boards". – Blaisorblade Jan 12 '13 at 17:27

The Raspberry Pi boards have either two or five LEDs on a corner of the board.

They are as follow:

enter image description here

  • ACT (or OK on the older Revision 1 boards): green, blinks on SD card access, off otherwise
  • PWR: red when powered on.

The following LEDs exist only on Model B boards:

  • FDX: green when Full Duplex ethernet is active
  • LNK: green when ethernet is connected, blinks on data transfer
  • 100 (or 10M on on the older Revision 1 boards): orange, indicates 100Mbps ethernet

Sources: adafruit, elinux

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