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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
1  
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
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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 20 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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1  
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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The LEDs on the Raspberry pi Model B are (where two names are given the first is the name printed on rev 1 boards and the second is the name printed on rev 2 boards).

  • PWR -- power light, this is just hardwired to the power supply.
  • OK/ACT -- Lights up when there is SD card activity (since SD card activity is usually intermittent this usually results in flashing). This LED is driven by the kernel and will not light up until a kernel is successfully loaded.
  • FDX -- Lights up if the Ethernet cable is connected and link is running in full duplex mode.
  • LNK -- Ethernet controller link/activity LED. Lights up when Ethernet cable is connected. Flashes when there is network activity.
  • 10M/100 -- Lights up when the Ethernet cable is connected and the link is running at 100Mbps.

On the Raspberry pi model A the silkscreen for the three ethernet related LEDs is still visible but the LEDs themselves are not present. The power and ACT LEDs are the same as later B models.

On the B+ and later models the LED arrangement has changed significantly.

  • The Ethernet LEDs (on models with Ethernet) are now on the Ethernet socket and are unlabeled, I'm not sure exactly what each LED means.
  • The ACT LED now lights up solidly before the kernel boots.
  • The power LED is now longer a dumb LED across the power supply but is instead connected to an undervoltage detection circuit and will turn off if the supply voltage is low.
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Additional info for B+ and later boards:

PWR lights up continuously if the PI has enough power (> 4.65V), light goes off below the voltage and it may blink then, signalizing it may not have enough power for some usb devices without own external power supply.

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That is only true for the B+ and later boards. For the original model A/B the power LED was just an LED+resistor across the 5V power rail. – Peter Green May 21 at 23:57

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