What do system LEDs signify?

Question

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?

Answer 1

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

Answer 2

Current RPi models have some of the following LEDs:

PWR (red)

Indicates that power has been provided to the board. On A+ and later models it will flash if the voltage drops below 4.63V

ACT (green)

Indicates SD card activity: flashes when read or write is in progress, otherwise steady ON (on Pi Zero) or OFF. Should flash intensively during boot. This is the only LED present on Pi Zero.

Additionally, ACT is flashed in specific patterns (check this if you suspect to have an old firmware) to indicate early boot problems.

LNK (green)

Indicates Ethernet (LAN) connection and activity: it is constantly ON when connected, and flashes on data transfers. Located on the PCB in model B, and on the Ethernet RJ-45 socket in later models.

100/1000 (yellow or green)

Indicates 100Mbit Ethernet link on 100Mbit models or 1000Mbit link on Raspberry Pi 3B+ and 4. It's OFF for lower speed connections. Located on the PCB in model B, and on the Ethernet RJ-45 socket in later models.

FDX (green or orange?)

Indicates Full Duplex Ethernet connection. Only exists in model B.

Answer 3

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

They are as follow:

• 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

Answer 4

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.

Answer 5

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.