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Is it possible to set some of four available user leds to show serial port activity, like we can set it to show mmc activity?

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Based on Ubuntu linux on BBB, but other Linux will work similarly.

The user LED mode can be accessed using the following command:

echo none | sudo tee /sys/class/leds/beaglebone\:green\:usr3/trigger > /dev/null

Where none can be replaced with one of the following:

none nand-disk mmc0 mmc1 timer oneshot heartbeat backlight gpio cpu0 default-on transient

I am unaware of any option to "connect" a user LED to a serial port. The only option I see is "manually" writing 0 or 1 to one of the brightness files upon receiving data in your program.

Optionally work around running as root by issuing a sudo chown ubuntu:ubuntu /sys/class/leds/beaglebone\:green\:usr3/brightness once.

In Perl you can blink the LED as follows:

my $ledState = 0;
while( 1 ) {
  $ledState ^= 1;   # toggle state
  open( LED , ">" , "/sys/class/leds/beaglebone\:green\:usr3/brightness" ) or die "Cannot open file for LED control: $!\n";;
  print LED $ledState;
  close( LED );
  sleep( 1 );
}
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