I am trying to run the "motor_test" example of the MKR motor carrier library, but when I am attaching the MKR 1000 to the carrier the red led next to power led is constantly on, see attachment. The board is being powered I measured it with a multi-meter but in the "M3+ M3-" sockets of the motors I don't measure voltage.
In the servo and encoder terminals I measure voltage.
When I am trying to upload either the flasher or the test_motor example the IDE message is successful upload but in the serial wont happen anything.
I am not talking about the power led. I am talking about the led which is next to it.
From those two the left one (yellow) is on when the on/off is on. The right one (red) I dont know and is constantly on.
There are two LEDs side by side. The one on the left (not lit in @xrek's picture) is the power LED. The one on the right (DL10) is an indicator used by the ATSAMD11 microcontroller on the MKR Motor Carrier.
The battery connector is 2S and 3S compatible, meaning it can be used with battery packs of nominal voltage 7.4 V or 11.1 V. You can't use a 1S LiPo battery as the power supply to the MKR Motor Carrier because it supplies less than the minimum voltage of 6.5 V specified on the Tech Specs page.
As far as I can tell, it's currently only used by the bootloader:
There is some provision to use it in the firmware:
but that isn't used anywhere. It also doesn't match well with the schematic, which shows the LED connected between the IO pin and +3.3V (and thus a LOW should be associated with the LED being on, not a HIGH as in that "led_on" function.
So I would guess that LED is a hardware feature that may be used more in future versions of the firmware, but already can be used as an indicator that no firmware is installed (in which case the bootloader will be running constantly, thus the blinking LED mentioned in the example sketches).
If you're looking into that code further, take note that the bootloader code is compiled using the "super compact" pin mapping, in which PA03 is Arduino pin 0, whereas the firmware code is compiled using the "standard" pin mapping, in which PA03 is Arduino pin 3. It's a little confusing that the LED is controlled using a different Arduino pin number in each of the programs.