It is usually no reason to expose the electronics, but in some occations it is necessary and for OEM systems it is unavoidable. This FAQ is mainly written for DVL OEM systems, but the content applies to the Signatures and DVLs also.
The board stack on the Signature and DVL consists of three boards (+ the sensor board), and two of them have LEDs to indicate their state.
Board stack for Signature and DVL
SEC/ INTERFACE BOARD - not in use if serial communication is used.
Yellow LED: ON: 100Mbit/s Ethernet. OFF: 10Mbit/s Ethernet
Orange LED: Ethernet Activity
Red and Green ON: Bootloader mode
Red OFF and Green ON: Normal mode
Red ON and Green OFF: Error
BBP / PROCESSING BOARD
Red LED: Quick blink when power is added (or reboot). Otherwise OFF.
Continuous red: Error
Green LED: Indicates that the instrument is in command mode. Turns off when the instrument is measuring
Yellow LED: Blink when the instrument pings (similar as the blue LED that is visible from outside if the instrument has a housing. Unlike the blue LED, the yellow cannot be turned off)