HP Designjet 3D Service Manual page 29

controller board, +5 and +12 VDC come from the PDB. One function of the
D6-D12 LEDs is that they turn on sequentially to show software download
progress. During normal operation, D13 will blink approximately once every
two seconds to indicate that the watchdog is monitoring the system and
everything is operational.
LED Label
D1 +3.3 VDC Supply
D2 +5 VDC Supply
D3 +12 VDC Supply
D6-D12 Debug LEDs (software use only)
D13 Coldfire processor heartbeat
Description
-13