3D Systems FIGURE 4 MODULAR User Manual page 9

Upper-chamber door-closed sensor - Opening the upper-
chamber door during printing will trip this sensor, causing the
print job to abort if the door is not closed within 15 seconds.
Also, if the door is open before a print job begins, this will
prevent the print job from beginning.
Catch-Tray Presence Sensor - Having the catch tray uninstalled
before printing will trip this sensor; and the printer will not be
able to be marked as "ready".
Catch tray removed for clarity
Print Platform Presence Sensor - Having the print platform
uninstalled before printing will trip this sensor; and the printer
will not be able to be marked as "ready".
Resin-Tray Presence Sensor - Having the resin tray uninstalled
before printing will trip this sensor; and the printer will not be
able to be marked as "ready".
3D Systems, Inc. 5 p/n 42-D116, Rev. K