IBM Personal Computer AT Manual To Operations page 184

Control Panel Reference for Single Sheets
Light
Fl
~
Forms
Mode
c::::J
Test
Mode
c::::J
Fl
~
Indicates
On - your printer is in the ready mode.
Flashing -
your printer is in the ready mode
but has been released by command from the
system.
If
this is the only light On, power is supplied
to the printer, and your printer is in the
not-ready mode.
Your printer is in the forms mode.
The printer is in the test mode.
On - your printer is out of paper.
Flashing continually -
your printer requires
service.
1-7 4 Color Printer