IBM Personal Computer AT Manual To Operations page 183

Summary
You have now seen how each control-panel button and
light works.
Your IBM Personal Computer Color Printer has four
modes: ready, not-ready, forms, and test. It can be in
only one of these modes at a time. The ready, forms,
and test modes are each indicated by a light with the
same name. When only the Power light is on, your
printer is in the not-ready mode.
The Ready button sets your printer to the ready mode to
accept data from your system, or to the not-ready mode
so that other modes may be set.
The Forms Mode button sets the printer to the forms
mode. The Initialize button causes the printer to leave
the forms mode and changes the top-of-form setting.
In the forms mode, the Line Feed button and the Form
Feed button produce very small paper movement. In
the not-ready mode, these buttons produce a large
amount of paper movement.
The Test Mode button sets your printer to the test mode
and starts the printer self-test to verify printer
operation.
The Power light indicates when power is supplied to
your printer. If only the Power light is On, your printer
is in the not-ready mode.
The Fault light glows steadily when your printer is out
of paper, and flashes continually if the power-on tests
detect a problem.
Color Printer 1-73