IBM RT PC Options Installation page 231

7. After the light comes on, wait up to four minutes while numbers
flash on and off on the two-digit display.
8. Is the red two-digit display blank (no numbers on it)?
Yes. Go to the next step.
No. Go to Step 37.
9. Adjust the brightness and contrast controls on the display, if
the display has these controls. If you need help operating the
display, see IBM RT PC Guide to Operations.
Note: If your system is connected to an IBM 5085 Graphics
Processor, the keyboard belongs to the 5081 Display at power on.
If you have not switched the keyboard to your display using the
Alt and Sw Keybd keys, see the 5080/RT PC Graphics System
Operation.
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Section 9. Checking Out the Option Installation 9-7