AT&T MERLIN LEGEND Communications System Release 2.0 Maintenance And Troubleshooting Manual page 156

Control Unit Problems
Red LED
When the red LED is on, the module is not in service because it is in standby
mode, being tested, or in an alarm condition. When a module resumes
normal operations, the red LED turns off.
Yellow LED
When the yellow LED is on, it usually means that at least one call is in
progress on that module. This is usually true for modules with trunks. When
this is the status, you must reset the board if you want to terminate any calls in
progress rather than waiting for them to terminate during a busy-out.
Green LED
The green LED is usually off. It maybe on during power up or when one of
the module Ioopback tests is running (Internal Loopback or CSU Loopback).
Sample LED Display
If a call is in progress on a module that is in a working, normal state, the
System Status display for that module appears as Roff Yon Goff.
If a slot is empty, R, Y, and G will appear without indicating a status. Slot 4 is
empty according to the sample screen in the last procedure.
Slot Status
Slot Status provides the following information about the module in a particular
slot:
LED status
errors (yes or no)
mode (initialization, standby, test, or normal)
alarms (yes or no)
maintenance busy (yes or no)
4-66 Checking System Status and Slot Status