With a loopback plug in the network port, any customer equipment connected to the NCC 2020 (or a data port of any DIU module assigned to the CSU) should see a loopback condition. If the customer equipment can report errors, it should indicate that no errors are being received.
A T1 loopback plug can be made by taking a male RJ-45 connector and placing two short jumper wires where a cable would normally go. One jumper is placed from pin one to pin five, the other jumper connects pin two to pin four. See Figure 5-5 for a drawing of a T1 loopback plug.
NOTE: When using a T1 loopback plug it may be necessary to temporarily change the timing selection in the CSU. If the CSU is set to recover network clock, it will be trying to recover a clock it is trying to recover, the clock frequency will drift. For this test, set the CSU clock source to Internal. Remember to change it back after the test.
T1 Loopback Plug
RJ -45 Connector
RJ-45 plug held with plastic latching tab unseen at bottom
Verilink NCC 2020 User Manual