HP 16500B User Manual page 114

If You Have a Problem
"Waiting for Trigger"
If a "don't care" trigger condition is set, this message indicates:
q
For an HP 16511B Logic Analyzer, only one of the two cards is receiving
its state clock. Refer to "Slow or Missing Clock."
q
For an HP 16510A,B Logic Analyzer, the pattern duration is probably set
to less than (<) instead of greater than (>). Since a "don't care" pattern is
always true, the "less than" condition is never satisfied. Set the trigger
correctly for the measurement that is desired.
516