HP 16550A User Reference page 233

OFF in the Format menu. Make sure the configuration is consistant
across menus.
This macro requires two unused patterns assigned to this machine. If the
user tries to select the n-bit serial pattern macro and there are not at least
two pattern resources assigned to the machine that are "unused" (defined as
BOTH all don't cares AND not used in the sequence) then this error results,
and the selection is ignored. Reconfigure the sequence levels to free up the
required resources.
Error Messages
Error Messages
11–5