Have you tried something like the following?

rman80 rcvcat rman/rman@rmandb target sys/password
-- or --
rman80 rcvcat rman/rman@rmandb
RMAN> connect target

The key to be sure to indicate your catalog database with "rcvcat"
For these examples, your target database is local. $ORACLE_SID (for UNIX) must be set as well for the target.

D.