rman target / 是连接了 target DB并进入了rman环境，这时还可以连接 catalog DB
rman nocatalog target /  是以nocatalog模式连接进入了target DB，这时是不可以再连接catalog DB了。详细如下：

[oracle@sar-vcom ~]$ rman target /

Recovery Manager: Release 10.2.0.1.0 - Production on 星期二 11月 24 10:04:54 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

connected to target database: CHARGE (DBID=1102464294)
RMAN> CONNECT CATALOG system/oracle

connected to recovery catalog database
RMAN> exit

Recovery Manager complete.
[oracle@sar-vcom ~]$ rman nocatalog target /

Recovery Manager: Release 10.2.0.1.0 - Production on 星期二 11月 24 15:55:28 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

connected to target database: CHARGE (DBID=1102464294)
using target database control file instead of recovery catalog

RMAN> CONNECT CATALOG system/oracle

RMAN-06900: WARNING: unable to generate V$RMAN_STATUS or V$RMAN_OUTPUT row
RMAN-06901: WARNING: disabling update of the V$RMAN_STATUS and V$RMAN_OUTPUT rows
ORACLE error from target database:
ORA-03135: connection lost contact

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-06445: cannot connect to recovery catalog after NOCATALOG has been used

RMAN> exit

Recovery Manager complete.