the reason you cannot connect to an early Oracle version si not a Toad
problem, but an Oracle Client problem. Starting around 9i (I think) the
Oracle client had checks built in to make sure that you didn’t try to
connect to an older version of the database.
If you have an 8i (or 9i) client, you’ll be able to connect down to
Oracle 7.3. I used to do this when I was running an 11g client but still
had to connect with 7.3.4, 8.0 and 8i databases. I installed the 8i
client and for the databases that I needed to use it with, I chose it as
the Oracle Home to use on the connections screen.
The only problem with this was the fact that Toad can only use one
Oracle Home at a time, so if I connected to an old database, I had to
use the same Oracle client to talk to newer versions, and sometimes they
refused the connection - because there’s a check in the database to see
that the client in use isn’t too old!
Oracle want everyone to be running the latest versions.