Thanks a lot for many who responded to my request for help:
what's the trick of adding a dumb terminal to RS6000.
The trick is that RS6000 needs a carrier signal. (One replay says that
AIX 3.1 doesn't need but 3.2 needs).
To solve the problem, two solutions:
1. use smit to add 'clocal', as a soft carier detect loop.
2. short circut 8-20pin on a DB25 on host side.
Of course, a cable of more than 3 pins will also work.
Thanks again, esp. to these netters: