Undo

[bhiker@free.fr]

Sorry for this newbie's question about undo.

I do not succeed to undo characters with emacs 21.3 under console.

Here is what I get:

* Under X, it may work:
C-_ is binded to the undo function (as returned by C-h c C-_) and works fine.
Besides it, at first try, M-x undo removes one character.
This character will appear again with the following M-x undo.

* Under a console text-based environment: 
C-_ is binded to DEL 

M-x undo behaves like under X: one character is deleted at the first try and 
will appear again at the second try.

So, how may I bind C-_ to the undo function for the console environment?
Thanks!