You are so correct retrolefty,
in fact I have not yet a null modem.
I have bought a gender changer thinking it would solve my problem, but not.
So I removed it, and used simple wires crossing tx and rx like a null modem between the RS232 to TTl converter and the serial cable from the barcode scanner.
It worked perfectly wired this way, so I expect it to work with the null modem (that I will buy soon).
Thanks for your comment.