Half-duplex VT420?

Since I'll be running another headless computer (HP 9000/380), I brought home my VT420. Nice terminal, but unfortunately this specimen will only talk, not listen. At first, I suspected my HP had lost its settings (the stupid boot PROM must be set via the keyboard to use a serial console). I proceeded to bring my HIL keyboard home and set it (without any screen) to use a "remote" console again, and the machine seemed to acknowledge that, beeping happily. But no response either. So I hooked it up to my Amiga. Typing at the keyboard, there was output in NComm, but doing the opposite didn't work at all. So we have only one-way communication. What could be the problem? The cable works fine, I'm getting -- En ligne avec Thor 2.6a. Allt ?r under kontroll, och caps lock ?r bredvid.