Hello Manuel.  You need to edit /etc/ttytab on the SunOS machine and
disable the console line by changing it to off.
then, enable ttya like this:
ttya	"/usr/etc/getty std.9600" unknown local secure

Now, when you reboot, you should see prom, boot and console messages on the
serial port.  You should also be able to login.  You  may need to change
the terminal settings on your terminal between the time the machine is in
the prom monitor and the time it's booted.  The prom speak n81, and SunOS
speaks e71.
-Brian
On Dec 13,  5:57am, Manuel Bouyer wrote:
} Subject: SunOS4 & serial console
} Hi,
} sorry for the off-topic question, but I think some of the guys on this list
} may have the solution (and the faster I solve this, the longer I'll hack
} on NetBSD :)
} I have some SunOS 4.1.4 machine which were installed from keyboard/screen,
} and now I'd like to use a serial console for them (connected to
} a serial port of a NetBSD machine).
} I've set input and output to ttya in the PROM, and the serial console
} work: I can see message from the PROM and type commands. I can also see
} kernel boot messages. But once init is started, outputs of /etc/rc is
} garbage on the serial console, just as if the speed, or number of
} start/stop/parity bits were wrong. If I boot single user, I also get
} garbage displayed, but what I type seems to be properly interpreted
} (if I type 'halt' I get the ok prompt).
} One the boot completes and getty is started, output is clean again and
} I can log in without problems.
} Any idea on how to solve this ?
} 
} --
} Manuel Bouyer <bouyer@antioche.eu.org>
} --
>-- End of excerpt from Manuel Bouyer