You mentioned the keyboard is plugged into the monitor - Do you mean a dumb terminal? Is the 'monitor' plugged into the serial port (a or b)?

Here is the info -

The definition of a headless system is one that does not have an attached
keyboard or does not have a Sun console. It will by default send the boot
information out serial port A if the keyboard or console are not attached.
Serial port B can also be used.

Attach using NULL modem cable to:

-An ascii terminal- For example, Wyse terminal
-Another Sun workstation that has a Sun console
-A Network terminal concentrator/server

Technical Support does not recommend booting off a serial port with a
modem attached or not having any type of console.

A console allows the ability to send a break signal dropping the
system to the OK prompt for diagnostics. The console device is
/dev/console.


What needs to be configured on Solaris
--------------------------------------

A. Open Boot Prom:

OK> setenv output-device ttya
OK> setenv input-device ttya

Note: the above is not necesary on tty A if the keyboard is disconnected.
It is necessary to change these setting for serial port B regardless.

Additional settings:
ttya-rts-dtr-off=false
ttya-ignore-cd=true

Syntax for changing Open Boot Prom value:

OK> setenv ttya-rts-dtr-off false
OK> setenv ttya-ignore-cd true

Use printenv to verify the following settings:

ttya-mode=9600,8,n,1,-
To change:
OK> setenv ttya-mode 9600,8,n,1,-

Run the "reset" command to make the above settings take effect

OK> reset


B. The terminal type, /etc/inittab and port monitors

Do not start port monitor services for the serial port. Verify with pmadm -l.
Make sure there are no listings for the tty port that is acting as the
console. (Solaris 2.5 ttya and ttyb have port monitor services enabled for
modem logins by default- These services need to be removed)

To remove type:

pmadm -p zsmon -r -s <SVCTAG>

SVCTAG will be ttya or ttyb.

/etc/inittab takes care of setting the console port characteristics
with the following line:

co:234:respawn:/usr/lib/saf/ttymon -g -h -p "`uname -n` console login: "
-T sun -d /dev/console -l console -m ldterm,ttcompat

By default the console terminal type is "sun" specified with the
-T option above. This means that for ascii terminals vi will not work.
Inorder for programs like vi to work the TERM variable needs to be
changed to match the "personality" of the terminal.
This can be done by modifing -T in /etc/initab to 'wyse30' (for example)
or changing the TERM variable in .login, .profile, or .cshrc or at
the command line, for example:

setenv TERM wyse30

C. Sending a Break Signal

Unpluging the serial cable is the equivalent to sending a break
and will bring the Sun box down to the OK prompt. If the cable was
not attached during bootup plugging it in may send a break. When
a Network Terminal Server is rebooted this will also send a break
and bring the Sun boxes down to the OK prompt.


What needs to be configured on the terminal
-------------------------------------------

Ascii terminals have a setup mode. For the wys30 <shift><setup> keys
will bring up the terminals parameters. Use the arrow keys and
the space bar to proceed thru the settings.

settings: 8 data bits, no parity, 1 stop bit (8, none, 1)
baud: Use 9600!
personality (modes) : vt100, wyse60, etc needs to match the Solaris
TERM variable

Related SRDB's
--------------

SRDB ID: 13758
SYNOPSIS: Headless 4.x connected to Network Terminal Server displays garbage until login.

INFODOC ID: 13760
SYNOPSIS: Ways to Send Break (L1-A) to remotely logged into serial console device.

INFODOC ID: 14672
SYNOPSIS: Solaris 2.4/2.5 Modem configuration

INFODOC ID: 13766
SYNOPSIS: Warning - ttymon cannot allocate controlling tty on /dev/console,

INFODOC ID: 13724
SYNOPSIS: Allowing root logins over modem lines in /etc/ttytab


Also:

OpenBoot Command Reference Manual
A. Setting Up a TIP connection