Sun enterprise 420R boot issue

Hi,
I am using sun enterprise 420R server. Problem is on the monitor i am not able to see anything. I tried Serial connection, on terminal(using serial) i could see that POST(power on start tests) tests are being done, after that "timeout waiting for arp rarp packet" message keep displaying forever and it never comes up.
Boot device is set to net.
By pressing CONTROL+B also not able to get ok prompt.
i could not disable the POST tests by pressing STOP+A.

Can anyone pls help how to change boot device to disk and how to get ok prompt.

Thanks in advance
kk