Problem booting rb230

Hello, I´m trying to boot an rb230 with routeros in a CF but with a serial modem connected at the serial0 port for dial backup. When I connect the power supply the rb230 seems to communicate with the modem (us robotics sporster) as the rd and sd leds on the modem blink, and the rb doesn´t boot. I tried disabling the system console but can´t solve the problem. I think the problem is with the Routerboot on the RB230, but i don´t know how to boot directly. How can I solve this?

Connect Null Modem cable to the RouterBOARD 230, login to the RouterBOARD BIOS by hitting any key,
click on,
k - boot key

set instead of any.
It should help.