The default console setting on the Model 712 is the video device and keyboard. HP initially offered no other method because they didn't want the inexpensive machine to be used as a server. Later, it was discovered that a solution existed to switch the console to the serial port.

Interrupt the boot process with <ESC> to access the boot administration menu. From there, start the ISL (initial system loader) with `boot pri isl`. Within the ISL, the console can be redirected to the serial port using a somewhat cumbersome command.

BOOT_ADMIN> boot pri isl
ISL> conspath 2/0/4.0x283

Power off the machine, connect a serial cable, and set the terminal software to 9600,8,Nn1. Power the system back on. From now on, the primary console will be the serial port.

• If HP-UX is not located at the primary boot address, it must be specified, e.g.

   BOOT_ADMIN> boot scsi.2.0 isl

• If a GUI is installed, such as CDE or HP OpenView, it will be displayed regardless of the console device.

Interrupt the boot process, goto isl and change the setting

BOOT_ADMIN> boot pri isl
IS> conspath 1/0/0.0

https://www.netbsd.org/ports/hppa/serialconsole-712.html