Excerpts from netbsd: 3-Jul-98 serial console in 1.3.2? Jukka
Marin@pyy.jmp.fi (601*)

> How do I enable serial port console in NetBSD 1.3.2 (i386)?  In the kernel
> config file there's the line
> 
> options         "CONSDEVNAME=\"com\"",CONADDR=0x3f8,CONSPEED=9600

console(4) should answer mosy of your questions.
In short: the console device selection will be passed
by the bootloader to the kernel (unless you have
pre-1.3 bootblocks).
Either you change the bootblocks to use a serial
console theirselfs (see boot_console(8)), or you
set it in the kernel config file as you did. In the
latter case you probably have to set "CONS_OVERRIDE"
too.

> Do I need to disable the corresponding com* port in the config file?

No - in contrary, it is strictly needed.

> options(4) mentions SERCONSOLE

That's in the "atari" section.

best regards
Matthias