How to use kgdb on development board with single UART port

[Dahua Mei <tatwah.mui%gmail.com@localhost>]

I'm trying to debug netbsd kernel in beagleboard rev. B. It has only
one UART port. Although I enabled KGDB in the conf file and commented
out DDB like following:

#options    DDB     # in-kernel debugger
#options        DDB_ONPANIC=1
#options    DDB_HISTORY_SIZE=100    # Enable history editing in DDB
options     KGDB
options     KGDB_DEVNAME="\"com\"",KGDB_DEVADDR=0x49020000,KGDB_DEVRATE=115200
#UART 3

GDB still reports timeout like the symptom described in
https://groups.google.com/forum/#!topic/fa.netbsd.tech.kern/k35CsZCDdpI

Could someone help me how I can use kgdb with only on UART port on the
board? Thanks in advance!