Error messgae when netbooting - ex0: device timeout

["Boo Geum Jung" <bgjung%etri.re.kr@localhost>]

Oweing to many help, I finally cross-compile and download netbsd to
sandpoint.
And executed well.
But when I try to netboot, the flowing error message is occurred.
What's the problem?

=================
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 1.5W (MYRAMDISK) #4: Mon Jul 30 21:36:58 KST 2001
    
root%bgjung3.etri.re.kr@localhost:/usr/src/sys/arch/sandpoint/compile/MYRAMDISK
CPU: PowerPC Version 0xffff800c (Revision 17.3)
total memory = 32768 KB
avail memory = 25552 KB
using 435 buffers containing 1740 KB of memory
mainbus0 (root)
pci0 at mainbus0 bus 0
pci0: i/o space, memory space enabled
vendor 0x1057 product 0x0004 (host bridge, revision 0x12) at pci0 dev 0
function
 0 not configured
pcib0 at pci0 dev 11 function 0
pcib0: vendor 0x10ad product 0x0565 (rev. 0x10)
vendor 0x10ad product 0x0105 (IDE mass storage, interface 0x8f, revision
0x05) a
t pci0 dev 11 function 1 not configured
ex0 at pci0 dev 16 function 0: 3Com 3c900B-TPO Ethernet (rev. 0x4)
ex0: interrupting at irq 3
ex0: MAC address 00:10:5a:22:9e:89
exphy0 at ex0 phy 24: 3Com internal media interface
exphy0: 10baseT, 10baseT-FDX, auto
isa0 at pcib0
com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
com0: console
com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo
pckbc0 at isa0 port 0x60-0x64
lpt0 at isa0 port 0x278-0x27b irq 7
fdc0 at isa0 port 0x3f0-0x3f7 irq 6 drq 2
biomask 0 netmask 8 ttymask 8
md0: internal 2048 KB image area
boot device: <unknown>
root on md0a dumps on md0b
no file system for md0 (dev 0x1100)
cannot mount root, error = 79
root device (default md0a): ex0
dump device:
file system (default generic):
root on ex0
nfs_boot: trying BOOTP
ex0: device timeout
ex0: device timeout
ex0: device timeout

B.G.Jung