Serial port connection (null modem) not working

[Fili Wiese <debian@oni.nl>]

Hello,

I'm having trouble setting up a serial connection between two servers
using a null modem cable.
This link is needed as a redundant connection for Heartbeat.
Both machines are indentical (both hardware and software).

Some info:
-----------------------------------------------------------------

#uname -a
Linux galvatron 2.6.18-5-686  #1 SMP Sun Aug 12 21:57:02 UTC 2007 i686
GNU/Linux

#ls -al /dev/ttyS0
crw-rw-rw- 1 root dialout 4, 64 2007-08-23 14:56 /dev/ttyS0

#groups
dailout (among others)

#setserial /dev/ttyS0
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4

#cat /proc/interrupts
4:          4          1    IO-APIC-edge  serial

#cat /proc/ioports
03f8-03ff : serial

#cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:0 rx:2 brk:2 RTS|DTR|DSR
1: uart:unknown port:000002F8 irq:3
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3

-----------------------------------------------------------------

In my humble view, everything seems to be set up perfectly.
However, an echo/cat test simply doesn't work:

server1: echo test >/dev/ttyS0
server2: cat </dev/ttyS0
(no output)

I'm really at a loss :S
Does anybody have a clue where to look next?

Maybe the (brand-new) null-modem cable is not wired properly but how do
I find that out?

Thanks in advance,

Fili