TTL null modem

Dear all,
is it possible to connect via cable a couple of ttl serial interfaces
present in the board?

Yes

@BBBB: thanks for useful reply !

@Marco: yes it's possible. You can use CON1, CON2 and CON3 but you need to
make some small external board with MAX323 for level adjustment (for
reference look at mini 2440
schematics:http://friendlyarm.net/dl.php?file=mini2440_schematic.zip at
page 6)

Thank you,

only for a sake of clearness, I need to connect CON1 to CON2 directly,
can I do it using a direct cable?

Hi Marco,

why you need to make a connection between CON1 and CON2? You can connect
but only TXD0, RXD0 with TXD1 nad RXD1. Don't connect VDD5V (could lead to
board damage).

Hi Marco,

sorry for mistake. Connection CON1 and CON2 could make a problems. Because
CON1 levels are adjusted by MAX3232 I'm not sure what happens if you
connect CON1 and CON2 (you can try but could make some problems ;)). Just
connect CON2 and CON3 this should work for you purpose.

Thanks,
I need it for testing purposes, I need to send messages across the serial
CON port and I would like to test the message sent reading the same
messages on an other port.
Thanks again.

Definitely; I have done it.  Connect CON2 TXD to CON3 RXD and then CON2 RXD
to CON3 TXD.  I found mating connectors on old sound board cables.  
You can then send data from one port to the other with the com test utility
or your own program.

My intention is to build a small opto-isolated interface for CON2 and CON3
to NMEA-0183 and to SeaTalk, respectively, for marine instrument data.