Hello.
You should not connect the RS232 pins to the Netduino input, because they uses different voltage levels.
The Netduino works with voltages in the 0..+3.3V range (+5V allowed for some inputs). The RS232 works also with negative voltages swinging from -10V to +10V (approx).
What you looking for is a RS232 to TTL converter shield, like this one. There are plenty of similar shields, so you may search for others. However, you should bear in mind that the shield should be equipped with the MAX232 chip (or equivalent).
Cheers
Thanks for the advice. I'll try and get hold of the shield you suggested (or similar). My device that I'm trying to connect to claims to be 5V for the serial communication, but I guess it probably still swings -ve, so maybe that's the problem. It's expecting the message from the Netduino to go negative and it doesn't.
Still, shouldn't I be able to connect the Netduino pin 0 to pin 1 and make that board talk to itself? Just as a way to convince myself that the board is working and the right messages are getting sent? Needless to say, it ain't working. The values I'm getting back on port.Read aren't the same as what I sent out with port.Write. (edit: nevermind, it's working now)