I use the RS232 shield on an Arduino Mega following the example on the shield wiki page.
The arduino serial monitor receive the “Goodnight moon” message and the terminal on the PC the “Hello World” message.
However, I cannot transmit back from the PC terminal.
Ok I found the reason
"Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15 …"
Actually a pity the shield cannot use the Mega serial ports on pins 14 to 19 …