Ok, so far there are several things that have been said that are incorrect so far.
1. Port 1 can be a MODBUS slave. It is set for 9600 baud, odd parity, and 1 stop bit.
2. You only need pins 4 & 5 jumpered for RS-232 on Port 2. Do not jumper any others. Pins 7 & 8 are the same potential, ground. You say your cable is right, did you swap pins 2 & 3 (RXD-TXD & TXD-RXD) or do they go through straight (RXD-RXD & TXD-TXD)?
3. Make sure your baud, parity, and stop bits are configured the same in your master device as in the PLC Port 2 setup.
If you are really struggling with this call AD's tech support, they can get you going probably in a few minutes (800-633-0405).