S5 to PC (using rs232/tty cable) serial data capture

Guys,
Did anyone tried capturing the serial data being transmitted "from PC to Siemens S5 CPU" which are connected via serial/tty cable?
I actually wanted to know the format of data being transmitted so that I will see which particular memory of S5 of the are being addressed when I change the "set point value of pressure".
Thanks!

The program in PC computer that communicates with the S5 PLC (via RS232/TTY cable) is using AS511/RK512/MODNET 1/N.
How do these protocols appear in a captured serial port?
I am intending to use PortMon software to capture the serial data.