1. Connect a 1756-CP3 or 1747-CP3 cable between your PC's serial port and the 1756-L61 controller.
2. In RSLinx Lite, configure a new "RS232 Serial Devices" driver. Be certain there are no other serial drivers (especially the 1747-PIC/AIC+ driver) in use. Set the driver to use the COM port to which the serial cable is connected.
3. Click the "Autoconfigure" button in the driver configuration window.
This always, always, always, always works unless you have a damaged controller, an intentionally differently configured controller, or a PC communication port problem. It's REALLY easy.
If you can't make this work, call your local Rockwell Automation distributor and get their technical specialist to help you.