I hate to jump onto someone else's post, but I have an issue that might be along these same lines.
I am using a USB to serial cable that I believe was bought for this very use, and I can connect to and run the L35E, my only problem is that, in RSLinx, the controller shows up as "Unrecognized Device" and has a yellow question mark in front of it. I want to connect to this controller via ethernet, but I do not have the options to set the IP address in RSLinx.
Shane