I am assuming you are using a 1747-CP3 cable to connect to the SLC-5/04. Close RSLogix500. Make sure your SLC-5/04 is powered on and is connected to your computer.
Open RSLinx and if there is a DFI driver installed, stop it and then delete it. Add the RS-232 DF1 Device Driver back to the configured drivers and select Auto-Configure when the communication settings popup appears. It should find the SLC-5/04 and update the communication settings. If it is successful, the driver list will show something like AB_DF1-1 DH+ Sta.0 COM1 RUNNING in your configured drivers list.