Go to the online help from inside Citect. Select Driver Help. Select Contents, open the "book" titled I/O Devices then select Allen Bradley, then SLC500 PLC's. I believe the 503 CPU had a DH485 port and a RS232 port. You can use either RSLInx or serial but be careful. If you chooes the serial method you will need to make changes to congifuration of the serial port, these changes will prevent RSLinx ( and therfore your programing software) from access that port in the future. Although it cost a little more I would suggest that you use the RSLinx method. You will need to buy thecorrect version of RSLinx and that will allow both Citect and Programming software to share the com port.