You could use the MiniDin if you have a PCMK card or a 1784-U2DHP USB to DH+ adapter, that would be the easiest.
Otherwise if the Serial port is configured for DH485 you could try to connect with a serial cable and set up a "PIC/AIC" driver in RSLinx (depending on your windows service pack and Linx rev, it may not work, dicey at best) or you could talk DH485 through that port with a 1747-UIC you would need a 25 pin to 9 pin adapter also with that.
Not sure if that port can be configured for Modbus or ASCII offhand on the PLC 5, if that was the case you would have to talk DH+.