make no assumptions
Be careful; the Provox "PCIU" unit that I expect you're connecting to has a long and complicated history of protocols and capabilities.
First, discard any thoughts of DH-485 protocol (the SLC-500 network). This is either Modbus or DF1 protocol.
Next, examine the 1785-KE's configuration and connection closely. Be certain it's a 1785-KE and not a 1771-KX1. Report on how many wires are connected and how. See if you can figure out the protocol configuration of the -KE.
If this is really a DF1 link, you could use the serial port on the controller with an appropriate signal converter (RS232 to RS485).
If you want to preserve the DF1 port for processor access, then a Prosoft card is the best choice. As mgvol indicated, the MVI56-DFCM is a DF1 protocol module for the ControlLogix chassis.