The 1761-L31 was one of the very smallest CompactLogix controllers in the first generation of that product. While it can run DH485 protocol on the serial port (with a signal converter, the 1761-NET-AIC), if you aren't familiar with CompactLogix I don't recommend getting it involved.
The Red Lion DataStationPlus has a reliable and mature DH485 driver, and
a specific cable. If this were my system, I would get a DSPSX with a Profibus DP slave expansion module (XCPBDP) and use that to implement the interface.
I prefer the DSPSX to the slightly-cheaper DSPLE because the embedded HMI web interface is such a useful diagnostic tool.