it can't be done ... please don't even try it - you can damage things ...
there's a LOT more to it than just the connection at the end of the cable ...
that little gray box near the end of the cable has special electronics in it ... the box on the 1784-PCM6 cable is specifically for DH+ (Data Highway Plus) ... it sounds like you need the 1784-PCM4 cable ... its little gray box is designed for DH-485 (Data Highway 485) ...
I'm assuming here that you already have the 1784-PCMK "credit card" to fit into a slot on your laptop ...
tip: if you buy a cable, be sure to get the proper "A" series or "B" series to match the card you already have ... the two series are different sizes - and they will NOT interchange ...
PS edit ... and welcome to the forum ...