You cannot communicate directly with the 5/15 from a serial port. The 5/15 port is DH+, pin 1=L2(Blue), pin 5=L1(White), pin 7=Shield. You'd need a 1784KT board for a desktop computer and a 1784CP cable. You may be able to see it on a DH+ network, I've never done this so I won't say it's possible. But if you don't have the other stuff, it's worth a try.