On the SLC-5/03, the top port is for a DH-485 network. It's an "RJ45" style connector that looks like Ethernet, but it is NOT ETHERNET.
It has a +24V DC pin on it and if you plug an Ethernet cable and Ethernet switch (or your laptop's Ethernet port) into it, you will smoke the connected Ethernet port.
All this stuff is prominent in the user manuals and is pretty much second nature to A-B users, but since you're coming to this from a novice standpoint I wanted to mention the first two most common errors.