Although EIA485 isn't common in day to day PC networking, it is common in industrial telemetry as it can work over a four wire bus and transmit up to 1.2km without repeaters. RS232 is only good for a few meters and even Ethernet runs out of steam at 100m. It's a bit of a pfaff when you only want to connect two things together by a 30cm bit of string though...
The instructions suggest using Cat 5 Ethernet cable for the 4 wire bus. Two are used for +12V and Ground power lines and the other two are the A and B serial data lines. They suggested using Ethernet cables because they have twisted pairs to eliminate interference. But I ignored that and just used 4 core flat telephone wire as it was such a short cable I was making. They also say that you're supposed to terminate the A and B wires at each end of the bus with a 100 Ohm resistor between them. I sort of did this by putting a 100 Ohm resistor at one end (inside the Tristar wiring box where it would be safe from being knocked). I didn't have two 100 Ohm resistors in my spares box so I didn't bother with the one at the other end... Seems to work ok without it. I've been receiving data from the SunSaver without problem so again it's probably only important for long wire runs.
With the SunSaver connected to the TriStar, you can use the MSView software to talk to any device on the EIA485 bus (up to 128 devices) using IP. The TriStar can be set to bridge the Ethernet and EIA485 networks (it just forwards the MODBUS packets to the devices on the EIA485 network). It's especially handy because with the exception of the TriStar MPPT-60, no other Morningstar products are IP enabled but now they can be. It opens up the possibility of using cheap and commonly available networking products to move the data around. For instance, I don't have Ethernet cable run around the house. I have a wi-fi transmitter and the TriStar is actually connected to the computer upstairs via an Ethernet switch and a pair of Netgear Ethernet over AC power adapters. This was more reliable than the wi-fi (which has a dead spot in that part of the house) and the CCTV data also travels over that switch and link.
So now my laptop upstairs can fetch performance data from the Tristar controller and also directly from the SunSaver controller. The two controllers share the same IP address on the Ethernet but have different MODBUS IDs so each controller responds only to it's own commands.
You have to run the custom settings wizard for each controller to change the default MODBUS ID (all Morningstar devices are preset to "1"). So I changed the SunSaver controller to be ID "2" and left the TriStar on its default of "1".
The EIA485 network needs external power of 12V DC so I connected the two power pins on the EIA485 connector plug to the variable lab power supply that I use to run the AA battery charger and mobile phone chargers. This converts the battery 24V to the needed 12V, although the EIA485 adapter isn't fussy and will work on any voltage between 8 and 16V, so you could just connect it to a 12V solar battery directly. But I'm running a 24V system so I have to use a DC-DC converter. The pair of adapters (RS232 and EIA485) together only consume about 20mA (or 0.5W power) so I can leave them running 24 hours (unlike the Toshiba laptop that consumed about 11W.
