4. Hardware setup
Although there is nothing complicated here, let's state it again: inverters are industrial machines built to deal with high voltages, so I recommend to carefully read the manuals of both the inverter and the piggyback module before opening the inverter. This is not the usual RTFM bullshit. This is serious.
As you have read the manuals, I won't repeat here how to perform the installation and wiring of the module ;-).
The only thing I would add is a mechanical consideration: To avoid that the serial cable be pulled violently if someone trips over it for example, I strongly advise that you secure it with a cable tie around the cable and a plastic washer (i cut one in a Pepsi bottle cap :-)), like this :
If you didn't select a wire-end converter, you'll have to solder the other end of the cable to a female DB-9 connector. Only Rx, Tx and Gnd have to be soldered to pins 2, 3 and 5.