NSI Microplex is a proprietary control protocol made by the NSI company- now owned by Leviton lighting. Microplex is a digital protocol, like DMX, but it is limited in the number of channels it can control. It uses a 3 pin XLR cable to send the data to the dimmers.
Now, NSI Microplex is a decent system if you just need a few lights are aren’t planning to expand that in the future. Some of the simple consoles that NSI makes even run off of power that is shot down the XLR cable- meaning that you don’t need to plug in the console with a power brick! However, the one problem you may find is that once you buy into the NSI Microplex system, there is no getting out! Microplex will not communicate with DMX devices, and vice versa- so keep that in mind. You can buy a converter if you’re desperate, but they are costly and may take quite a bit of setup to get working properly.
So, if you’re looking into buying an NSI system, I probably wouldn’t suggest it due to it’s limitations- it doesn’t play well with others! However, if you’ve already got a Microplex system- all is not lost, and you can expand it and add DMX devices if you need to later!