I've received enough email questions from new laser folks that I think a tutorial on this topic would be helpful.
Let's look at what a "Connection" really is with BCL.
BCL is no different from any other app that tries to connect to a COM Port. If the 'settings' are correct, a connection can be made. Settings refers to baud rate, parity, stop bit, data bits and handshake parameters.
In human terms, these settings are like a key. If you're settings are correct, you can connect to a COM Port. If your key is correct, it will unlock a door to a hotel room (you'll see a minute why I chose to relate this key to a hotel with many rooms).
So, if you can connect to a COM Port with BCL, what does that mean? It simply means you've connected to a COM Port and that's all. Does it mean you've connected to your laser machine? No. Your laser machine may or may not be connected to that COM Port.
When BCL starts up, it fetches all the available COM Ports from Windows and populates these in the COM Port dropdown list. You may see more than one com port in this list. If you do, it simply means Windows has detected multiple items that are connected (hence available) to various COM Ports. Your laser machine is one of them. A USB drive may be another. Something else may be connected to a third COM Port.
But which one did you connect to with BCL?
Let's go back to our hotel key. Like COM Ports on your PC, you can have many and like a hotel, there are many doors to many rooms. So now with your key, you've unlocked a door to a hotel room, and you step inside the room. But who is in the room (which device have you connected to?). You won't know until you say "Hello, who's in this room?". If it's your laser machine, it will say back to BCL "Hello it's me, your laser machine and to prove it, here is my Grbl welcome message: Grbl 0.9i ['$' for help]". Ahhh so now you know you are connected to your laser machine.
Let's say by mistake, you've connected to the wrong COM Port. What will happen? What will you see in BCL? We don't know because we don't know which device you've connected to and we don't know how that device was programmed to respond, if at all, to a successful connection. More than likely you won't see anything in the BCL Receive window as far as a message from the device you've connected to.
So, the lesson here is this: when you try to connect to your laser machine, you MUST verify that you get the Grbl "welcome message" (Grbl 0.9i ['$' for help]) so that you know for sure that you're connected to your laser machine.
Another important reason to always read this welcome message is to make sure you have the correct Grbl firmware installed (flashed) on your Nano for BCL (0.9i or 0.9j). You might be switching back and forth with using different software and have forgotten to flash back to the correct Grbl for BCL use!
If you do not get this welcome message, do NOT try and use BCL as it will not work and any behavior (right or wrong) will not mean anything. If you don't get that welcome message, you've got to figure out why and work on getting it before you can start to use BCL.