TL;DR* Here's what the license entails:
1. Anyone can copy, modify and distribute this software.
2. You have to include the license and copyright notice with each and every distribution.
3. You can use this software privately.
4. You can use this software for commercial purposes.
5. If you dare build your business solely from this code, you risk open-sourcing the whole code base.
6. If you modify it, you have to indicate changes made to the code.
7. Any modifications of this code base MUST be distributed with the same license, GPLv3.
8. This software is provided without warranty.
9. The software author or license can not be held liable for any damages inflicted by the software.
More information on about the LICENSE can be found here
Zaphod77,
I really appreciate you taking the time to address my concerns. The software community really can be a wonderful help to others.
Regarding your latest comment . . .
They would definitely be able to modify MY version of the Open Source firmware and be able to install it on a device (in my case it would be any 3D printer that employs the Marlin Open Source firmware). But if they wanted their installation of my version to make use of the additional capabilities I have added to it, which is sending commands to the attached extra microprocessor and having that microprocessor control certain pieces of new hardware that I intend to have on MY version of a 3D printer, they would have to, on their own, attach a microprocessor and the additional hardware as well as WRITE the code for that attached microprocessor.
It's that additional code which runs only on that separate microprocessor and makes no use of any Open Source software that I hope I can keep from being Open Source. Others may well be able to figure out what that code does, how it does it, and write their own, but I don't see that there's any requirement that I simply give it away.
I'm also expecting there to be a very unique piece of hardware as part of my design and which also would be controlled by that software residing on the additional microprocessor. That hardware is unique enough that I would be pursuing a patent on it. That would not prohibit anyone from figuring out what that (potentially patented) hardware does and figuring out how to have something else do what it does and how to have the software (that they would write) control that hardware. But if they wanted to incorporate MY unique hardware they may well need to license it from me or purchase it from me.
But since that hardware does not really preclude anyone from building a 3D printer that does everything MY new 3D printer will do, I would think there would be nothing that violates the Open Source principles.
Yes? No?