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GIT is one of a group of publicly accessible repositories for software, firmware, and documentation which implement 'versioning'. Versioning keeps track of changes to files and let's one follow those changes, revert to previous versions, and create branches so that varieties of the code can be made, and also merge branches back into the main branch.
Wiki entry: https://en.wikipedia.org/wiki/Git_%28software%29
SourceForge is another such web based versioning repository and should be familiar to MERG members as the home of JMRI: http://jmri.sourceforge.net,
The most popular web based Git repository is currently GitHub https://github.com; although there are plenty of other web, intranet and local based Git applications to choose from.
Comparison between SourceForge vs GitHub: http://recomparison.com/comparisons/101554/sourceforge-vs-github/
To help understand how Git may be helpful to MERG here is a beginners article on GitHub:
General-purpose input/output (GPIO) is a generic pin on an integrated circuit or computer board whose behavior — including whether it is an input or output pin — is controllable by the user at run time. GPIO pins have no predefined purpose, and go unused by default.
This is a company that has written a Microsoft Basic program to control a model railway from a computer.
Is a common point in circuits normally 0 volts, but can be higher than 0 volt.