FP16: Some common terminology
If your background is not in the electronics field, we may confuse you a bit with the way we refer to power supply pins, grounds, "common" terminals etc. In electronics we generally quote all voltages in relation to an assumed common or reference terminal. That common terminal is often called "ground", "gnd" or "circuit ground". It may or may not actually be tied to a physical ground point, for instance mains power supply earth.
In SPLat boards we usually label the "ground" terminal "0V". On boards that are powered off an external DC power supply (which is most off-the-shelf boards), 0V is the terminal connected to the negative terminal of the power supply. Measured relative to that pin, therefore, the positive power supply pin will be 24V (or 12V or whatever the supply voltage is) positive. We then say "we have a +12V supply". All boards contain a 5V regulator chip to feed the microprocessor and/or other logic chips. Its output voltage is said to be +5V, again measured relative to the 0V terminal.
Similarly, all input and output voltages are also measured relative to the 0V terminal, and will in virtually all cases be positive voltages. For example, on most SPLat models the input voltage threshold for digital inputs, and analog input and output voltages, are measured relative to 0V.