Endstop connections on RAMPS 1.4 board
Just bought a used Prusa i3 printer with a RAMPS 1.4 board (that might be redundant, not entirely sure). It checks out OK, although I need to do some cleanup and calibration.
Looking at the endstop connections on the board, there seem to be seven possible connection positions. Let's number them 1 though 7, with 1 nearest the power/heater connections side of the board.
There seem to be two positions for each axis if I'm understanding: X X' Y Y' Z Z' and ?.
The endstops, which DO work, are connected at locations 2, 3 and 5, which I believe are X', Y and Z.
Question: What's the difference between X and X' (or Y or Z for that matter)?
Question: What's the 7th position for?
Thanks.