Hello,
There are three types of axis "switches" used in cnc; soft limits, hard limits, and home switches.
Hard limits are the widest set apart, do NOT connect to the software--except to let it know, "Houston, we have a
PROBLEM!" In other words, these are electrically wired to shut things down safely regardless of what the computer or software might "want"... (Note that shutting down safelymay be MORE than just dropping the power immediately. Think of what will happen if a cutter is engaged in the cut when you power down the only thing--the drives--holding it in position. Climb cutting anyone!?!?
Inside of the hard limits are two more switches called soft limits. These are what has been described in the answers so far... Think of them like the "warning track" in a Baseball outfield. They ARE wired into the computer, and generally stop motion from CONTINUING in the same direction. Most better controls WILL allow motion in the opposite direction, which is one answer to your question. Some controls require you to acknowledge the soft limit alarm/fault before allowing you to move off the switch.
Still others will have a toggle in the software which does the same thing.
Soft limits are also capable of being put into the computer control itself--without the REAL switches on the machine. This is becoming more common, because it is obviously less expensive.
It also better allows the computer to do one of the important things which distinguish soft limits from hard limits. Which is to decellerate and MAINTAIN position. (Hard limits just stop motion immediately, and you have to reset and re-home to get back to work.) When soft limits are part of the software in the control they are usually called stroke limits, or stored stroke limits.
And they require the third type of switch, which is the home switch, or switches. This is a single switch per axis, which is very accurate. On more expensive machines, the home switch is electrically "and-d" with the encoder index pulse for precise repeatability. ("And"ing means BOTH the index AND the switch have to be on for the circuit to register as truly actuated. It is a common digital boolean function.) The home switch "location" is used by the computer to determine "where" the soft limits--stored stroke limits-- belong.
So in an ideal machine, there will be 5 switches PER axis! (Remember that 3 are more common nowadays, due to the use of stroke limits for the soft limit function.) Even less expensive machines will use mechnical stops for the hard limits--you can get away with this ploy with steppers, b ut with servos you should have REAL hard limit switches AND mechanical stops!
Some machines will use one of the soft limit switches for BOTH the soft limit--at the home end-- and as a home switch. These machines will often have two computer imputs for limits; limit hi and limit lo. And even less expensive technique is to use 2 switches in one NORMALLY-CLOSED "loop", so only one computer imput is needed. This is very typical of many of the machines shown here in "the Zone"...
You DO want your limit switches to be NC--Normally closed. That way, if a wire breaks, it will be seen as a switch opening, and stop the machine. This also prevents themachine from starting when a wire has broken between uses of the machine. Switches CAN ALSO fail in the closed shorted mode, but this is less common, and most designs DO use the NC mode...
One consideration to give your limits is the ability to allow for over-travel. Especially the soft limits. Because the axis might be moving quickly when the switch is sensed and it will take a bit of distance to slow, then stop the axis. Try not to set up your switches so the machine can get "behind" them, however. This can lead to all kinds of "fun" things!
Hope this helps,
Ballendo
P.S. "Types" may not be the best word. There are many kinds of switches, and "types" here means their USE, not their construction or actuating methodology...