Hevert-

There are usually two separate systems that can stop or pause the machine.

The first one is the E-Stop circuit. Generally, this is a contactor which is held in by an unbroken loop of normally closed switches. The switches include the various E-Stop buttons, and sometimes a safety interlock on the electric box. The E-Stop is designed so that it always definitely cuts power and stops the motion of the machine. This is an uncontrolled stop - the position of the machine is unknown after this event. Using a loop of normally closed switches ensures that if any one switch opens, the stop signal occurs. Generally the computer is notified after this event occurs by some form of fault input, but is not involved in the shutdown itself.

The second system is just an input pin (or pins) to the controlling computer. This can include proximity sensors or limit switches, and pause buttons. The software on the computer reads this input, and takes the appropriate action - usually bringing the motion to a controlled pause.

A proximity sensor or limit switch at the end of an axis can also be used for as a home switch. These are often wired in the second system as an input to the computer, which then uses them for both home and limit.

Because the computer can know which axis is in motion, it is possible to wire more than one sensor or switch to an input and still have a combined homing and limit sensor system.

So, hitting a limit switch does not remove all power - it tells the computer to execute a stop. If you haven't hit a hard limit or other problem, then you can potentially resume.

Hitting the E-stop does remove all power, and you cannot resume.

The discussion of relays for the proximity sensors or limit switches is about how best to interface multiple sensors to the computer's input pins. The sensors and switches work better at higher voltages (12v or 24v) than the computer's inputs (5v or 3.3v), and there are several different approaches to combining switches, adjusting the voltages, and avoiding overloading the computer's inputs.

I got pissed off with the limited I/O points & low field voltage on parallel port.
Now playing with MESA 5i25 FPGA card with a 7i76 daughter board. Apart from all the required points for 5 axis stepper, VFD pwm points, it still has 12 output & 36 input with a choice of field voltage of 5 to 32V.

So you are saying stop procrastinating and trying to understand this stuff and just get on with it?

Sounds good to me.

Hey, Darren Sayler was just a lower level finisher and managed to build one.

Go easy on me Pete.....I think proxys are a good idea. Order now so that stuff is in when you need it. Get a laser crosshair ordered too. Mine took 6 weeks from China.