Optimum Limit and Homing Switches

Hi all,

Assuming Paraport inputs are not a problem (I have 3x parallel ports available, 15 input pins) what is the optimum number and layout of switches per axis? I will be using 3 of these inputs for spindle speed feedback (speed 1, and offset speed 2 for direction, plus keyphasor for angle refrence), 1 for e-stop, and 1 for a common stepper motor protection signal.

Am I right to believe:
2 x limit (one at either end, leaving enough room for overrun)
1 x homing (not so sure where this should be positioned)

Is there any benefit in keeping the two limits separate? Joining them will free up some of the inputs in case I ever decide on a 4th (rotary) axis.

Cheers, Scott.

If you have enough inputs I would recommend using separated limits and homing. The advantages are that you could signal the controller and inform the user what limit and direction was it tripped. This is normally obvious looking at the machine, but it could be a great help in large machines or in case of a cable issue.

The homing is generally between the limits (it can reach the homing before it trips the limit one). I believe - not fully sure - that you can also use one of the limits also as your home (when homing, the controller will assume that the limit is the homing switch).

(I have 3x parallel ports available, 15 input pins)

You have a lot more than that, setting a port as 'in' gives you pins 2-9 as input
eg
loadrt hal_parport cfg="0x378 out 0xc000 in 0xc800 in"

www.linuxcnc.org/docs/2.4/html/hal_parallel_port.html

regards

Thanks emcPT,

I thought about combining the two limits of each axis, as the machine will know which direction it was travelling in when the limit circuit was broken, and hence which switch it most likely touched. To add a layer of certainty, you could also tell the machine which areas of the travel the switches are in:

Not knowing HAL yet, so in English, for X axis:

-Annunciate X MinLim IF X travel direction is -ve AND tool in range 0% to 3% of X travel.

-Annunciate X MaxLim IF X travel direction is +ve AND tool in range 97% to 100% of X travel.

This will save 3 inputs on a 3 axis machine, and give pretty reliable data given the two factors. I'd be tempted to trip the machine without the factors, just on the circuit going open.. Just using this logic for operator annunciation.

Like you say though if there is a wiring fault, it's not 100% self diagnostic, but it's not that hard to find a fault in such a simple circuit, so I'm willing to sacrifice that convenience.

I'd leave the Homing switches separate, as it does seem to make sense.

Homing- where typically is the home position for a mill and lathe?

Cheers, Scott

ArcEye wrote:

(I have 3x parallel ports available, 15 input pins)

You have a lot more than that, setting a port as 'in' gives you pins 2-9 as input
eg
loadrt hal_parport cfg="0x378 out 0xc000 in 0xc800 in"

www.linuxcnc.org/docs/2.4/html/hal_parallel_port.html

regards

Well, that is good news! I didn't know you could swap them from Out to In... Fantastic!

Perhaps scrap my last post and I'll look into wiring individually..

Cheers, Scott

I've just read through the guide in the link you sent, a nice start to understanding HAL!

Cheers Scott