multiple solid state home/limit switches on one input

[will gilmore]

I'm building a small lathe (http://cnczone.com/forums/showthread.php?t=92337) and I'm now trying to figure out home/limit switches. Lathe will have Mach3, steppers and most likely G540 or less likely G203v drivers.

I'd like to admit upfront that I'm not very knowledgeable about electronics. Please feel free to correct or advise in anyway, I won't be offended.

Repeatability and reliability are my main concerns. I plan on having one home switch for each axis and using software limits. For this reason I would like my home switch to act as normally closed.

Solid state seems to offer the best repeatability. I'm considering the following for home switches.

Optical: digikey 365-1234-ND http://search.digikey.com/scripts/dk...me=365-1234-ND
Hall effect: digikey 480-1993-ND http://search.digikey.com/scripts/dk...me=480-1993-ND

If I understand their function both switches normally (slot unblocked) conduct from the output wire to the ground wire. If the slot is blocked the circuit opens. This functions the same way as a NC mechanical switch.

My question is how do I connect two (or more) of these switches to one input pin?

[BobWarfield]

Diodes, my friend, that's future! (Appologies to The Graduate which referred to Plastics).

See here: http://www.cnccookbook.com/CCMillCNCHomeSwitches.htm

Crude diagram at top of page for wiring together some IH optical switches. There's probably better ways than the diodes, but they are cheap and cheerful.

Also, you don't want to wire all the homes together. Mach3 wants to see each axis on its own. Wire the limits together if you like.

Best,

BW

[will gilmore]

The way I read that diagram is that pin 2 (sig) is held low by the resistor. When a switch is activated it sends a high voltage which brings sig to high. The diode allows the high voltage to get to pin 2 but does not allow it to travel back to the other switch because the output of the non activated switch would be low. If I'm reading this correctly (BIG if) the switches are acting as NO and if there was a failure you wouldn't know until your axis over traveled.

I thought it was common practice to have all home switches on one pin? I figured mach would know which switch it hit.

[BobWarfield]

Will, the Mach3 Mill manual has a good discussion of how to wire the homes and limits and of how Mach3 treats them. You have a lot of good choices.

Let's start with Limits. It's ok to wire them all together, because a limit is a limit and you just want the machine to stop if any limit is hit.

This is not true of a Home switch. Home is all about the machine finding it's reference coordinates in a highly accurate and repeatable way. You walk up to the machine, boot it up, and "Ref" the axes. Now Mach3 knows exactly where it is on the DRO.

Let's say you crash, have a power failure, or otherwise cause your machine to get "lost". Your part is still in the vise and you want to continue. Without home switches, the best you can do is use an edgefinder or similar tool to find a known location on the part. With Home switches, you REF the axes and the machine knows where it is again.

So what happens if we wire all the Home switches together? Well, it means we have to REF each axis independently, because if we move them all at once, we have no idea which one triggered the Home. This is perfectly ok, but it just means REF'ing requires 3 operations if you wire things up that way. If you provide Mach with 3 inputs, one per axis, you can use a "RefAll" function and one touch of a button will reference all three axes automagically.

That's pretty cool!

OK, now here is another thing to note about my Home switches. They are combined Home/Limit, and that is why those particular diodes are shown that way. I've not yet decided which configuration I want, and I may even leave out the diode approach so I can do simultaneous Home. In that case, each of my axes has 2 switches. One is a limit for one end, and one is a combined home/limit for the other end.

Lots to think about. Grab the Mach3 reference and ponder it. Let us know if you have more questions.

Best,

BW

[will gilmore]

Bob, you are a true asset to the DIY CNC community. I've spent hours reading and learning from your website. Thanks for taking the time to help.

This is going on a lathe so only 2 axes to home. Also, I'm planning on having gang tooling so I would want the Z to home first anyway. I wasn't planning on using limit switches. I'm planning on using a G540 so only 4 inputs. One for spindle speed sensor. If I use two separate inputs for homes I only have one left.

I'll go back to read all the Mach3 and Gecko docs again tonight. If I was using 2 NC mechanical home switches I would wire them in series and home one axis at a time. I'm new to solid state electronics and trying to figure out how to make the solid state switches (transistors?) act like NC mechanical switches in series.