# Limit Switch Woes



## TomS (Apr 2, 2019)

I have a CNC lathe that was running well until yesterday.  Not convinced it's related but I installed one of those inexpensive eBay tachometers and for some unknown reason my limit switch circuit is dead.  Mach3 does not recognize an open switch or the E-stop (E-stop switch is wired in series).  The lights on the Diagnostic Screen do not come on when the switches are tripped.  I've checked the limit switch wiring circuit and I have continuity up through the BoB inputs.  Pressing the E-stop has no effect even though it breaks the circuit.  I've changed input pin numbers, without changing any wiring, and nothing.  In desperation I swapped the input and Com wires, still nothing.  

If I understand the LS circuitry I should have about 3vdc at the limit switches in normal run mode (switches are wired NC).  Tripping a switch the circuit voltage drops to 0vdc and activates the E-stop circuit.  Testing with my voltmeter and in normal run mode I have 0vdc at the switches and when tripped I have 3vdc at the switches.  The switches are acting like they are wired NO.  I'm thinking that my BoB is faulty but not thoroughly convinced because I've wired the LS input to other input pins that are working.  I'm stumped here.  Any suggestions on what I should test next?  Or buy a BoB?

BTW - the tachometer works great.


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## markba633csi (Apr 2, 2019)

Does powering down the tach have any effect on the problem?  Rule that out first


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## TomS (Apr 2, 2019)

markba633csi said:


> Does powering down the tach have any effect on the problem?  Rule that out first



We're thinking along the same lines.  I removed the tach feed into the BoB.  No change; limit switches still not recognized.

BTW - I mis-spoke when I said other inputs were working.  The tach input wasn't working either.  The tach itself shows RPM but Mach does not.  It appears that all of the inputs are dead except voltage is flowing in the LS circuit.  New BoB?


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## markba633csi (Apr 2, 2019)

I'm thinking maybe the board got hit with a static discharge.  Unless you have a scope and can go down to component level I think replacing it is your only option
You sure the BoB has power?  Sounds almost like a regulator is dead


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## TomS (Apr 2, 2019)

markba633csi said:


> I'm thinking maybe the board got hit with a static discharge.  Unless you have a scope and can go down to component level I think replacing it is your only option
> You sure the BoB has power?  Sounds almost like a regulator is dead



Thanks.  That's what I was thinking.  A replacement board is fairly inexpensive, thank god.


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## spumco (Apr 5, 2019)

If you haven't already bought a new BOB, try hooking the LS chain to a different input pin and seeing if Mach sees them again.  If so, I'd say the original pin has a fried optocoupler.

That assuming, of course, that you have a spare input pin.


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## TomS (Apr 6, 2019)

spumco said:


> If you haven't already bought a new BOB, try hooking the LS chain to a different input pin and seeing if Mach sees them again.  If so, I'd say the original pin has a fried optocoupler.
> 
> That assuming, of course, that you have a spare input pin.



I tried each of the input pins with the same result.  All of the inputs are "dead".  My tach is also connected to one of the inputs for a spindle index signal.  That isn't working either.  I ordered a replacement BoB that should arrive today. 

I'm using an old BoB and UC100 that I bought for my CNC mill a few years ago.  It worked when I first started up my lathe then the limit switch circuit quit.  I found this out when I homed my lathe and it ran through the home switch into the hard stop.  No mechanical damage thankfully.


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## spumco (Apr 6, 2019)

Well, if every input is dead I'd say you're in the market for a new BOB...

I know you just ordered a new BOB, but If you like UCCNC they've just come out with a new motion controller + BOB combo board with high-speed isolated inputs.  Looks pretty nice.

I feel your electrical pain, BTW.  I'm in the beginning stages of switching from a UC400ETH + PMDX126 to a UC300ETH + UB1/UD1 setup.

-Ralph


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## TomS (Apr 6, 2019)

Thanks for the tip.  I thought about upgrading but I didn't want to go through a complete rewire of the control cabinet at this time.  And the replacement Bob was only $20.


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## spumco (Apr 7, 2019)

You're going to have to rewire it when you build a 20 station live tooling turret and sub-spindle, why not just bite the bullet now?

Do it for the children.


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## TomS (Apr 7, 2019)

Too funny!  I'm still learning how to touch off my cutting tools so a turret and sub-spindle are way over my head. LOL


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## TomS (Apr 9, 2019)

I installed the new BoB and I'm up and running, except!  The homing sequence doesn't work.  It worked before with the old board and the replacement board is identical.  When I start the homing sequence the axis moves to the switch and trips.  I go to the Diagnostics screen and select OverRide Limits but Mach will not reset and allow me to back off the limit switch.

I checked my configuration and it is correct.  I can jog the X and Y axis and the spindle turns.  Short of reloading Mach any suggestions?


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## JimDawson (Apr 9, 2019)

Press <ctrl> and jog maybe?  I think that overrides the limits.


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## TomS (Apr 9, 2019)

JimDawson said:


> Press <ctrl> and jog maybe?  I think that overrides the limits.



Thanks but that didn't work.  Couple of other things that I didn't mention in my post above.  When I click on OverRide Limits then press the Reset button the message "Limit Switch Tripped" goes away.  The other thing is the Estop indicator light is coming on when the limits are tripped even though it is not enabled.  It won't clear unless I shut down Mach and restart.  Maybe I need to enable it in order to clear it.  

Another question: Can I use any Com terminal on the BoB in lieu of the Com terminal on the Input block?  I'm asking because the Input terminal block has five connection points but potentially I can have five inputs (pins 10,11,12,13 and 15).  My old board had the same number of terminals on the Input block and the last one was marked "Com".  This new board is not marked.


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## TomS (Apr 9, 2019)

Fixed!  Evidently Mach3 activates the Estop circuit when a limit is tripped even though the Estop is not configured in Ports and Pins setup.  Because I didn't have my Estop configured I couldn't reset and therefore over riding the limits doesn't work.  I configured my Estop and I can now home the machine.

Now on to sorting out the eratic tachometer output.  It never ends!


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