View Full Version : Newbie connecting limit switches

03-25-2008, 07:59 AM
when trying to figure out how to connect the limit switches I came out with a big concern: the pinout diagram shows just 'IN' pins for each switch, there is no 'commom out' so... from where should come the + current to close the contact?
Do I need a external +5V power supply for this? Or should the supply voltage for the switches come from somewhere in the controller board?


The Blight
03-25-2008, 12:28 PM
What breakout board are you using? Most boards have a +5V supply.

03-25-2008, 12:39 PM
Use pin 10 and take it back to common, use normally closed switches and hook in series.
Set TCNC to active high, if a switch opens pin 10 to common is lost, stopping the machine.

The Blight
03-25-2008, 03:19 PM
Does pin 10 have a pull-up?

03-25-2008, 03:33 PM
thanks Blight, thanks Chester
I've looked carefully and the board really have a '5VLOG' output ... :o

Now my switch are simple open/close with no built-in resistor. I know the parallel port can safely handle 20mA input signals, much more current can burn the things out. So do I need to use some resistor in serial with the limit switch to limit the current of the input signal?

The Blight
03-25-2008, 06:34 PM
There is a better way to make this. Use a pull-up resistor between the +V and the input (about 10k), and mount the switch between the input and gnd (N/O). This way when the switch closes, the pin will be pulled to gnd (low). When the switch is open, it will be pulled high.

Take a look at this link: http://www.seattlerobotics.org/encoder/mar97/basics.html

If you have a PMDX-122, all inputs have weak pull-ups and only needs to be connected to gnd.

03-25-2008, 07:00 PM
I have a cnc4pc break out board, so I'm not sure if there is a resistor involved or not. Sorry I couldn't be more help. Good luck with your project.

03-26-2008, 06:52 AM
great, I will wire things that way, thanks!

One last question: can the limit and home switches be the same? TCNC can handle it?

03-26-2008, 07:41 AM
Yes, it will work for "HomeAsLimit" when you set the function. HOWEVER, do one axis at a time and get it to working before you move on to the next one. Also, you will not set a limit switch for that position where you have the home switch. The software will treat the switch appropriately (home or limit) as needed.