Electronics > Beginners

Rules to Establish Pull Up/Pull Down

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Larsson55:
I have a BOB (breakout board) for a cnc machine I’m building with a number of input /output pins. On the outputs I have to set the COM jumper to either 5VDC or GND and on the input pins I have to set the jumpers to either pull up or pull down.

I know the meaning of the pull up/dn and COM but how do I find out and understand what I need? Lets say I have a limit switch which is normally closed and when it opens it will stop the cnc. Are any rules here to tell me how to set my jumpers and why?

I like to educate myself and understand why I need to set a jumper to a certain position

Thank you

Zero999:
It depends on whether the switch connects between the input and 0V or +V.

Larsson55:
Let me see if I can get this correct. So if the switch connects between input and +V I will set the dip switch to pull down otherwise if the switch connects between input and 0V the dip will be set to pull up.

ledtester:

--- Quote from: Larsson55 on June 27, 2022, 11:50:47 pm ---Let me see if I can get this correct. So if the switch connects between input and +V I will set the dip switch to pull down otherwise if the switch connects between input and 0V the dip will be set to pull up.

--- End quote ---

Basically that's it. For instance, see this diagram (from https://circuitdigest.com/tutorial/pull-up-and-pull-down-resistor ):

[attach=1]

The resistor determines what the input will be when the switch is open.

Btw, if you select the wrong kind of resistor what will happen is that your switch won't change the input when pressed.

Zero999:
Two other points:

The power supply connected to the switch and pull-up/down must share a common 0V/ground with the logic circuit, which is probably a microcontroller or PLC inside our machine.

The pull-up/down resistor must pass a minimum wetting current to ensure the switch's contacts reliably close and avoid noise. In this instance it shouldn't matter as DIP switches are designed to work with low currents. 10k is probably a reasonable value which will give 0.5mA.

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