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I've installed the GPIO python library to read/write to GPIO pins. To get the state of the pins I'm using "gpio readall" which displays a table with all the pins from the RPI and I can see which pin is HIGH and which one is LOW.

The problem i'm facing is that if I connect a ribbon cable to RPI (the other end of the cable is not connected) and run the "gpio readall" command multiple times one after another I see different values for the pins... the values are randomly changing... but if I remove the ribbon cable and run the same command multiple times I get all the time the same result...

I'm expecting to see the same values with or without the ribbon cable connected...

Do you have any idea why is that ?

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  • What connected to the other side? What are you measuring? Id there is nothing connected the cable it self pick up on electro magnetic differences and as suggested is "floating"
    – Piotr Kula
    May 16, 2013 at 21:39
  • 2
    congratulations, you've just invented Radio =)
    – lenik
    May 17, 2013 at 2:31

2 Answers 2

Reset to default
7

Pins cannot be left floating, the value will be indeterminate without at least SOME external hardware (pull--up or -down resistors, etc.). The ribbon cable may act as an antenna and pick up additional noise that may otherwise not be present, giving more variation.

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  • This. If there's nothing on the far end of the ribbon cable, you can't make assumptions about what the GPIOs will read from it.
    – Ian Howson
    May 17, 2013 at 7:23
1

It is actually possible to set up the RasPi input ports with/without pull up/pull down resistors. check the GPIO register descriptions available on the net.

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