How to connect 2 raspberry pi by their GPIO Pins?

[aywizard]

I tried to connect two raspberry pi by their GPIO pins, using one as input, another one as output. This is how I connected them:

I simply connect two Raspberry Pi's GPIO Pin 9 with a 10K resistor.

I am using python to test the connection, the problem is the input side can not get the correct input, does anyone know why?

The python code for input side(on python console):

Code: Select all

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(9, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.input(9) 

The python code for output side(on python console):

Code: Select all

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(9, GPIO.OUT, initial=GPIO.LOW)
GPIO.output(9, GPIO.HIGH) 

The problem is when I set the pull_up_down parameter in input side to GPIO.PUD_DOWN, no matter how I change the output pin, the input always get 0. If I change the pull_up_down parameter to GPIO.PUD_OFF, the input is floating. It seems the input GPIO pin can not get any input from the output pin side, is there anything wrong with my connecting or my code?

Thank you!

[ghp]

Hello,
you need also to connect the ground lines.
Regards,
Gerhard

[aywizard]

Hello,
you need also to connect the ground lines.
Regards,
Gerhard

Wow, that works!
Would you mind explaining to me why their ground lines need to be connected? I think all ground pins are 0V, why we need to connect them?

Thank you very much!

[rpdom]

Would you mind explaining to me why their ground lines need to be connected? I think all ground pins are 0V, why we need to connect them?

0V on one circuit needs to be connected to 0V on the other circuit to make sure they are both measuring from the same point.

[GerardWassink]

For the receiving RPi to be able to 'see' a signal, there must be a certain voltage on it's input. Every voltage, also in this case, is measured between two points. In this case one must be the 'Common Ground' or GND. That's why they must be connected.

HTH,

Gerard

[DougieLawson]

Would you mind explaining to me why their ground lines need to be connected? I think all ground pins are 0V, why we need to connect them?

Because electrons flow in circles. If you have electrons going out on one wire they need another wire to come back.

[aywizard]

I think I understand it now. Thanks a lot for all of you!

[mikerr]

See also connecting them via the serial pins,
before writing python-serial code:
viewtopic.php?f=44&t=51633

[boyoh]

I tried to connect two raspberry pi by their GPIO pins, using one as input, another one as output. This is how I connected them:

I simply connect two Raspberry Pi's GPIO Pin 9 with a 10K resistor.

I am using python to test the connection, the problem is the input side can not get the correct input, does anyone know why?

The python code for input side(on python console):

Code: Select all

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(9, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.input(9) 

The python code for output side(on python console):

Code: Select all

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(9, GPIO.OUT, initial=GPIO.LOW)
GPIO.output(9, GPIO.HIGH) 

The problem is when I set the pull_up_down parameter in input side to GPIO.PUD_DOWN, no matter how I change the output pin, the input always get 0. If I change the pull_up_down parameter to GPIO.PUD_OFF, the input is floating. It seems the input GPIO pin can not get any input from the output pin side, is there anything wrong with my connecting or my code?

Thank you!

Your reason for connecting two Pi's
Is it for a project,t or for experimenting
Yes you have to common the 0-of each
Pi's, If it is for a project, I suggest you
you use Opto Isolators, you will not need
to common the 0- There will be no physical
connection between Pi's
The other alternative would to use one
power supply for both Pi's.
I would like to know what is you reason
for inter connecting two Pi's

[baksteen-13]

do you need a 10K resistor or can it be less?
i have a 3k3 one. will that work?

[mahjongg]

No it doesn't matter the resistor is there purely for safety in case one PI decides to program the GPIO as an output.
When programmed as input the input resistance is very very high, so using 1K 10K or even 100K doesn't matter much.

[Prajwal MR]

One thing is worrying me soo much here ! During boot up, some gpio's will go high(pulled up). Now what if both the pins on each raspberry pi (input pin and output pin) that are connected, go high during bootup, will it no lead to an exceeding voltage and damage the pi ?