After several maddening errors it occurs to me that there is a problem in my GPIO setup, to wit, RPi.GPIO is failing silently.

To test this theory I got my multimeter, set it to volts, put one probe in ground and another probe in the GPIO port (GPIO 17 in this case) and get 0 volts coming through.

Ordinarily I would think there is a loose conection, or a problem with the board (I am using the Adafruit T-Cobbler http://www.adafruit.com/products/1105 ) but I am able to move a servo with this command (using servoblaster) (which does not use the RPi.GPIO library)

echo p1-22=50 > /dev/servoblaster

My question is - am I testing the GPIO correctly? Is there a better way to test if I am getting any current through the GPIO setup?

The code I am using (Python) is as follows

import RPi.GPIO as GPIO
import time


dirPin = 4
stepPin = 17

GPIO.setup(dirPin, GPIO.OUT)
GPIO.setup(stepPin, GPIO.OUT)

for i in range(0, 3):
   GPIO.output(dirPin, 1)
   GPIO.output(stepPin, 1)
   GPIO.output(dirPin, 1)
   GPIO.output(stepPin, 0)   
  • Without any code, there's not a lot we could say, other than "it should work".
    – Gerben
    Dec 8, 2013 at 12:21
  • Whoops! My bad, I added the code Dec 8, 2013 at 13:25
  • Just to make sure. BCM GPIO 4 and 17, means physical pin 7 and 11. Can't see anything wrong with the code. Maybe try GPIO.setmode(GPIO.BOARD) (and change pinnumbers to 7 & 11 of course). Pretty weird. Sorry I can't help.
    – Gerben
    Dec 9, 2013 at 13:45

1 Answer 1


One method of testing this would be to connect an LED to the pin and see if you can flash it. I use an LED bar graph like this one from adafruit.

LED bar graph

I mounted this and the corresponding resistors to a piece of stipboard, and added a set of male and female headers to each end (by using both male and female headers on both ends, I can hook it up easily no matter what type of pins the Pi or microcontroller I am testing has). You could do this with plain LEDs as well, but I would suggest at least 8. This allows testing a full port (microntrollers group pins into ports) at once.

One thing I did notice in your code is the long sleep 15000 time:


Was this so you could test the voltage? Because currently you are sleeping for more than 4hrs.

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