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I have an Arduino Uno (R3) monitoring a magnetometer connected via USB to a PI (B) running latest Raspian. My Pi Python program scans the USB every second reading an integer value via ser = serial.Serial('/dev/ttyACM0', 57600).

This works fine when run from an active terminal. However I want to run the Python program starting at boot. Now the Python program fails to read the USB state.

I have tried starting the program via both rc.local and crontab. In either case the Python script is happy, reading/writing its data files, but fails when it tries to access USB. I have inserted a wait of up to 2 min assuming that USB may take time to initialize.

I wonder whether the USB is disabled until active login? Am I simply being dim here? :-)

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Sorted it. The problem was that I was not flushing the buffer first

ser = serial.Serial('/dev/ttyACM0', 57600)
ser.flushInput()
  • Perhaps also consider setting timeout when you open the device – scruss May 31 '17 at 15:21

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