I'm trying to connect 16x2 LCD screen to clone Arduino UNO via PF8574T backpack. 4 cables are connected to +5V, GND, SDA and SDL pins.

When I tried this I2C scanner code from arduino.cc. It returned strange characters in serial monitor at first couple of tries. And now, when I try computer hangs.

When I use A4 A5 pins on the Arduino instead of SDA and SDL, i2c reported the device is on 0x27. It is ok. But if i keep LCD connected to Arduino, my iMac shutdowns after for a while. In error log i see there was a problem regarding USB serial device. I assume it as a power problem.

I'm on OS X Yosemite. Why this can be happening and how can I fix?

Note: Computer is hanging as soon as the Arduino usb cable connected.

  • Update: When I removed the LCD, the it connected without error so I could delete I2C scanner sketch.
    – wizofwor
    Apr 27, 2015 at 18:52
  • Update2: I have updated my question. It seems as a power problem.
    – wizofwor
    Apr 28, 2015 at 7:47

1 Answer 1


Sometimes my system hangs in the way you describe ("hanging as soon as the Arduino usb cable connected") if I mistakenly connect the +ve and -ve supply lines to an i2c device the wrong way round.

  • It seems there is short in the backback circuit. Or may be the pin markings are wrong.
    – wizofwor
    Apr 28, 2015 at 7:48

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