I'm trying to connect 16x2 LCD screen to clone Arduino UNO via PF8574T backpack. 4 cable 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. 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.