PCF8574P with Wire library driving 7 segment display

So I'd like to get the basics of I2C 1st before doing something more complicated.

To my surprise, I couldn't find any basic tutorial about that with great explanation.

Setup: 4x4 matrix keypad connected directly to arduino, 7 segment connected to PCF8574P (I don't know what difference there is between P and not P).

I want to use my 4x4 keypad to show what I pressed on 7 segment.

I understand how to use the matrix keypad and 7 segment by it's own but I have do not understand too well the I2C.

"Proto code"

Get keycode, encode to 7 segment code, send via I2C to 7 segment.

I've checked this http://garagelab.com/profiles/blogs/tutorial-arduino-i-o-port-expander-with-pcf8574 and it seems "easy".

From what I've gathered first you need to begin transmission to address (how do I know the address?) then you send the data (send binary number as decimal,so to turn on all 8 pins you need to send 255?), and end the transmission.

Oxmaster: From what I've gathered first you need to begin transmission to address (how do I know the address?) then you send the data (send binary number as decimal,so to turn on all 8 pins you need to send 255?), and end the transmission.

Read the datasheet for the chip. It will tell you the address and how to use the chip. Use Google to find the datasheet. Typically an I/O Expander will have a register to set which pins are Input and which are Output. To set 8 pins HIGH you will want to set those pins as Output first.