The PCF8574 IO Expansion Board is used as remote 8-bit I/O expander for I2C-bus. Up to 8 PCF8574 IO Expansion Board can be connected to the I2C-bus, providing up to 64 I/O ports.
The PCF8574 IO Expansion Board features I2C pinheader on one side, and I2C connector on the opposite side.
Hence, it's more flexible to connect the board to your development system. The board also supports I2C cascading, allowing the use of multi module connected to the I2C bus at the same time by connecting the pinheader and connector.
 
It has 8 I/O pins
Three I2C address select jumpers mean up to 8 expanders to one bus for 64 total GPIO added
Each pin can be an input with light pull-up or an output sink
IRQ output will automatically alert you when input pins change value
This chip does not have a pin direction register. You cannot set the pins to be input or output - instead each pin has two possible states. Basically you can think of it as an open-drain output with a 100K resistor pull-up built in.
Option one: Lightly pulled up 'input' - by default it will read as a high logic level, but connecting the GPIO to ground will cause it to read as a low logic level.
Option two: Strong 20mA low-driving transistor sink output. This means the output is 'forced' to be low and will always read as a low logic level.