Notes for users with a Pollin.de interface board
http://www.pollin.de/shop/dt/NDU4OTgxOTk-/Bausaetze_Module/Bausaetze/LCD_I2C_Modul.html

The pollin interface board will not work with de default library.
To get it working two control lines to the LCD need to be changed.

Open file "LiquidCrystal_I2C.h" with a text editor like Notepad (not WordPad !)
 
In that file look for:
#define En B00010000  // Enable bit
#define Rw B00100000  // Read/Write bit
#define Rs B01000000  // Register select bit

Replace these lines by:
#define En B01000000  // Enable bit
#define Rw B00100000  // Read/Write bit
#define Rs B00010000  // Register select bit



People at Pollin also have misunderstood the PCF8574 Datasheet and list the wrong addresses on their PCB
For PCF8574A the addressing is:
				
Jp3	Jp2	Jp1		
A2	A1	A0	Dec	Hex	
L	L	L	56	0x38
L	L	H	57	0x39
L	H	L	64	0x40
L	H	H	74	0x4A
H	L	L	75	0x4B
H	L	H	76	0x4C
H	H	L	77	0x4D
H	H	H	78	0x4E

They also seem to ship boards with a PCF8574
For PCF8574 the addressing is:

Jp3	Jp2	Jp1		
A2	A1	A0	Dec	Hex	
L	L	L	32	0x20
L	L	H	33	0x21
L	H	L	34	0x22
L	H	H	35	0x23
H	L	L	36	0x24
H	L	H	37	0x25
H	H	L	38	0x26
H	H	H	39	0x27


They have also chosen two rather high pull-up resistors (10K) for the I2C lines. Usually two 4K7 resistors should do the job.
Please note that on a I2C bus only one device should have the pull-up resistors installed!

I hope this helps in getting your LCD working.

Mario