I bought one of those very cheap I2C modules
from eBay for connecting to the back of a standard LCD so you can access it using I2C and reduce pins count needed. I downloaded the standard 'LiquidCrystal_I2C' library for controlling the PCF8574P chip used on this board from here
but found it did not work.
After trying several other LiquidCrystal_I2C libraries I gave up and decided to find out what the problem was and discovered the pin order connecting to the LCD from the PCF8574P was nibble reversed compared to what the library was expecting and the LED control logic was also reversed.
PCF8574P Expected Actual
P0 D4 Rs
P1 D5 R/W
P2 D6 En
P3 D7 LED
P4 En D4
P5 R/W D5
P6 Rs D6
P7 LED D7
I changed a few lines and added nibble swap code to the library for other who may have suffered the same problem.
EDIT: Forgot to say that despite the eBay example saying the I2C address is 0x20 it was 0x27 on mine.