Raspberry PI with HD44780 LCD and I2C Interface Board

Sven Bachmann
After buying an HD44780 compatible LCD together with an I2C interface board (which has an PCF8574T soldered on it) I tried to output some text in C on it but the display just stayed black - or after changing some GPIOs, it stayed blue. I measured every pin of the PCF8574T and got the following scheme: PCF8574T <-> HD44780 P0 -- RS P1 -- R/W P2 -- EN P4 -- DB4 P5 -- DB5 P6 -- DB6 P7 -- DB7 R/W should be mapped to GND if the HD44780 should not talk back to the CPU.