The datasheet for the NXP PCA8574/74A contains the following line on addressing (p.6):
When AD2, AD1 and AD0 are held to VDD or VSS, the same address as the PCF8574 or PCF8574A is applied.
I'm afraid I don't understand what is meant here. Can someone clarify please?
Also, while address ranges are given as (depending on inputs AD0..AD2)
PCF8574: 0x20 - 0x27
PCF8574A: 0x38 - 0x3F
both the text and fig.7 suggest that the device address can freely be chosen between 0x01 and 0x7F (though I don't see how this would be done).
Or am I reading this wrong?