Thanks Ray . I was not aware that stations should be “standard” (not GPIO). So S01 is mapped to P0 in PCF8574 (on the expansion), S02 mapped to P1…..and so on, until S08 mapped to P7 .
Question is why you did not use the spare pins on “main” PCF8574. According to the schematic, only two pins are used . One for B1 and the other for B3, so it seems there are 6 unused pins .