Advantech ITA-1611 Series User Manual page 59

PCA9554 register:
In write transmission, command byte is the first byte after the address byte. It is used
as a pointer to decide which register to read/write.
PCA9554 has four registers to control GPIO.
PCA9554 register 0:
This register is read only. It shows input logic level of the pin, and has nothing to do
with the input or output direction defined by Register 3. It will not respond to write
operation.
Default value "X" is determined by external logic level. Usually, it is 1, indicating there
is internal pull-up resistor but no external signal.
If a certain GPIO Pin is set to Input. you can read corresponding Input value by read-
ing corresponding bit of register 0.
49 ITA-1611 User Manual