nomada, personally I would write 01010101 to each of the pcf8574's then read back the contents of the input register. With all inputs open circuited you should read the exact same patern as you wrote to it. Then I would change the pattern to 10101010 and read the contents once again making sure the readback is the same as the programed value. After this I would write all 1's to the pcf8574's and start reading them in a normal situation. The verify siquence could ba a subroutine you would run at a timed interval to ensure the integrity of the reading ability of the pcf8574's.

Dave Purola,
N8NTA