What this 8-bit I/O expander does on the bus
The NXP PCF8574ADW is an 8-bit I/O expander controlled over an I²C bus at 100 kHz standard-mode clock speed. It gives a host microcontroller eight extra push-pull I/O lines without burning GPIO pins — useful when you've run out of port pins on a small MCU or need to drive a bank of LEDs, read switches, or control relays across a ribbon cable. Each I/O pin can sink up to 25 mA — enough to light an LED directly through a series resistor — or source 1 mA for logic-level signalling. The interrupt output (INT) pulls low when any input changes state, so the host doesn't have to poll the bus; it can sleep until a button press or sensor edge wakes it up. Power-on reset (POR) ensures the outputs start in a known-high state after the supply comes up.
You'll find it in PLC I/O modules, HVAC controller boards, vending machines, and any equipment where a remote bank of I/O needs to talk back to a central processor over a two-wire bus.
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