16 GPIOs on two wires — the I²C I/O expander for panel and control boards
The PCA9555DWRG4: It gives you 16 push-pull GPIOs, each capable of sinking 25 mA or sourcing 10 mA, controllable over a two-wire bus at clock frequencies up to 400 kHz. A built-in power-on reset (POR) forces all outputs to a known state at power-up, which keeps relays and indicators from glitching. The interrupt output (INT) signals the host when an input changes, so you do not have to poll the bus.
400 kHz I²C fast mode — bus timing and throughput
The PCA9555DWRG4 supports standard (100 kHz) and fast-mode (400 kHz) I²C clock rates. At 400 kHz, a full register read of all 16 inputs takes about 200 µs, fast enough for panel-button scanning, LED multiplexing, or reading DIP-switch banks in a motor drive. The bus interface is SMBus-compatible, so it works with SMBus hosts that expect timeout and packet-error checking. The I²C address is set by three hardware address pins (A0, A1, A2), giving up to eight devices on the same bus for 128 GPIOs total.
The I/O pins sink 25 mA or source 10 mA.