PCF8574T I2C Interface 8-bit I/O expander The PCF8574/74A provides general-purpose remote I/O expansion via the two-wire bidirectional I2C bus (serial clock (SCL), serial data (SDA)).

Note that most backpack modules are centered around PCF8574T (SO16 package of PCF8574 in DIP16 package) with a default slave address of 0x27. If your backpack holds a PCF8574AT chip, then the default ...

Hi Rob, I'm using the Waveshare GPIO PCF8574T connected to a 4x4 keypad with I2CKeypad.h connected to a NANO. I placed keyPad.begin(KEYPAD_ADDRESS); in the setup() section and uint8_t idx = keyPad.