diff --git a/src/devices/expander_pca9555.rs b/src/devices/expander_pca9555.rs
new file mode 100644
index 0000000..abed2fc
--- /dev/null
+++ b/src/devices/expander_pca9555.rs
@@ -0,0 +1,105 @@
+use crate::hal;
+use hal::i2c::I2c;
+use hal::prelude::*;
+use stm32h7xx_hal::prelude::{
+    _embedded_hal_blocking_i2c_Read as Read, _embedded_hal_blocking_i2c_Write as Write,
+    _embedded_hal_blocking_i2c_WriteRead as WriteRead,
+};
+
+/// Data corresponding to a single PCA9555 I/O Expander
+///
+/// In the future we can add:
+/// - chained-device support (similar to pca9685)
+/// - output configuration
+/// - INT pin support for non-polling behavior
+///
+/// For now this is a minimal driver for providing access
+/// to 16 additional inptus.
+pub struct Pca9555<I2CN> {
+    i2c: I2c<I2CN>,
+    address: u8,
+    states: [bool; 16],
+}
+
+#[derive(Debug)]
+pub enum I2cError<W, WR> {
+    Write(W),
+    WriteRead(WR),
+}
+
+#[expect(type_alias_bounds)]
+pub type WriteReadError<W: Write + WriteRead> =
+    I2cError<<W as Write>::Error, <W as WriteRead>::Error>;
+
+impl<I2CN> Pca9555<I2CN>
+where
+    I2c<I2CN>: Write + WriteRead + Read,
+{
+    pub fn new(address: u8, mut bus: I2c<I2CN>) -> Self {
+        let base_address = 0x20;
+        let dev_addr = base_address | (address & 0x07);
+        let mut dev = Pca9555 {
+            i2c: bus,
+            address: dev_addr,
+            states: Default::default(),
+        };
+
+        // Configuration
+        // Default config appears to be:
+        // configuration = input
+        // polarity = not-inverted
+        // So for our initial implementation.. no config is necessary
+
+        // Any attempts to write here will cause an infinite loop during if device is not
+        // present and serial callback hasn't been started. So maybe we don't do this here yet.
+
+        // This was here to test why device wasn't sending ACK (but it wasn't on the PCB ...)
+        // Will remove these comments, and finish driver once I have appropriate HW
+        // crate::delay::CycleDelay::new().delay_ms(30u8);
+        // let _ = dev
+        //     .i2c
+        //     .write(dev.address, &[Register::ConfigurationPort0 as u8, 0xff]);
+
+        // Fill internal data with current states on hardware.
+        // dev.update();
+
+        dev
+    }
+
+    /// Polls the device, reading both of it's input registers to fill the
+    /// internal state data
+    // pub fn update(&mut self) -> Result<(), WriteReadError<W>> {
+    pub fn update(&mut self) {
+        // Register pairs flip automatically after reads.
+        // So we can basically just send the `InputPort0` command,
+        // and read two data bytes back every time we want new data
+        let mut raw_states = [0u8; 2];
+        let _ = self
+            .i2c
+            .write_read(self.address, &[Register::InputPort0 as u8], &mut raw_states);
+
+        let raw_union: u16 = (raw_states[0] as u16) | ((raw_states[1] as u16) << 8);
+        for i in 0..16 {
+            self.states[i] = (raw_union & (1 << i)) != 0;
+        }
+    }
+
+    /// Returns the current state recorded for a particular pin
+    pub fn get_state(&self, index: usize) -> bool {
+        let idx = index.min(15);
+        self.states[idx]
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[repr(u8)]
+enum Register {
+    InputPort0 = 0x00,
+    InputPort1 = 0x01,
+    OutputPort0 = 0x02,
+    OutputPort1 = 0x03,
+    PolarityInversion0 = 0x04,
+    PolarityInversion1 = 0x05,
+    ConfigurationPort0 = 0x06,
+    ConfigurationPort1 = 0x07,
+}
diff --git a/src/devices/mod.rs b/src/devices/mod.rs
index 8f58ac2..e71d384 100644
--- a/src/devices/mod.rs
+++ b/src/devices/mod.rs
@@ -1,4 +1,5 @@
 pub mod encoder;
+pub mod expander_pca9555;
 pub mod led_driver_pca9685;
 pub mod mpr121;
 pub mod multiplexer;