diff --git a/src/main.rs b/src/main.rs
index fb18bd1..08f0583 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,6 +7,7 @@ mod option_window;
 mod ram;
 mod state;
 mod window;
+mod twosio;
 
 use std::collections::HashMap;
 
diff --git a/src/twosio.rs b/src/twosio.rs
new file mode 100644
index 0000000..154aa52
--- /dev/null
+++ b/src/twosio.rs
@@ -0,0 +1,108 @@
+use bitflags::bitflags;
+use eframe::egui::{Ui, ComboBox};
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    card::{Card, SettingsUi},
+    register,
+};
+
+bitflags! {
+    #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+    struct Status: u8 {
+        const RDRF = 0b0000_0001; 
+        const TDRE = 0b0000_0010;
+        const DCD  = 0b0000_0100;
+        const CTS  = 0b0000_1000;
+        const FE   = 0b0001_0000;
+        const OVRN = 0b0010_0000;
+        const PE   = 0b0100_0000;
+        const IRQ  = 0b1000_0000;
+    }
+}
+
+#[derive(Serialize, Deserialize)]
+pub struct TwoSioSettings {
+    start_addr: u8,
+}
+
+pub struct TwoSio {
+    start_addr: u8,
+    port1_status: Status,
+    port1_recv_data: u8,
+    port2_status: Status,
+    port2_recv_data: u8,
+}
+
+impl Card for TwoSio {
+    fn new(settings: ron::Value) -> Result<TwoSio, anyhow::Error> {
+        let settings: TwoSioSettings = settings.into_rust().unwrap();
+        Ok(Self {
+            start_addr: settings.start_addr,
+            port1_status: Status::TDRE,
+            port1_recv_data: 0,
+            port2_status: Status::TDRE,
+            port2_recv_data: 0,
+        })
+    }
+
+    fn default_settings() -> String {
+        ron::to_string(&TwoSioSettings {
+            start_addr: 0,
+        })
+        .unwrap()
+    }
+
+    fn get_settings(&self) -> String {
+        ron::to_string(&TwoSioSettings {
+            start_addr: self.start_addr,
+        }).unwrap()
+    }
+
+    fn settings_ui(settings: ron::Value) -> anyhow::Result<impl SettingsUi>
+    where
+        Self: Sized,
+    {
+        Ok(TwoSioSettingsUi {
+            settings: settings.into_rust().unwrap(),
+        })
+    }
+
+    fn read_io(&mut self, address: u8) -> Option<u8> {
+        if address & 0xF8 == self.start_addr {
+            match address & 0x2 {
+                0 => Some(self.port1_status.bits()),
+                1 => Some(self.port1_recv_data),
+                2 => Some(self.port2_status.bits()),
+                3 => Some(self.port2_recv_data),
+                _ => unreachable!(),
+            }
+        } else {
+            None
+        }
+    }
+}
+
+pub struct TwoSioSettingsUi {
+    settings: TwoSioSettings,
+}
+
+impl SettingsUi for TwoSioSettingsUi {
+    fn draw_ui(&mut self, ui: &mut Ui) {
+        ui.horizontal(|ui| {
+            ui.label("Base address");
+            ComboBox::from_id_source("2sio_base").selected_text(format!("{:#x}", self.settings.start_addr)).show_ui(ui, |ui| {
+                for i in 0..64 {
+                    let start_addr = (i as u8) * 4;
+                    ui.selectable_value(&mut self.settings.start_addr, start_addr, format!("{:#x}", start_addr));
+                }
+            });
+        });
+    }
+
+    fn serialize_settings(&self) -> String {
+        ron::to_string(&self.settings).unwrap()
+    }
+}
+
+register!(TwoSio, "88-2SIO");