diff --git a/src/config.rs b/src/config.rs
index dee5363..1120822 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -130,13 +130,13 @@ impl From<ExpanderConfig> for u8 {
 ///     .unwrap();
 /// ```
 #[must_use = "Configuration changes are not applied unless committed"]
-pub struct Configurator<'e, EI: ExpanderInterface> {
+pub struct Configurator<'e, EI: ExpanderInterface + Send> {
     expander: &'e mut Expander<EI>,
     expander_config_dirty: bool,
     banks: [BankConfig; 7],
 }
 
-impl<'e, EI: ExpanderInterface> Configurator<'e, EI> {
+impl<'e, EI: ExpanderInterface + Send> Configurator<'e, EI> {
     pub(crate) fn new(expander: &'e mut Expander<EI>) -> Self {
         Self {
             expander,
diff --git a/src/expander/immediate.rs b/src/expander/immediate.rs
index f4fd86b..fd617d7 100644
--- a/src/expander/immediate.rs
+++ b/src/expander/immediate.rs
@@ -14,23 +14,12 @@ use registers::valid_port;
 pub struct ImmediateIO<M, EI>(M, PhantomData<EI>)
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface;
-
-// Unsafety: This is only needed because the presence of PhantomData<EI> causes the struct to no
-// longer be Sync, because EI is often not Sync since it owns a global resource (e.g. SPI device).
-// However, the EI is actually owned by the Expander which is in the mutex which normally
-// re-instates Sync-ness. PhantomData is there to shut up the unused type parameter error.
-unsafe impl<M, EI> Sync for ImmediateIO<M, EI>
-where
-    M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
-{
-}
+    EI: ExpanderInterface + Send;
 
 impl<M, EI> ImmediateIO<M, EI>
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
+    EI: ExpanderInterface + Send,
 {
     pub(crate) fn new(expander: Expander<EI>) -> Self {
         ImmediateIO(M::new(expander), PhantomData)
@@ -54,7 +43,7 @@ where
 impl<M, EI> ExpanderIO for ImmediateIO<M, EI>
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
+    EI: ExpanderInterface + Send,
 {
     fn write_port(&self, port: u8, bit: bool) {
         self.0.lock(|ex| ex.write_port(port, bit).unwrap())
diff --git a/src/expander/mod.rs b/src/expander/mod.rs
index 59dbcde..27a309a 100644
--- a/src/expander/mod.rs
+++ b/src/expander/mod.rs
@@ -18,7 +18,7 @@ pub struct Expander<EI: ExpanderInterface> {
     pub(crate) config: ExpanderConfig,
 }
 
-impl<EI: ExpanderInterface> Expander<EI> {
+impl<EI: ExpanderInterface + Send> Expander<EI> {
     /// Create a new `Expander`.
     ///
     /// Takes ownership of the `ExpanderInterface` which it should use to communicate with the
diff --git a/src/expander/transactional.rs b/src/expander/transactional.rs
index d99a00c..eb6729f 100644
--- a/src/expander/transactional.rs
+++ b/src/expander/transactional.rs
@@ -48,7 +48,7 @@ pub enum Strategy {
 pub struct TransactionalIO<M, EI>
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
+    EI: ExpanderInterface + Send,
 {
     expander: M,
     issued: AtomicUsize,
@@ -58,21 +58,10 @@ where
     _ei: PhantomData<EI>,
 }
 
-// NOTE(unsafe): This is only needed because the presence of PhantomData<EI> causes the struct to
-// no longer be Sync, because EI is often not Sync since it owns a global resource (e.g. SPI
-// device).  However, the EI is actually owned by the Expander which is in the mutex which normally
-// re-instates Sync-ness. PhantomData is there to shut up the unused type parameter error.
-unsafe impl<M, EI> Sync for TransactionalIO<M, EI>
-where
-    M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
-{
-}
-
 impl<M, EI> TransactionalIO<M, EI>
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
+    EI: ExpanderInterface + Send,
 {
     pub(crate) fn new(expander: Expander<EI>) -> Self {
         TransactionalIO {
@@ -160,7 +149,7 @@ where
 impl<M, EI> ExpanderIO for TransactionalIO<M, EI>
 where
     M: IOMutex<Expander<EI>>,
-    EI: ExpanderInterface,
+    EI: ExpanderInterface + Send,
 {
     fn write_port(&self, port: u8, bit: bool) {
         let or_bit = 1 << port;
diff --git a/src/interface.rs b/src/interface.rs
index baca2ff..d8f1e4f 100644
--- a/src/interface.rs
+++ b/src/interface.rs
@@ -121,9 +121,8 @@ pub(crate) mod test_spy {
 
     use super::ExpanderInterface;
     use registers::RegisterAddress;
-    use std::cell::RefCell;
     use std::fmt;
-    use std::rc::Rc;
+    use std::sync::{Arc, Mutex};
 
     #[derive(Clone, Copy, Debug, PartialEq)]
     pub enum TestRegister {
@@ -134,15 +133,15 @@ pub(crate) mod test_spy {
     }
 
     pub struct TestSpyInterface {
-        registers: Rc<RefCell<Vec<TestRegister>>>,
-        reads: Rc<RefCell<Vec<u8>>>,
+        registers: Arc<Mutex<Vec<TestRegister>>>,
+        reads: Arc<Mutex<Vec<u8>>>,
     }
 
     impl TestSpyInterface {
         pub fn new() -> Self {
             let mut new = Self {
-                registers: Rc::new(RefCell::new(Vec::new())),
-                reads: Rc::new(RefCell::new(Vec::new())),
+                registers: Arc::new(Mutex::new(Vec::new())),
+                reads: Arc::new(Mutex::new(Vec::new())),
             };
             new.reset();
             new
@@ -151,8 +150,8 @@ pub(crate) mod test_spy {
         pub fn reset(&mut self) {
             use self::TestRegister::*;
 
-            self.reads.borrow_mut().clear();
-            let mut regs = self.registers.borrow_mut();
+            self.reads.lock().unwrap().clear();
+            let mut regs = self.registers.lock().unwrap();
             regs.clear();
             regs.resize(0x60, Forbidden);
 
@@ -179,21 +178,21 @@ pub(crate) mod test_spy {
         }
 
         pub fn get(&self, addr: u8) -> TestRegister {
-            self.registers.borrow()[addr as usize]
+            self.registers.lock().unwrap()[addr as usize]
         }
 
         pub fn set(&mut self, addr: u8, val: TestRegister) {
-            self.registers.borrow_mut()[addr as usize] = val;
+            self.registers.lock().unwrap()[addr as usize] = val;
         }
 
         pub fn reads(&self) -> Vec<u8> {
-            self.reads.borrow().clone()
+            self.reads.lock().unwrap().clone()
         }
     }
 
     impl ExpanderInterface for TestSpyInterface {
         fn write_register(&mut self, addr: RegisterAddress, value: u8) -> Result<(), ()> {
-            let mut regs = self.registers.borrow_mut();
+            let mut regs = self.registers.lock().unwrap();
             let enc_addr = u8::from(addr);
             assert!(enc_addr <= 0x5F);
             match regs[enc_addr as usize] {
@@ -204,8 +203,8 @@ pub(crate) mod test_spy {
             Ok(())
         }
         fn read_register(&mut self, addr: RegisterAddress) -> Result<u8, ()> {
-            self.reads.borrow_mut().push(addr.into());
-            let regs = self.registers.borrow();
+            self.reads.lock().unwrap().push(addr.into());
+            let regs = self.registers.lock().unwrap();
             let enc_addr = u8::from(addr);
             assert!(enc_addr <= 0x5F);
             match regs[enc_addr as usize] {
@@ -240,14 +239,14 @@ pub(crate) mod test_spy {
     }
 
     pub struct SemanticTestSpyInterface {
-        ports: Rc<RefCell<Vec<TestPort>>>,
+        ports: Arc<Mutex<Vec<TestPort>>>,
     }
 
     impl SemanticTestSpyInterface {
         pub fn new(init: Vec<bool>) -> Self {
             assert!(init.len() == 32 - 4);
             Self {
-                ports: Rc::new(RefCell::new(
+                ports: Arc::new(Mutex::new(
                     init.into_iter()
                         .map(|b| TestPort::Reset(b))
                         .collect::<Vec<_>>(),
@@ -264,7 +263,8 @@ pub(crate) mod test_spy {
         pub fn peek_all(&self) -> Vec<TestPort> {
             use self::TestPort::*;
             self.ports
-                .borrow()
+                .lock()
+                .unwrap()
                 .iter()
                 .cloned()
                 .map(|v| match v {
@@ -277,7 +277,8 @@ pub(crate) mod test_spy {
         pub fn peek_bits(&self) -> Vec<bool> {
             use self::TestPort::*;
             self.ports
-                .borrow()
+                .lock()
+                .unwrap()
                 .iter()
                 .cloned()
                 .map(|v| match v {
@@ -289,7 +290,7 @@ pub(crate) mod test_spy {
         fn write_port(&self, port: u8, bit: bool) {
             use self::TestPort::*;
             let idx = port as usize - 4;
-            let slot_ref = &mut self.ports.borrow_mut()[idx];
+            let slot_ref = &mut self.ports.lock().unwrap()[idx];
             *slot_ref = match slot_ref {
                 Reset(_) | BlindWrite(_) => BlindWrite(bit),
                 Read(_) | ReadWrite(_) => ReadWrite(bit),
@@ -299,7 +300,7 @@ pub(crate) mod test_spy {
         fn read_port(&self, port: u8) -> bool {
             use self::TestPort::*;
             let idx = port as usize - 4;
-            let slot_ref = &mut self.ports.borrow_mut()[idx];
+            let slot_ref = &mut self.ports.lock().unwrap()[idx];
             let (upd, ret) = match slot_ref {
                 Reset(b) | Read(b) => (Read(*b), *b),
                 ReadWrite(b) => (ReadWrite(*b), *b),
@@ -316,7 +317,8 @@ pub(crate) mod test_spy {
                 f,
                 "[ {} ]",
                 self.ports
-                    .borrow()
+                    .lock()
+                    .unwrap()
                     .iter()
                     .map(|&v| format!("{:?}", v))
                     .collect::<Vec<_>>()