diff --git a/src/ophyd_async/core/_derived_signal.py b/src/ophyd_async/core/_derived_signal.py
new file mode 100644
index 000000000..be8b1aada
--- /dev/null
+++ b/src/ophyd_async/core/_derived_signal.py
@@ -0,0 +1,154 @@
+import asyncio
+import dataclasses
+from collections.abc import Iterator
+from typing import Generic, TypeVar, get_type_hints
+
+import numpy as np
+
+from ._device import Device
+from ._protocol import AsyncMovable
+from ._signal import SignalR, SignalRW, soft_signal_rw
+from ._signal_backend import Array1D, SignalBackend
+from ._status import AsyncStatus
+from ._utils import T, get_origin_class
+
+RawSignalsT = TypeVar("RawSignalsT")
+ParametersSignalsT = TypeVar("ParametersSignalsT")
+RawT = TypeVar("RawT")
+DerivedT = TypeVar("DerivedT")
+ParametersT = TypeVar("ParametersT")
+
+
+class Transform(Generic[RawT, DerivedT, ParametersT]):
+    def forward(self, raw: RawT, parameters: ParametersT) -> DerivedT: ...
+    def inverse(self, derived: DerivedT, parameters: ParametersT) -> RawT: ...
+
+
+F_contra = TypeVar("F_contra", bound=float | Array1D[np.float64], contravariant=True)
+
+
+# TODO: should this be a TypedDict?
+@dataclasses.dataclass
+class SlitsRaw(Generic[F_contra]):
+    top: F_contra
+    bottom: F_contra
+
+
+@dataclasses.dataclass
+class SlitsDerived(Generic[F_contra]):
+    gap: F_contra
+    centre: F_contra
+
+
+@dataclasses.dataclass
+class SlitsParameters:
+    gap_offset: float
+
+
+class SlitsTransform(Transform[SlitsRaw, SlitsDerived, SlitsParameters]):
+    def forward(
+        self, raw: SlitsRaw[F_contra], parameters: SlitsParameters
+    ) -> SlitsDerived[F_contra]:
+        return SlitsDerived(
+            gap=raw.top - raw.bottom + parameters.gap_offset,
+            centre=(raw.top + raw.bottom) / 2,
+        )
+
+    def inverse(
+        self, derived: SlitsDerived[F_contra], parameters: SlitsParameters
+    ) -> SlitsRaw[F_contra]:
+        half_gap = (derived.gap - parameters.gap_offset) / 2
+        return SlitsRaw(
+            top=derived.centre + half_gap,
+            bottom=derived.centre - half_gap,
+        )
+
+
+def _get_dataclass_args(method) -> Iterator[type]:
+    for k, v in get_type_hints(method):
+        cls = get_origin_class(v)
+        if k != "return" and dataclasses.is_dataclass(cls):
+            yield cls
+
+
+async def _get_dataclass_from_signals(cls: type[T], device: Device) -> T:
+    coros = {}
+    for field in dataclasses.fields(cls):
+        sig = getattr(device, field.name)
+        assert isinstance(
+            sig, SignalR
+        ), f"{device.name}.{field.name} is {sig}, not a Signal"
+        coros[field.name] = sig.get_value()
+    results = await asyncio.gather(*coros.values())
+    kwargs = dict(zip(coros, results, strict=True))
+    return cls(**kwargs)
+
+
+class DerivedBackend(Generic[RawT, DerivedT, ParametersT]):
+    def __init__(
+        self,
+        device: Device,
+        transform: Transform[RawT, DerivedT, ParametersT],
+    ):
+        self._device = device
+        self._transform = transform
+        self._raw_cls, self._param_cls = _get_dataclass_args(self._transform.forward)
+
+    async def get_parameters(self) -> ParametersT:
+        return await _get_dataclass_from_signals(self._param_cls, self._device)
+
+    async def get_raw_values(self) -> RawT:
+        return await _get_dataclass_from_signals(self._raw_cls, self._device)
+
+    async def get_derived_values(self) -> DerivedT:
+        raw, parameters = await asyncio.gather(
+            self.get_raw_values(), self.get_parameters()
+        )
+        return self._transform.forward(raw, parameters)
+
+    async def set_derived_values(self, derived: DerivedT):
+        assert isinstance(self._device, AsyncMovable)
+        await self._device.set(derived)
+
+    async def calculate_raw_values(self, derived: DerivedT) -> RawT:
+        return self._transform.inverse(derived, await self.get_parameters())
+
+    def derived_signal(self, variable: str) -> SignalRW[float]:
+        return SignalRW(DerivedSignalBackend(self, variable))
+
+
+class DerivedSignalBackend(SignalBackend[float]):
+    def __init__(self, backend: DerivedBackend, variable: str):
+        self._backend = backend
+        self._variable = variable
+        super().__init__(float)
+
+    async def get_value(self) -> float:
+        derived = await self._backend.get_derived_values()
+        return getattr(derived, self._variable)
+
+    async def put(self, value: float | None, wait: bool):
+        derived = await self._backend.get_derived_values()
+        # TODO: we should be calling locate on these as we want to move relative to the
+        # setpoint, not readback
+        setattr(derived, self._variable, value)
+        await self._backend.set_derived_values(derived)
+
+
+class Slits(Device):
+    def __init__(self, name=""):
+        self._backend = DerivedBackend(self, SlitsTransform())
+        # Raw signals
+        self.top = soft_signal_rw(float)
+        self.bottom = soft_signal_rw(float)
+        # Parameter
+        self.gap_offset = soft_signal_rw(float)
+        # Derived signals
+        self.gap = self._backend.derived_signal("gap")
+        self.centre = self._backend.derived_signal("centre")
+        super().__init__(name=name)
+
+    @AsyncStatus.wrap
+    async def set(self, derived: SlitsDerived[float]) -> None:
+        raw: SlitsRaw[float] = await self._backend.calculate_raw_values(derived)
+        await asyncio.gather(self.top.set(raw.top), self.bottom.set(raw.bottom))
diff --git a/src/ophyd_async/core/_protocol.py b/src/ophyd_async/core/_protocol.py
index d703d0011..189db5e16 100644
--- a/src/ophyd_async/core/_protocol.py
+++ b/src/ophyd_async/core/_protocol.py
@@ -127,6 +127,14 @@ def unstage(self) -> AsyncStatus:
         """
 
 
+@runtime_checkable
+class AsyncMovable(Protocol):
+    @abstractmethod
+    def set(self, value) -> AsyncStatus:
+        """Return a ``Status`` that is marked done when the device is done moving."""
+        ...
+
+
 C = TypeVar("C", contravariant=True)