diff --git a/src/main/java/home/gateway/adapters/heatpump/ModbusSetpointRegisters.java b/src/main/java/home/gateway/adapters/heatpump/ModbusSetpointRegisters.java
new file mode 100644
index 0000000..8950f93
--- /dev/null
+++ b/src/main/java/home/gateway/adapters/heatpump/ModbusSetpointRegisters.java
@@ -0,0 +1,42 @@
+package home.gateway.adapters.heatpump;
+
+import home.gateway.adapters.modbus.ModbusClient;
+import home.gateway.application.SetpointRegisters;
+import java.io.IOException;
+
+/** Setpoint registers of the controller: whole values in sign-magnitude words, read with FC3 and written with FC6. */
+public class ModbusSetpointRegisters implements SetpointRegisters {
+
+    private static final int SIGN_BIT = 0x8000;
+    private static final int MAX_MAGNITUDE = 0x7FFF;
+
+    private final ModbusClient modbus;
+    private final int unitId;
+
+    public ModbusSetpointRegisters(ModbusClient modbus, int unitId) {
+        this.modbus = modbus;
+        this.unitId = unitId;
+    }
+
+    @Override
+    public int read(int register) throws IOException {
+        int[] words = modbus.readHoldingRegisters(unitId, register, 1);
+        if (words.length != 1) {
+            throw new IOException("Expected 1 word from register " + register + ", got " + words.length);
+        }
+        return (int) Scale.UNITS.decode(words[0]);
+    }
+
+    @Override
+    public void write(int register, int value) throws IOException {
+        modbus.writeSingleRegister(unitId, register, encode(value));
+    }
+
+    private static int encode(int value) {
+        int magnitude = Math.abs(value);
+        if (magnitude > MAX_MAGNITUDE) {
+            throw new IllegalArgumentException("Setpoint does not fit 15 bits: " + value);
+        }
+        return value < 0 ? magnitude | SIGN_BIT : magnitude;
+    }
+}
diff --git a/src/test/java/home/gateway/adapters/heatpump/ModbusSetpointRegistersTest.java b/src/test/java/home/gateway/adapters/heatpump/ModbusSetpointRegistersTest.java
new file mode 100644
index 0000000..677a5ad
--- /dev/null
+++ b/src/test/java/home/gateway/adapters/heatpump/ModbusSetpointRegistersTest.java
@@ -0,0 +1,98 @@
+package home.gateway.adapters.heatpump;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import home.gateway.adapters.modbus.ModbusClient;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ModbusSetpointRegistersTest {
+
+    private final FakeClient client = new FakeClient();
+    private final ModbusSetpointRegisters registers = new ModbusSetpointRegisters(client, 7);
+
+    @Test
+    void readsOneHoldingRegisterOfItsUnit() throws IOException {
+        client.word = 8;
+
+        assertEquals(8, registers.read(4143));
+        assertEquals(List.of("read 7:4143+1"), client.calls);
+    }
+
+    @Test
+    void decodesSignMagnitudeOnRead() throws IOException {
+        client.word = 0x8005;
+
+        assertEquals(-5, registers.read(4143));
+    }
+
+    @Test
+    void writesOneRegisterOfItsUnit() throws IOException {
+        registers.write(4143, 9);
+
+        assertEquals(List.of("write 7:4143=9"), client.calls);
+    }
+
+    @Test
+    void encodesNegativeValuesAsSignMagnitude() throws IOException {
+        registers.write(4143, -5);
+
+        assertEquals(List.of("write 7:4143=" + 0x8005), client.calls);
+    }
+
+    @Test
+    void rejectsValuesThatDoNotFitFifteenBits() {
+        assertThrows(IllegalArgumentException.class, () -> registers.write(4143, 0x8000));
+        assertThrows(IllegalArgumentException.class, () -> registers.write(4143, -0x8000));
+        assertEquals(List.of(), client.calls);
+    }
+
+    @Test
+    void propagatesWriteFailure() {
+        client.failure = new IOException("timeout");
+
+        assertThrows(IOException.class, () -> registers.write(4143, 9));
+    }
+
+    @Test
+    void propagatesReadFailureAndShortAnswer() {
+        client.failure = new IOException("timeout");
+        assertThrows(IOException.class, () -> registers.read(4143));
+
+        client.failure = null;
+        client.empty = true;
+        assertThrows(IOException.class, () -> registers.read(4143));
+    }
+
+    private static final class FakeClient implements ModbusClient {
+        final List<String> calls = new ArrayList<>();
+        int word;
+        boolean empty;
+        IOException failure;
+
+        @Override
+        public int[] readHoldingRegisters(int unitId, int address, int count) throws IOException {
+            if (failure != null) {
+                throw failure;
+            }
+            calls.add("read " + unitId + ":" + address + "+" + count);
+            return empty ? new int[0] : new int[] {word};
+        }
+
+        @Override
+        public int[] readInputRegisters(int unitId, int address, int count) {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public void writeSingleRegister(int unitId, int address, int value) throws IOException {
+            if (failure != null) {
+                throw failure;
+            }
+            calls.add("write " + unitId + ":" + address + "=" + value);
+        }
+    }
+}