diff --git a/README.md b/README.md
index 9107596..7d90502 100644
--- a/README.md
+++ b/README.md
@@ -50,11 +50,13 @@ device behavior grounded in the **real v4.2 firmware dump**
   9600 8N1), never closer. A virtual null-modem has no UART, so unpaced
   writes would land at the host in microsecond bursts no real board could
   produce; vRIO's writer thread schedules each byte against a monotonic
-  slot deadline instead, so e.g. the 45-byte CheckRequest response takes
-  the same ~47 ms it takes real hardware.
-- `CheckRequest` → one `BoardOk` CheckReply per board (the 11 boards from the
-  legacy firmware's table). `VersionRequest` → configurable version,
-  default **4.2**.
+  slot deadline instead, so e.g. the 51-byte CheckRequest response takes
+  the same ~53 ms it takes real hardware.
+- `CheckRequest` → the real board's init handshake: `TestModeChange` **enter**,
+  one `BoardOk` CheckReply per board (the 11 boards from the legacy firmware's
+  table), then `TestModeChange` **exit**. Hosts wait (≤5 s per step) on both
+  test-mode packets and send nothing while test mode is active, so the exit is
+  mandatory. `VersionRequest` → configurable version, default **4.2**.
 - `ResetRequest` re-zeroes the targeted axis (or all).
 - A NAK re-sends the last event up to **4 times**, then gives up with a
   RESTART byte — the real board's retry budget.
diff --git a/src/VRio.Core/Device/VRioDevice.cs b/src/VRio.Core/Device/VRioDevice.cs
index 9fdbdb6..b2c4f1c 100644
--- a/src/VRio.Core/Device/VRioDevice.cs
+++ b/src/VRio.Core/Device/VRioDevice.cs
@@ -13,7 +13,9 @@ namespace VRio.Core.Device;
 /// <para>Wire behavior (mirroring what RIOJoy expects from the real board):</para>
 /// <list type="bullet">
 ///   <item>ACKs every well-formed inbound packet, NAKs one with a bad checksum.</item>
-///   <item>CheckRequest → a BoardOk CheckReply per known board.</item>
+///   <item>CheckRequest → TestModeChange ENTER, a BoardOk CheckReply per known
+///   board (the self-test stream), then TestModeChange EXIT — the init
+///   handshake the real board performs and the game waits (≤5s per step) on.</item>
 ///   <item>VersionRequest → VersionReply with the configured firmware version
 ///   (default 4.2, matching the real board's dumped EPROM).</item>
 ///   <item>AnalogRequest → AnalogReply with the current five axis values.</item>
@@ -239,9 +241,14 @@ public sealed class VRioDevice
         switch (packet.Command)
         {
             case RioCommand.CheckRequest:
-                Logged?.Invoke("RX CheckRequest → all boards OK");
+                // Init handshake (verified against a real v4.2 board tap): the
+                // host waits ≤5s for TestModeChange ENTER before anything else,
+                // and sends no requests at all until the matching EXIT arrives.
+                Logged?.Invoke("RX CheckRequest → test mode enter, all boards OK, exit");
+                Send(PacketBuilder.TestModeChange(1));
                 foreach ((byte number, string _) in RioAddressSpace.Boards)
                     Send(PacketBuilder.CheckReply(RioStatusType.BoardOk, number));
+                Send(PacketBuilder.TestModeChange(0));
                 break;
 
             case RioCommand.VersionRequest:
diff --git a/tests/VRio.Core.Tests/VRioDeviceTests.cs b/tests/VRio.Core.Tests/VRioDeviceTests.cs
index 28ac905..a98688b 100644
--- a/tests/VRio.Core.Tests/VRioDeviceTests.cs
+++ b/tests/VRio.Core.Tests/VRioDeviceTests.cs
@@ -79,22 +79,35 @@ public class VRioDeviceTests
     }
 
     [Fact]
-    public void CheckRequest_reports_every_board_ok()
+    public void CheckRequest_enters_test_mode_reports_boards_then_exits()
     {
         var device = new VRioDevice();
         var wire = new Wire(device);
 
         Send(device, PacketBuilder.Build(RioCommand.CheckRequest));
 
-        Assert.Equal(RioAddressSpace.Boards.Count, wire.Packets.Count);
-        Assert.All(wire.Packets, p =>
+        // The init handshake: TestModeChange ENTER, one CheckReply per board,
+        // TestModeChange EXIT. The game waits on both test-mode packets and
+        // stays mute forever if the EXIT never arrives.
+        Assert.Equal(RioAddressSpace.Boards.Count + 2, wire.Packets.Count);
+
+        RioPacket enter = wire.Packets[0];
+        Assert.Equal(RioCommand.TestModeChange, enter.Command);
+        Assert.Equal(new byte[] { 1 }, enter.Payload);
+
+        RioPacket exit = wire.Packets[wire.Packets.Count - 1];
+        Assert.Equal(RioCommand.TestModeChange, exit.Command);
+        Assert.Equal(new byte[] { 0 }, exit.Payload);
+
+        var checks = wire.Packets.GetRange(1, RioAddressSpace.Boards.Count);
+        Assert.All(checks, p =>
         {
             Assert.Equal(RioCommand.CheckReply, p.Command);
             Assert.Equal((byte)RioStatusType.BoardOk, p.Payload[0]);
         });
         Assert.Equal(
             RioAddressSpace.Boards.Select(b => b.Number),
-            wire.Packets.Select(p => p.Payload[1]));
+            checks.Select(p => p.Payload[1]));
     }
 
     [Fact]