diff --git a/pylabrobot/barcode_scanners/keyence/keyence_backend.py b/pylabrobot/barcode_scanners/keyence/keyence_backend.py
index e79501fda71..92dee9935c6 100644
--- a/pylabrobot/barcode_scanners/keyence/keyence_backend.py
+++ b/pylabrobot/barcode_scanners/keyence/keyence_backend.py
@@ -73,20 +73,37 @@ async def initialize(self):
       )
 
   async def send_command(self, command: str) -> str:
-    """Send a command to the barcode scanner and return the response.
-    Keyence uses carriage return \r as the line ending by default."""
+    """Send a command and return its reply, accumulated byte-by-byte up to the \r terminator.
+
+    Replies are bare \r-terminated and silence is a valid empty reply (e.g. no barcode), so
+    stop at \r or the first byte-less read rather than reading a fixed byte count."""
 
     await self.io.write((command + "\r").encode(self.serial_messaging_encoding))
-    response = await self.io.read()
-    return response.decode(self.serial_messaging_encoding).strip()
+    buf = bytearray()
+    while True:
+      chunk = await self.io.read()
+      if not chunk:
+        break  # port timeout elapsed with no byte: reply done, or none coming
+      buf.extend(chunk)
+      if chunk == b"\r":
+        break
+    return buf.decode(self.serial_messaging_encoding).strip()
 
   async def stop(self):
     await self.io.stop()
 
   async def scan_barcode(self) -> Barcode:
-    data = await self.send_command("LON")
-    if data.startswith("NG"):
-      raise BarcodeScannerError("Barcode reader is off: cannot read barcode")
-    if data.startswith("ERR99"):
-      raise BarcodeScannerError(f"Error response from barcode reader: {data}")
-    return Barcode(data=data, symbology="unknown", position_on_resource="front")
+    try:
+      data = await self.send_command("LON")
+      if data.startswith("NG"):
+        raise BarcodeScannerError("Barcode reader is off: cannot read barcode")
+      if data.startswith("ERR99"):
+        raise BarcodeScannerError(f"Error response from barcode reader: {data}")
+      return Barcode(data=data, symbology="unknown", position_on_resource="front")
+    finally:
+      # LON latches the read beam on; release it whether the read succeeded or raised.
+      # try/except so a LOFF failure can't mask the scan's result.
+      try:
+        await self.send_command("LOFF")
+      except Exception:
+        logger.warning("Failed to turn off barcode reader beam (LOFF)", exc_info=True)
diff --git a/pylabrobot/barcode_scanners/keyence/keyence_backend_tests.py b/pylabrobot/barcode_scanners/keyence/keyence_backend_tests.py
new file mode 100644
index 00000000000..69f7285f564
--- /dev/null
+++ b/pylabrobot/barcode_scanners/keyence/keyence_backend_tests.py
@@ -0,0 +1,96 @@
+import unittest
+from unittest.mock import AsyncMock, call
+
+from pylabrobot.barcode_scanners.backend import BarcodeScannerError
+from pylabrobot.barcode_scanners.keyence.keyence_backend import (
+  HAS_SERIAL,
+  KeyenceBarcodeScannerBackend,
+)
+from pylabrobot.resources.barcode import Barcode
+
+
+@unittest.skipUnless(HAS_SERIAL, "pyserial is not installed")
+class TestKeyenceSendCommand(unittest.IsolatedAsyncioTestCase):
+  """send_command must read a full \\r-terminated reply, not a single byte."""
+
+  def setUp(self):
+    self.backend = KeyenceBarcodeScannerBackend(port="COM1")
+    self.backend.io.write = AsyncMock()  # type: ignore
+    self.backend.io.read = AsyncMock()  # type: ignore
+
+  async def test_writes_carriage_return_terminated_command(self):
+    self.backend.io.read.side_effect = [b"\r"]  # type: ignore
+    await self.backend.send_command("RMOTOR")
+    self.backend.io.write.assert_awaited_once_with(b"RMOTOR\r")  # type: ignore
+
+  async def test_accumulates_reply_until_carriage_return(self):
+    # The bug: a single io.read() returned one byte and truncated the reply.
+    self.backend.io.read.side_effect = [b"M", b"O", b"T", b"O", b"R", b"O", b"N", b"\r"]  # type: ignore
+    response = await self.backend.send_command("RMOTOR")
+    self.assertEqual(response, "MOTORON")
+
+  async def test_stops_reading_at_terminator(self):
+    # Once \r lands the reply is complete; reading again would block on the next command.
+    self.backend.io.read.side_effect = [b"O", b"K", b"\r"]  # type: ignore
+    await self.backend.send_command("LOFF")
+    self.assertEqual(self.backend.io.read.await_count, 3)  # type: ignore
+
+  async def test_byteless_read_is_an_empty_reply(self):
+    # A byte-less read means the port timeout elapsed with no reply (e.g. no barcode).
+    self.backend.io.read.side_effect = [b""]  # type: ignore
+    response = await self.backend.send_command("LON")
+    self.assertEqual(response, "")
+
+
+@unittest.skipUnless(HAS_SERIAL, "pyserial is not installed")
+class TestKeyenceScanBarcode(unittest.IsolatedAsyncioTestCase):
+  """scan_barcode must release the LON-latched read beam with LOFF, success or failure."""
+
+  def setUp(self):
+    self.backend = KeyenceBarcodeScannerBackend(port="COM1")
+    self.replies: dict[str, str] = {}
+    self.raise_on: set[str] = set()
+
+    def _send(command: str) -> str:
+      if command in self.raise_on:
+        raise RuntimeError(f"simulated failure on {command}")
+      return self.replies.get(command, "")
+
+    self.backend.send_command = AsyncMock(side_effect=_send)  # type: ignore[method-assign]
+
+  async def test_success_returns_barcode_then_releases_beam(self):
+    self.replies = {"LON": "ABC123", "LOFF": ""}
+    barcode = await self.backend.scan_barcode()
+    self.assertIsInstance(barcode, Barcode)
+    self.assertEqual(barcode.data, "ABC123")
+    self.backend.send_command.assert_has_calls([call("LON"), call("LOFF")])  # type: ignore
+
+  async def test_releases_beam_when_reader_off(self):
+    self.replies = {"LON": "NG", "LOFF": ""}
+    with self.assertRaises(BarcodeScannerError):
+      await self.backend.scan_barcode()
+    self.backend.send_command.assert_has_calls([call("LON"), call("LOFF")])  # type: ignore
+
+  async def test_releases_beam_on_error_response(self):
+    self.replies = {"LON": "ERR99", "LOFF": ""}
+    with self.assertRaises(BarcodeScannerError):
+      await self.backend.scan_barcode()
+    self.backend.send_command.assert_has_calls([call("LON"), call("LOFF")])  # type: ignore
+
+  async def test_loff_failure_does_not_mask_successful_scan(self):
+    # A failing LOFF is logged, never propagated, so the scan result survives.
+    self.replies = {"LON": "ABC123"}
+    self.raise_on = {"LOFF"}
+    barcode = await self.backend.scan_barcode()
+    self.assertEqual(barcode.data, "ABC123")
+
+  async def test_loff_failure_does_not_mask_scan_error(self):
+    # The reader error must propagate, not the swallowed LOFF failure.
+    self.replies = {"LON": "NG"}
+    self.raise_on = {"LOFF"}
+    with self.assertRaises(BarcodeScannerError):
+      await self.backend.scan_barcode()
+
+
+if __name__ == "__main__":
+  unittest.main()