diff --git a/pslab/external/motor.py b/pslab/external/motor.py
index 1fba065..d77e839 100644
--- a/pslab/external/motor.py
+++ b/pslab/external/motor.py
@@ -125,21 +125,32 @@ def import_timeline_from_csv(self, filepath: str) -> List[List[int]]:
         Returns
         -------
         List[List[int]]
-            A timeline consisting of servo angle values per timestep.
+            A timeline consisting of servo angle values per timestep, with one
+            angle per servo of this arm.
         """
         timeline = []
 
         with open(filepath, mode="r", newline="") as csvfile:
             reader = csv.DictReader(csvfile)
+            columns = [f"Servo{i}" for i in range(1, RoboticArm.MAX_SERVOS + 1)]
+            if reader.fieldnames is None or any(
+                column not in reader.fieldnames for column in columns
+            ):
+                raise ValueError("CSV must contain the Servo1-Servo4 columns")
             for row in reader:
                 angles = []
-                for key in ["Servo1", "Servo2", "Servo3", "Servo4"]:
-                    value = row[key]
-                    if value == "null":
+                for column in columns:
+                    value = row.get(column)
+                    # Short rows from older exports leave trailing servos unset.
+                    if value in (None, "", "null"):
                         angles.append(None)
                     else:
                         angles.append(int(value))
-                timeline.append(angles)
+                if any(angle is not None for angle in angles[len(self.servos) :]):
+                    raise ValueError(
+                        f"Timeline sets angles for more than {len(self.servos)} servos"
+                    )
+                timeline.append(angles[: len(self.servos)])
 
         return timeline
 
@@ -157,6 +168,12 @@ def export_timeline_to_csv(
             Directory path where the CSV file will be saved. The filename
             will include a timestamp to ensure uniqueness.
         """
+        for i, row in enumerate(timeline):
+            if len(row) > RoboticArm.MAX_SERVOS:
+                raise ValueError(
+                    f"Timestep {i} has more than {RoboticArm.MAX_SERVOS} angles"
+                )
+
         timestamp = datetime.now().strftime("%Y-%m-%d_%H-%M-%S")
         filename = f"Robotic_Arm{timestamp}.csv"
         filepath = os.path.join(folderpath, filename)
@@ -165,5 +182,7 @@ def export_timeline_to_csv(
             writer = csv.writer(csvfile)
             writer.writerow(["Timestep", "Servo1", "Servo2", "Servo3", "Servo4"])
             for i, row in enumerate(timeline):
+                # Pad to four servos so every row matches the header.
+                row = list(row) + [None] * (RoboticArm.MAX_SERVOS - len(row))
                 pos = ["null" if val is None else val for val in row]
                 writer.writerow([i] + pos)
diff --git a/tests/test_robotic_arm.py b/tests/test_robotic_arm.py
new file mode 100644
index 0000000..815440e
--- /dev/null
+++ b/tests/test_robotic_arm.py
@@ -0,0 +1,79 @@
+"""Tests for pslab.external.motor.RoboticArm CSV import and export.
+
+These tests do not require a connected PSLab.
+"""
+
+from unittest.mock import MagicMock
+
+import pytest
+
+from pslab.external.motor import RoboticArm, Servo
+
+
+def make_arm(servo_count: int) -> RoboticArm:
+    pwm = MagicMock()
+    return RoboticArm([Servo(f"SQ{i + 1}", pwm) for i in range(servo_count)])
+
+
+def write_csv(path, rows):
+    header = "Timestep,Servo1,Servo2,Servo3,Servo4\n"
+    path.write_text(header + "".join(f"{row}\n" for row in rows))
+    return str(path)
+
+
+@pytest.mark.parametrize("servo_count", [1, 2, 3, 4])
+def test_export_import_round_trip(tmp_path, servo_count):
+    arm = make_arm(servo_count)
+    timeline = [
+        [10 * (i + 1) for i in range(servo_count)],
+        [None] + [90] * (servo_count - 1),
+    ]
+
+    arm.export_timeline_to_csv(timeline, str(tmp_path))
+    (exported,) = tmp_path.glob("*.csv")
+
+    assert arm.import_timeline_from_csv(str(exported)) == timeline
+
+
+def test_export_pads_rows_to_four_servos(tmp_path):
+    make_arm(2).export_timeline_to_csv([[10, 20]], str(tmp_path))
+    (exported,) = tmp_path.glob("*.csv")
+
+    assert exported.read_text().splitlines()[1] == "0,10,20,null,null"
+
+
+def test_imported_timeline_runs_on_a_smaller_arm(tmp_path):
+    arm = make_arm(2)
+    path = write_csv(tmp_path / "t.csv", ["0,10,20,null,null"])
+
+    arm.run_schedule(arm.import_timeline_from_csv(path), time_step=0)
+
+    assert [servo.angle for servo in arm.servos] == [10, 20]
+
+
+def test_import_accepts_short_rows(tmp_path):
+    path = write_csv(tmp_path / "t.csv", ["0,10,20"])
+
+    assert make_arm(2).import_timeline_from_csv(path) == [[10, 20]]
+
+
+def test_import_rejects_angles_for_missing_servos(tmp_path):
+    path = write_csv(tmp_path / "t.csv", ["0,10,20,30,null"])
+
+    with pytest.raises(ValueError, match="more than 2 servos"):
+        make_arm(2).import_timeline_from_csv(path)
+
+
+def test_import_rejects_missing_servo_columns(tmp_path):
+    path = tmp_path / "t.csv"
+    path.write_text("Timestep,Servo1,Servo2\n0,10,20\n")
+
+    with pytest.raises(ValueError, match="Servo1-Servo4"):
+        make_arm(2).import_timeline_from_csv(str(path))
+
+
+def test_export_rejects_more_than_four_angles(tmp_path):
+    with pytest.raises(ValueError, match="more than 4 angles"):
+        make_arm(4).export_timeline_to_csv([[1, 2, 3, 4, 5]], str(tmp_path))
+
+    assert list(tmp_path.iterdir()) == []