diff --git a/motor_test.py b/motor_test.py
index 3084a50..0c43fcd 100644
--- a/motor_test.py
+++ b/motor_test.py
@@ -25,6 +25,7 @@ class Driver:
         self.msgs = 0
         self.other = []         # last few lines that weren't $MAll, for debugging
         self.alive = True
+        self.error = None       # set if the serial port fails in the reader thread
         threading.Thread(target=self._rx, daemon=True).start()
         self.send("$pwm:0,0,0,0#")
         self.send("$mtype:1#")          # 1 = Yahboom 520 motor
@@ -51,7 +52,11 @@ class Driver:
     def _rx(self):
         buf = b""
         while self.alive:
-            buf += self.ser.read(self.ser.in_waiting or 1)
+            try:
+                buf += self.ser.read(self.ser.in_waiting or 1)
+            except serial.SerialException as e:
+                self.error = str(e)
+                return
             while b"#" in buf:
                 line, buf = buf.split(b"#", 1)
                 i = line.rfind(b"$")
@@ -78,6 +83,8 @@ def hold(d, ch, value, seconds):
     """Keep re-sending the PWM so it holds even if the board has a command timeout."""
     end = time.perf_counter() + seconds
     while time.perf_counter() < end:
+        if d.error:
+            sys.exit(f"\nSerial error: {d.error}")
         d.pwm(ch, value)
         time.sleep(0.1)
 
@@ -88,6 +95,8 @@ def hand(d):
     print("Turn ONE wheel by hand, one full turn FORWARD (as if the robot drives toward the tape).")
     print("Then the other wheel. Ctrl-C when done.\n")
     while True:
+        if d.error:
+            sys.exit(f"\nSerial error: {d.error}")
         t, n, _ = d.get()
         m3 = t[2] - base[2]
         m4 = t[3] - base[3]
@@ -102,15 +111,17 @@ def spin(d):
         print(f"\n--- {name} ---")
         for p in SPIN_STEPS:
             hold(d, ch, p, 0.5)                     # let it spin up
-            t0, _, _ = d.get(); s0 = time.perf_counter()
+            t0, n0, _ = d.get(); s0 = time.perf_counter()
             hold(d, ch, p, 1.0)                     # measure over 1 s
-            t1, _, _ = d.get(); s1 = time.perf_counter()
+            t1, n1, other = d.get(); s1 = time.perf_counter()
             cps = (t1[ch] - t0[ch]) / (s1 - s0)
             rpm = cps / TICKS_PER_REV * 60
-            print(f"pwm {p:+5d}  ->  {cps:+8.0f} counts/s   {rpm:+6.1f} wheel rpm")
+            print(f"pwm {p:+5d}  ->  {cps:+8.0f} counts/s   {rpm:+6.1f} wheel rpm   {n1 - n0:4d} msgs")
         d.stop()
         time.sleep(1.0)
-    print("\nDone.")
+    _, _, other = d.get()
+    print("\nnon-encoder lines from the board:", other if other else "none")
+    print("Done.")
 
 
 def main():