diff --git a/artiq/coredevice/exceptions.py b/artiq/coredevice/exceptions.py
index d0f54f2fa..edaf8f14e 100644
--- a/artiq/coredevice/exceptions.py
+++ b/artiq/coredevice/exceptions.py
@@ -26,6 +26,17 @@ class RTIOSequenceError(ARTIQException):
     The offending event is discarded and the RTIO core keeps operating.
     """
 
+class RTIOCollisionError(ARTIQException):
+    """Raised when an event is submitted on a given channel with the same
+    coarse timestamp as the previous one but with a different fine timestamp.
+
+    Coarse timestamps correspond to the RTIO system clock (typically around
+    125MHz) whereas fine timestamps correspond to the RTIO SERDES clock
+    (typically around 1GHz).
+
+    The offending event is discarded and the RTIO core keeps operating.
+    """
+
 class RTIOOverflow(ARTIQException):
     """Raised when at least one event could not be registered into the RTIO
     input FIFO because it was full (CPU not reading fast enough).
diff --git a/artiq/test/coredevice/rtio.py b/artiq/test/coredevice/rtio.py
index 43acdc237..b12f98629 100644
--- a/artiq/test/coredevice/rtio.py
+++ b/artiq/test/coredevice/rtio.py
@@ -5,7 +5,8 @@ from math import sqrt
 
 from artiq.language import *
 from artiq.test.hardware_testbench import ExperimentCase
-from artiq.coredevice.exceptions import RTIOUnderflow, RTIOSequenceError
+from artiq.coredevice.exceptions import (RTIOUnderflow, RTIOSequenceError,
+                                         RTIOCollisionError)
 
 
 class RTT(EnvExperiment):
@@ -235,7 +236,7 @@ class CoredeviceTest(ExperimentCase):
             self.execute(SequenceError)
 
     def test_collision_error(self):
-        with self.assertRaises(runtime_exceptions.RTIOCollisionError):
+        with self.assertRaises(RTIOCollisionError):
             self.execute(CollisionError)
 
     def test_watchdog(self):