diff --git a/artiq/gateware/test/drtio/test_switching.py b/artiq/gateware/test/drtio/test_switching.py
new file mode 100644
index 000000000..66a9c9e69
--- /dev/null
+++ b/artiq/gateware/test/drtio/test_switching.py
@@ -0,0 +1,165 @@
+import unittest
+from types import SimpleNamespace
+import random
+
+from migen import *
+
+from artiq.gateware.drtio import *
+from artiq.gateware.drtio import rt_serializer, rt_packet_repeater
+from artiq.gateware import rtio
+from artiq.gateware.rtio import cri
+from artiq.coredevice.exceptions import *
+from artiq.gateware.test.drtio.packet_interface import PacketInterface
+
+
+class DummyTransceiverPair:
+    def __init__(self, nwords):
+        a2b_k = [Signal() for _ in range(nwords)]
+        a2b_d = [Signal(8) for _ in range(nwords)]
+        b2a_k = [Signal() for _ in range(nwords)]
+        b2a_d = [Signal(8) for _ in range(nwords)]
+
+        self.alice = SimpleNamespace(
+            encoder=SimpleNamespace(k=a2b_k, d=a2b_d),
+            decoders=[SimpleNamespace(k=k, d=d) for k, d in zip(b2a_k, b2a_d)],
+            rx_ready=1
+        )
+        self.bob = SimpleNamespace(
+            encoder=SimpleNamespace(k=b2a_k, d=b2a_d),
+            decoders=[SimpleNamespace(k=k, d=d) for k, d in zip(a2b_k, a2b_d)],
+            rx_ready=1
+        )
+
+
+class DummyRXSynchronizer:
+    def resync(self, signal):
+        return signal
+
+
+class DUT(Module):
+    def __init__(self, nwords):
+        self.transceivers = DummyTransceiverPair(nwords)
+
+        self.submodules.tsc_master = rtio.TSC("async")
+        self.submodules.master = DRTIOMaster(self.tsc_master,
+                                             self.transceivers.alice)
+        self.master.rt_controller.csrs.link_up.storage.reset = 1
+
+        rx_synchronizer = DummyRXSynchronizer()
+        self.submodules.tsc_satellite = rtio.TSC("sync")
+        self.submodules.satellite = DRTIOSatellite(
+            self.tsc_satellite, self.transceivers.bob, rx_synchronizer)
+        self.satellite.reset.storage.reset = 0
+        self.satellite.reset.storage_full.reset = 0
+        self.satellite.reset_phy.storage.reset = 0
+        self.satellite.reset_phy.storage_full.reset = 0
+
+        self.pt = PacketInterface("s2m", nwords*8)
+        self.pr = PacketInterface("m2s", nwords*8)
+        rep_if = SimpleNamespace(
+            rx_rt_frame=self.pt.frame, rx_rt_data=self.pt.data,
+            tx_rt_frame=self.pr.frame, tx_rt_data=self.pr.data)
+        self.submodules.repeater = rt_packet_repeater.RTPacketRepeater(
+            self.tsc_satellite, rep_if)
+        self.comb += self.satellite.cri.connect(self.repeater.cri)
+
+
+class Testbench:
+    def __init__(self):
+        self.dut = DUT(2)
+        self.now = 0
+
+    def init(self):
+        yield from self.dut.master.rt_controller.csrs.underflow_margin.write(100)
+        while not (yield from self.dut.master.link_layer.rx_up.read()):
+            yield
+        yield from self.get_buffer_space()
+
+    def get_buffer_space(self):
+        csrs = self.dut.master.rt_controller.csrs
+        yield from csrs.o_get_buffer_space.write(1)
+        yield
+        while (yield from csrs.o_wait.read()):
+            yield
+        r = (yield from csrs.o_dbg_buffer_space.read())
+        return r
+
+    def delay(self, dt):
+        self.now += dt
+
+    def write(self, channel, data):
+        mcri = self.dut.master.cri
+        yield mcri.chan_sel.eq(channel)
+        yield mcri.timestamp.eq(self.now)
+        yield mcri.o_data.eq(data)
+        yield
+        yield mcri.cmd.eq(cri.commands["write"])
+        yield
+        yield mcri.cmd.eq(cri.commands["nop"])
+        yield
+        status = yield mcri.o_status
+        while status & 0x1:
+            yield
+            status = yield mcri.o_status
+        if status & 0x2:
+            return "underflow"
+        if status & 0x4:
+            return "destination unreachable"
+
+
+class TestSwitching(unittest.TestCase):
+    clocks = {"sys": 8, "rtio": 5, "rtio_rx": 5,
+              "rio": 5, "rio_phy": 5,
+              "sys_with_rst": 8, "rtio_with_rst": 5}
+
+    def test_switching(self):
+        tb = Testbench()
+
+        def test():
+            yield from tb.init()
+            tb.delay(200)
+            yield from tb.write(1, 20)
+            for _ in range(40):
+                yield
+
+        current_request = None
+
+        def get_request():
+            nonlocal current_request
+            while current_request is None:
+                yield
+            r = current_request
+            current_request = None
+            return r
+
+        def expect_buffer_space_request(destination, space):
+            packet_type, field_dict, trailer = yield from get_request()
+            self.assertEqual(packet_type, "buffer_space_request")
+            self.assertEqual(trailer, [])
+            self.assertEqual(field_dict["destination"], destination)
+            yield from tb.dut.pt.send("buffer_space_reply", space=space)
+
+        def expect_write(timestamp, channel, data):
+            packet_type, field_dict, trailer = yield from get_request()
+            self.assertEqual(packet_type, "write")
+            self.assertEqual(trailer, [])
+            self.assertEqual(field_dict["timestamp"], timestamp)
+            self.assertEqual(field_dict["chan_sel"], channel)
+            self.assertEqual(field_dict["short_data"], data)
+
+        @passive
+        def send_replies():
+            yield from expect_buffer_space_request(0, 1)
+            yield from expect_write(200, 1, 20)
+            yield from expect_buffer_space_request(0, 1)
+
+            unexpected = yield from get_request()
+            self.fail("unexpected packet: {}".format(unexpected))
+
+        def receive(packet_type, field_dict, trailer):
+            nonlocal current_request
+            self.assertEqual(current_request, None)
+            current_request = (packet_type, field_dict, trailer)
+
+        run_simulation(tb.dut,
+            {"sys": test(), "rtio": tb.dut.pr.receive(receive), "rtio_rx": send_replies()}, self.clocks)