diff --git a/artiq/gateware/drtio/rt_packets.py b/artiq/gateware/drtio/rt_packets.py
new file mode 100644
index 000000000..dff21b6f5
--- /dev/null
+++ b/artiq/gateware/drtio/rt_packets.py
@@ -0,0 +1,146 @@
+from migen import *
+from migen.genlib.fsm import *
+from migen.genlib.record import *
+
+class PacketLayoutManager:
+    def __init__(self, alignment):
+        self.alignment = alignment
+        self.layouts = dict()
+        self.types = dict()
+    
+    def add_type(self, name, *fields, pad=True):
+        self.types[name] = len(self.types)
+        layout = [("ty", 8)] + list(fields)
+        misalignment = layout_len(layout) % self.alignment
+        if misalignment:
+            layout.append(("packet_pad", self.alignment - misalignment))
+        self.layouts[name] = layout
+
+
+def get_m2s_layouts(alignment):
+    plm = PacketLayoutManager(alignment)
+    plm.add_type("echo_request")
+    plm.add_type("set_time", ("timestamp", 64))
+    plm.add_type("write", ("timestamp", 64),
+                          ("channel", 16),
+                          ("address", 16),
+                          ("data_len", 8),
+                          ("short_data", 8))
+    plm.add_type("fifo_level_request", ("channel", 16))
+    return plm
+
+
+def get_s2m_layouts(alignment):
+    plm = PacketLayoutManager(alignment)
+    plm.add_type("echo_reply")
+    plm.add_type("fifo_level_reply", ("level", 24))
+    return plm
+
+
+class ReceiveDatapath(Module):
+    def __init__(self, ws, plm):
+        # inputs
+        self.frame = Signal()
+        self.data = Signal(ws)
+
+        # control
+        self.packet_buffer_load = Signal()
+
+        # outputs
+        self.frame_r = Signal()
+        self.data_r = Signal()
+        self.packet_type = Signal(8)
+        self.packet_last = Signal()
+        self.packet_as = dict()
+
+        # # #
+
+        # input pipeline stage - determine packet length based on type
+        lastword_per_type = [layout_len(l)//ws - 1
+                             for l in plm.layouts.values()]
+        packet_last_n = Signal(max=max(lastword_per_type)+1)
+        self.sync += [
+            self.frame_r.eq(self.frame),
+            self.data_r.eq(self.data),
+            If(self.frame & ~self.frame_r,
+                self.packet_type.eq(self.data[:8]),
+                packet_last_n.eq(Array(lastword_per_type)[self.data[:8]])
+            )
+        ]
+
+        # bufferize packet
+        packet_buffer = Signal(max(layout_len(l)
+                                   for l in plm.layouts.values()))
+        w_in_packet = len(packet_buffer)//ws
+        packet_buffer_count = Signal(max=w_in_packet+1)
+        self.sync += \
+            If(self.packet_buffer_load,
+                Case(packet_buffer_count, 
+                     {i: packet_buffer[i*ws:(i+1)*ws].eq(self.data_r)
+                      for i in range(w_in_packet)}),
+                packet_buffer_count.eq(packet_buffer_count + 1)
+            ).Else(
+                packet_buffer_count.eq(0)
+            )
+        self.comb += self.packet_last.eq(packet_buffer_count == packet_last_n)
+
+        # cast packet
+        for name, layout in plm.layouts.items():
+            self.packet_as[name] = Record(layout)
+            self.comb += self.packet_as[name].raw_bits().eq(packet_buffer)
+
+
+class RTPacketSatellite(Module):
+    def __init__(self, nwords):
+        ws = 8*nwords
+        self.rx_rt_frame = Signal()
+        self.rx_rt_data = Signal(ws)
+
+        self.tx_rt_frame = Signal()
+        self.tx_rt_data = Signal(ws)
+
+        # # #
+
+        rx_plm = get_m2s_layouts(ws)
+        rx_dp = ReceiveDatapath(ws, rx_plm)
+        self.submodules += rx_dp
+        self.comb += [
+            rx_dp.frame.eq(self.rx_rt_frame),
+            rx_dp.data.eq(self.rx_rt_data)
+        ]
+
+        fsm = FSM(reset_state="WAIT_FRAME")
+        self.submodules += fsm
+
+        continuation = Signal()
+        continuation_r = Signal()
+        frame_r_r = Signal()
+        self.sync += [
+            continuation_r.eq(continuation),
+            frame_r_r.eq(rx_dp.frame_r)
+        ]
+        fsm.act("WAIT_INPUT",
+            If(rx_dp.frame_r,
+                If(~frame_r_r | continuation_r,
+                    continuation.eq(1),
+                    packet_buffer_load.eq(1),
+                    If(rx_dp.packet_last,
+                        Case(rx_dp.packet_type, {
+                            rx_plm.types["echo_request"]: NextState("ECHO"),
+                            "default": NextState("ERROR_UNKNOWN_TYPE")
+                        })
+                    )
+                ).Else(
+                    NextState("ERROR_FRAME_MISSED")
+                )
+            )
+        )
+        fsm.state("ECHO",
+            NextState("WAIT_INPUT")
+        )
+        fsm.state("ERROR_FRAME_MISSED",
+            NextState("WAIT_INPUT")
+        )
+        fsm.state("ERROR_UNKNOWN_TYPE",
+            NextState("WAIT_INPUT")
+        )