diff --git a/artiq/gateware/drtio/rt_packets.py b/artiq/gateware/drtio/rt_packets.py
index dff21b6f5..199cc2071 100644
--- a/artiq/gateware/drtio/rt_packets.py
+++ b/artiq/gateware/drtio/rt_packets.py
@@ -1,15 +1,24 @@
+from types import SimpleNamespace
+
 from migen import *
 from migen.genlib.fsm import *
-from migen.genlib.record import *
+
+
+def layout_len(l):
+    return sum(e[1] for e in l)
+
 
 class PacketLayoutManager:
     def __init__(self, alignment):
         self.alignment = alignment
         self.layouts = dict()
         self.types = dict()
+        self.type_names = dict()
     
     def add_type(self, name, *fields, pad=True):
-        self.types[name] = len(self.types)
+        type_n = len(self.types)
+        self.types[name] = type_n
+        self.type_names[type_n] = name
         layout = [("ty", 8)] + list(fields)
         misalignment = layout_len(layout) % self.alignment
         if misalignment:
@@ -32,11 +41,18 @@ def get_m2s_layouts(alignment):
 
 def get_s2m_layouts(alignment):
     plm = PacketLayoutManager(alignment)
+    plm.add_type("error", ("code", 8))
     plm.add_type("echo_reply")
     plm.add_type("fifo_level_reply", ("level", 24))
     return plm
 
 
+error_codes = {
+    "frame_missed": 0,
+    "unknown_type": 1
+}
+
+
 class ReceiveDatapath(Module):
     def __init__(self, ws, plm):
         # inputs
@@ -56,8 +72,8 @@ class ReceiveDatapath(Module):
         # # #
 
         # input pipeline stage - determine packet length based on type
-        lastword_per_type = [layout_len(l)//ws - 1
-                             for l in plm.layouts.values()]
+        lastword_per_type = [layout_len(plm.layouts[plm.type_names[i]])//ws - 1
+                             for i in range(len(plm.layouts))]
         packet_last_n = Signal(max=max(lastword_per_type)+1)
         self.sync += [
             self.frame_r.eq(self.frame),
@@ -84,10 +100,73 @@ class ReceiveDatapath(Module):
             )
         self.comb += self.packet_last.eq(packet_buffer_count == packet_last_n)
 
-        # cast packet
+        # dissect 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)
+            fields = SimpleNamespace()
+            idx = 0
+            for field_name, field_size in layout:
+                setattr(fields, field_name, packet_buffer[idx:idx+field_size])
+                idx += field_size
+            self.packet_as[name] = fields
+
+
+class TransmitDatapath(Module):
+    def __init__(self, ws, plm):
+        self.ws = ws
+        self.plm = plm
+
+        # inputs
+        self.packet_buffer = Signal(max(layout_len(l)
+                                        for l in plm.layouts.values()))
+        w_in_packet = len(self.packet_buffer)//ws
+        self.packet_len = Signal(max=w_in_packet+1)
+
+        # control
+        self.stb = Signal()
+        self.done = Signal()
+
+        # outputs
+        self.frame = Signal()
+        self.data = Signal(ws)
+
+        # # #
+
+        packet_buffer_count = Signal(max=w_in_packet+1)
+        self.sync += [
+            self.done.eq(0),
+            self.frame.eq(0),
+            packet_buffer_count.eq(0),
+
+            If(self.stb & ~self.done,
+                If(packet_buffer_count == self.packet_len,
+                    self.done.eq(1)
+                ).Else(
+                    self.frame.eq(1),
+                    Case(packet_buffer_count, 
+                         {i: self.data.eq(self.packet_buffer[i*ws:(i+1)*ws])
+                          for i in range(w_in_packet)}),
+                    packet_buffer_count.eq(packet_buffer_count + 1)
+                )
+            )
+        ]
+
+    def send(self, ty, **kwargs):
+        idx = 8
+        value = self.plm.types[ty]
+        for field_name, field_size in self.plm.layouts[ty][1:]:
+            try:
+                fvalue = kwargs[field_name]
+                del kwargs[field_name]
+            except KeyError:
+                fvalue = 0
+            value = value | (fvalue << idx)
+            idx += field_size
+        if kwargs:
+            raise ValueError
+        return [
+            self.packet_buffer.eq(value),
+            self.packet_len.eq(idx//self.ws)
+        ]
 
 
 class RTPacketSatellite(Module):
@@ -109,7 +188,15 @@ class RTPacketSatellite(Module):
             rx_dp.data.eq(self.rx_rt_data)
         ]
 
-        fsm = FSM(reset_state="WAIT_FRAME")
+        tx_plm = get_s2m_layouts(ws)
+        tx_dp = TransmitDatapath(ws, tx_plm)
+        self.submodules += tx_dp
+        self.comb += [
+            self.tx_rt_frame.eq(tx_dp.frame),
+            self.tx_rt_data.eq(tx_dp.data)
+        ]
+
+        fsm = FSM(reset_state="WAIT_INPUT")
         self.submodules += fsm
 
         continuation = Signal()
@@ -123,7 +210,7 @@ class RTPacketSatellite(Module):
             If(rx_dp.frame_r,
                 If(~frame_r_r | continuation_r,
                     continuation.eq(1),
-                    packet_buffer_load.eq(1),
+                    rx_dp.packet_buffer_load.eq(1),
                     If(rx_dp.packet_last,
                         Case(rx_dp.packet_type, {
                             rx_plm.types["echo_request"]: NextState("ECHO"),
@@ -135,12 +222,18 @@ class RTPacketSatellite(Module):
                 )
             )
         )
-        fsm.state("ECHO",
-            NextState("WAIT_INPUT")
+        fsm.act("ECHO",
+            tx_dp.send("echo_reply"),
+            tx_dp.stb.eq(1),
+            If(tx_dp.done, NextState("WAIT_INPUT"))
         )
-        fsm.state("ERROR_FRAME_MISSED",
-            NextState("WAIT_INPUT")
+        fsm.act("ERROR_FRAME_MISSED",
+            tx_dp.send("error", code=error_codes["frame_missed"]),
+            tx_dp.stb.eq(1),
+            If(tx_dp.done, NextState("WAIT_INPUT"))
         )
-        fsm.state("ERROR_UNKNOWN_TYPE",
-            NextState("WAIT_INPUT")
+        fsm.act("ERROR_UNKNOWN_TYPE",
+            tx_dp.send("error", code=error_codes["unknown_type"]),
+            tx_dp.stb.eq(1),
+            If(tx_dp.done, NextState("WAIT_INPUT"))
         )