diff --git a/software/glasgow/access/direct/demultiplexer.py b/software/glasgow/access/direct/demultiplexer.py
index eebc0967d..76f1320c3 100644
--- a/software/glasgow/access/direct/demultiplexer.py
+++ b/software/glasgow/access/direct/demultiplexer.py
@@ -151,7 +151,9 @@ async def claim_interface(self, applet, mux_interface, args, pull_low=set(), pul
 
 class DirectDemultiplexerInterface(AccessDemultiplexerInterface):
     def __init__(self, device, applet, mux_interface,
-                 read_buffer_size=None, write_buffer_size=None):
+                 read_buffer_size=None, write_buffer_size=None,
+                 read_packets_per_xfer=_packets_per_xfer, read_xfers_per_queue=_xfers_per_queue,
+                 write_packets_per_xfer=_packets_per_xfer, write_xfers_per_queue=_xfers_per_queue):
         super().__init__(device, applet)
 
         self._write_buffer_size = write_buffer_size
@@ -162,6 +164,11 @@ def __init__(self, device, applet, mux_interface,
         self._pipe_num   = mux_interface._pipe_num
         self._addr_reset = mux_interface._addr_reset
 
+        self.read_packets_per_xfer = read_packets_per_xfer
+        self.read_xfers_per_queue = read_xfers_per_queue
+        self.write_packets_per_xfer = write_packets_per_xfer
+        self.write_xfers_per_queue = write_xfers_per_queue
+
         config_num = self.device.usb_handle.getConfiguration()
         for config in self.device.usb_handle.getDevice().iterConfigurations():
             if config.getConfigurationValue() == config_num:
@@ -214,7 +221,7 @@ async def reset(self):
         # streaming data, there are no overflows. (This is perhaps not the best way to implement
         # an applet, but we can support it easily enough, and it avoids surprise overflows.)
         self.logger.trace("FIFO: pipelining reads")
-        for _ in range(_xfers_per_queue):
+        for _ in range(self.read_xfers_per_queue):
             self._in_tasks.submit(self._in_task())
         # Give the IN tasks a chance to submit their transfers before deasserting reset.
         await asyncio.sleep(0)
@@ -229,7 +236,7 @@ async def _in_task(self):
                     self.logger.trace("FIFO: read pushback")
                     await self._in_pushback.wait()
 
-        size = self._in_packet_size * _packets_per_xfer
+        size = self._in_packet_size * self.read_packets_per_xfer
         data = await self.device.bulk_read(self._endpoint_in, size)
         self._in_buffer.write(data)
 
@@ -277,7 +284,7 @@ async def read(self, length=None, *, flush=True):
 
     def _out_slice(self):
         # Fast path: read as much contiguous data as possible, up to our transfer size.
-        size = self._out_packet_size * _packets_per_xfer
+        size = self._out_packet_size * self.write_packets_per_xfer
         data = self._out_buffer.read(size)
 
         if len(data) < self._out_packet_size:
@@ -293,7 +300,7 @@ def _out_slice(self):
 
     @property
     def _out_threshold(self):
-        out_xfer_size = self._out_packet_size * _packets_per_xfer
+        out_xfer_size = self._out_packet_size * self.write_packets_per_xfer
         if self._write_buffer_size is None:
             return out_xfer_size
         else:
@@ -330,7 +337,7 @@ async def write(self, data):
         # The write scheduling algorithm attempts to satisfy several partially conflicting goals:
         #  * We want to schedule writes as early as possible, because this reduces buffer bloat and
         #    can dramatically improve responsiveness of the system.
-        #  * We want to schedule writes that are as large as possible, up to _packets_per_xfer,
+        #  * We want to schedule writes that are as large as possible, up to write_packets_per_xfer,
         #    because this reduces CPU utilization and improves latency.
         #  * We never want to automatically schedule writes smaller than _out_packet_size,
         #    because they occupy a whole microframe anyway.
@@ -338,17 +345,17 @@ async def write(self, data):
         # We use an approach that performs well when fed with a steady sequence of very large
         # FIFO chunks, yet scales down to packet-size and byte-size FIFO chunks as well.
         #  * We only submit a write automatically once the buffer level crosses the threshold of
-        #    `_out_packet_size * _packets_per_xfer`. In this case, _slice_packet always returns
-        #    `_out_packet_size * n` bytes, where n is between 1 and _packet_per_xfer.
+        #    `_out_packet_size * write_packets_per_xfer`. In this case, _slice_packet always returns
+        #    `_out_packet_size * n` bytes, where n is between 1 and write_packets_per_xfer.
         #  * We submit enough writes that there is at least one write for each transfer worth
-        #    of data in the buffer, up to _xfers_per_queue outstanding writes.
+        #    of data in the buffer, up to write_xfers_per_queue outstanding writes.
         #  * We submit another write once one finishes, if the buffer level is still above
         #    the threshold, even if no more explicit write calls are performed.
         #
-        # This provides predictable write behavior; only _packets_per_xfer packet writes are
+        # This provides predictable write behavior; only write_packets_per_xfer packet writes are
         # automatically submitted, and only the minimum necessary number of tasks are scheduled on
         # calls to `write`.
-        while len(self._out_tasks) < _xfers_per_queue and \
+        while len(self._out_tasks) < self.write_xfers_per_queue and \
                     len(self._out_buffer) >= self._out_threshold:
             self._out_tasks.submit(self._out_task(self._out_slice()))
 
@@ -357,16 +364,16 @@ async def flush(self, wait=True):
 
         # First, we ensure we can submit one more task. (There can be more tasks than
         # _xfers_per_queue because a task may spawn another one just before it terminates.)
-        if len(self._out_tasks) >= _xfers_per_queue:
+        if len(self._out_tasks) >= self.write_xfers_per_queue:
             self._out_stalls += 1
-        while len(self._out_tasks) >= _xfers_per_queue:
+        while len(self._out_tasks) >= self.write_xfers_per_queue:
             await self._out_tasks.wait_one()
 
-        # At this point, the buffer can contain at most _packets_per_xfer packets worth
+        # At this point, the buffer can contain at most write_packets_per_xfer packets worth
         # of data, as anything beyond that crosses the threshold of automatic submission.
         # So, we can simply submit the rest of data, which by definition fits into a single
         # transfer.
-        assert len(self._out_buffer) <= self._out_packet_size * _packets_per_xfer
+        assert len(self._out_buffer) <= self._out_packet_size * self.write_packets_per_xfer
         if self._out_buffer:
             data = bytearray()
             while self._out_buffer:
diff --git a/software/glasgow/applet/internal/fixed_throughput/__init__.py b/software/glasgow/applet/internal/fixed_throughput/__init__.py
new file mode 100644
index 000000000..932b2e0b2
--- /dev/null
+++ b/software/glasgow/applet/internal/fixed_throughput/__init__.py
@@ -0,0 +1,119 @@
+import logging
+import asyncio
+import argparse
+import time
+from amaranth import *
+
+from ... import *
+
+
+class FixedThroughputSubtarget(Elaboratable):
+    def __init__(self, rate_reg, in_fifo):
+        self.rate_reg = rate_reg
+        self.in_fifo = in_fifo
+
+    def elaborate(self, platform):
+        m = Module()
+
+        data_rate_count = Signal(8)
+        data_valid = Signal()
+        data = Signal(8)
+        overflow = Signal()
+
+        m.d.comb += data.eq(Cat(overflow, Const(0, 7)))
+
+        # delta sigma ish
+        m.d.sync += Cat(data_rate_count, data_valid).eq(data_rate_count + self.rate_reg + 1)
+
+        with m.If(data_valid):
+            with m.If(self.in_fifo.w_rdy):
+                m.d.comb += [
+                    self.in_fifo.w_data.eq(data),
+                    self.in_fifo.w_en.eq(1),
+                ]
+            with m.Else():
+                m.d.sync += overflow.eq(1)
+
+        return m
+
+
+class FixedThroughputApplet(GlasgowApplet):
+    logger = logging.getLogger(__name__)
+    help = "evaluate fixed read throughput performance"
+    description = """
+    Evaluate fixed read throughput performance and check for in FIFO overflows 
+    """
+
+    @classmethod
+    def add_build_arguments(cls, parser, access):
+        pass
+
+    def build(self, target, args):
+        self.mux_interface = iface = \
+            target.multiplexer.claim_interface(self, args=None, throttle="none")
+
+        rate_reg, self.__addr_rate = target.registers.add_rw(8)
+
+        subtarget = iface.add_subtarget(
+            FixedThroughputSubtarget(
+                rate_reg,
+                iface.get_in_fifo(auto_flush=False)
+            )
+        )
+
+    @classmethod
+    def add_run_arguments(cls, parser, access):
+        parser.add_argument(
+            "--rpkts", metavar="READ-PACKETS", type=int,
+            help="How many packets per read transfer")
+
+        parser.add_argument(
+            "--rxfers", metavar="READ-TRANSFERS", type=int,
+            help="How many read transfers to have active at a time")
+
+    async def run(self, device, args):
+        kwargs = {}
+        if args.rpkts is not None:
+            kwargs["read_packets_per_xfer"] = args.rpkts
+        if args.rxfers is not None:
+            kwargs["read_xfers_per_queue"] = args.rxfers
+
+        return await device.demultiplexer.claim_interface(self, self.mux_interface, args=None, **kwargs)
+
+    @classmethod
+    def add_interact_arguments(cls, parser):
+        parser.add_argument(
+            "rate_mbps", metavar="rate", type=float,
+            help="data rate in Mbps")
+
+    async def interact(self, device, args, iface):
+        data_rate = round(args.rate_mbps/8 * 1e6 / 48e6 * 256) - 1
+        await device.write_register(self.__addr_rate, data_rate)
+        await iface.reset()
+
+        try:
+            count = 0
+            begin = time.time()
+            overflow = False
+
+            while not overflow:
+                data = await iface.read()
+                data_list = list(data)
+
+                count += len(data_list)
+                overflow = 1 in data_list
+
+        finally:
+            duration = time.time() - begin
+            mbps = count*8 / duration / 1e6
+            expected_mbps = (data_rate+1)/256 * 8 * 48
+
+            print(f"{overflow=}")
+            print(f"Elapsed: {duration}")
+            print(f"Mbps: {mbps}")
+            print(f"Expected Mbps: {expected_mbps}")
+
+    @classmethod
+    def tests(cls):
+        from . import test
+        return test.FixedThroughputAppletTestCase
diff --git a/software/glasgow/applet/internal/fixed_throughput/test.py b/software/glasgow/applet/internal/fixed_throughput/test.py
new file mode 100644
index 000000000..2192ec654
--- /dev/null
+++ b/software/glasgow/applet/internal/fixed_throughput/test.py
@@ -0,0 +1,8 @@
+from ... import *
+from . import FixedThroughputApplet
+
+
+class FixedThroughputAppletTestCase(GlasgowAppletTestCase, applet=FixedThroughputApplet):
+    @synthesis_test
+    def test_build(self):
+        self.assertBuilds()
diff --git a/software/pyproject.toml b/software/pyproject.toml
index edcad145e..bf120cc5b 100644
--- a/software/pyproject.toml
+++ b/software/pyproject.toml
@@ -78,6 +78,7 @@ glasgow = "glasgow.cli:run_main"
 [project.entry-points."glasgow.applet"]
 selftest = "glasgow.applet.internal.selftest:SelfTestApplet"
 benchmark = "glasgow.applet.internal.benchmark:BenchmarkApplet"
+fixed-throughput = "glasgow.applet.internal.fixed_throughput:FixedThroughputApplet"
 
 analyzer = "glasgow.applet.interface.analyzer:AnalyzerApplet"
 uart = "glasgow.applet.interface.uart:UARTApplet"