csaxs_bec/devices/epics/delay_generator_csaxs/ddg_2.py

@@ -22,7 +22,10 @@ Burst mode is enabled:
 - Burst period is set to the exposure time.
 """
 
+from __future__ import annotations
+
 import time
+from typing import TYPE_CHECKING
 
 from bec_lib.logger import bec_logger
 from bec_server.scan_server.scans.scan_base import ScanInfo as ScanServerScanInfo
@@ -42,6 +45,9 @@ from csaxs_bec.devices.epics.delay_generator_csaxs.delay_generator_csaxs import
 )
 from csaxs_bec.devices.utils.utils import fetch_scan_info
 
+if TYPE_CHECKING:  # pragma: no cover
+    from bec_lib.devicemanager import DeviceManagerBase, ScanInfo
+
 logger = bec_logger.logger
 
 ########################
@@ -113,6 +119,96 @@ class DDG2(PSIDeviceBase, DelayGeneratorCSAXS):
     distributes the trigger to the detectors. The DDG2 is triggered by the DDG1 through the EXT/EN channel.
     """
 
+    # Methods exposed to the BEC client, i.e. callable as dev.ddg2.<name>()
+    USER_ACCESS = ["get_readout_times", "set_readout_times", "effective_readout_times"]
+
+    def __init__(
+        self,
+        name: str,
+        prefix: str = "",
+        scan_info: ScanInfo | None = None,
+        device_manager: DeviceManagerBase | None = None,
+        readout_times: dict[str, float] | None = None,
+        **kwargs,
+    ):
+        """
+        Args:
+            readout_times: per-channel-pair readout times in seconds, merged over
+                DEFAULT_READOUT_TIMES. Only the pairs given are overridden, e.g.
+                ``{"ab": 3.0e-3}``. See set_readout_times() for what this controls.
+        """
+        super().__init__(
+            name=name, prefix=prefix, scan_info=scan_info, device_manager=device_manager, **kwargs
+        )
+        self._readout_times: dict[str, float] = dict(DEFAULT_READOUT_TIMES)
+        if readout_times:
+            self.set_readout_times(**readout_times)
+
+    def effective_readout_times(self) -> dict[str, float]:
+        """Return the readout times actually in force for the upcoming scan.
+
+        The scan may request a readout time of its own; it wins when it is larger than
+        this device's configured value. Scans default readout_time to 0, so the
+        configured value is the floor rather than a fallback.
+
+        The result defines the exposure window -- period minus gap -- that every
+        detector in the scan is expected to integrate over, whether it reads the gate
+        (Falcon) or runs its own timer from the rising edge (Eiger).
+
+        Returns:
+            dict[str, float]: effective readout time per channel pair, in seconds.
+        """
+        scan_readout = 0.0
+        if self.scan_parameters is not None:
+            scan_readout = float(getattr(self.scan_parameters, "readout_time", 0.0) or 0.0)
+        return {ch: max(scan_readout, rt) for ch, rt in self._readout_times.items()}
+
+    def get_readout_times(self) -> dict[str, float]:
+        """Return the readout times per channel pair, in seconds.
+
+        Returns:
+            dict[str, float]: a copy of the effective readout times.
+        """
+        return dict(self._readout_times)
+
+    def set_readout_times(self, **channels: float) -> dict[str, float]:
+        """Set the readout time of one or more channel pairs, in seconds.
+
+        The readout time is subtracted from the exposure time to obtain the pulse
+        width, while the burst period stays at the exposure time. Raising it therefore
+        widens the gap between consecutive detector triggers and shortens the exposure
+        by the same amount; the frame rate is unaffected.
+
+        This matters for detectors that need a minimum dead time between frames. The
+        default of 0.2 ms is shorter than some detectors can handle -- FalconcSAXS, for
+        instance, declares MIN_READOUT = 3 ms -- and a value that is too short shows up
+        as silently dropped frames rather than as an error.
+
+        Channel pair 'ab' is the one multiplexed to the detectors, so it is normally the
+        only one worth changing. The new value applies from the next on_stage, i.e. the
+        next scan; it does not alter a scan already staged.
+
+        Args:
+            **channels: channel pair to readout time in seconds, e.g. ``ab=3.0e-3``.
+
+        Returns:
+            dict[str, float]: the effective readout times after the update.
+
+        Raises:
+            ValueError: for an unknown channel pair or a non-positive value.
+        """
+        for channel, value in channels.items():
+            if channel not in self._readout_times:
+                raise ValueError(
+                    f"Unknown channel pair {channel!r}; expected one of "
+                    f"{sorted(self._readout_times)}"
+                )
+            if value <= 0:
+                raise ValueError(f"readout time for {channel!r} must be > 0, got {value}")
+            self._readout_times[channel] = float(value)
+        logger.info(f"DDG {self.name} readout times set to {self._readout_times}")
+        return self.get_readout_times()
+
     def on_init(self) -> None:
         """Initialize the device"""
         self.scan_parameters: ScanServerScanInfo | None = None
@@ -193,9 +289,14 @@ class DDG2(PSIDeviceBase, DelayGeneratorCSAXS):
 
         # NOTE Check if the exposure time is longer than all readout times.
         # Raise a ValueError if requested exposure time is too short.
-        if any(exp_time <= rt for rt in DEFAULT_READOUT_TIMES.values()):
+        # NOTE The gap is the scan's readout_time when the scan asks for one, otherwise this
+        # device's configured value. Scans default readout_time to 0, hence the max().
+        # Every detector must derive its exposure window from the same number -- see
+        # effective_readout_times().
+        readouts = self.effective_readout_times()
+        if any(exp_time <= rt for rt in readouts.values()):
             raise ValueError(
-                f"Exposure time {exp_time} is too short for the readout times {DEFAULT_READOUT_TIMES}"
+                f"Exposure time {exp_time} is too short for the readout times {readouts}"
             )
 
         #########################################
@@ -208,8 +309,13 @@ class DDG2(PSIDeviceBase, DelayGeneratorCSAXS):
         if self.burst_period.get() != exp_time:
             self.burst_period.put(exp_time)
 
-        # Calculate the pulse width for the channel pair 'ab'
-        burst_pulse_width = exp_time - DEFAULT_READOUT_TIMES["ab"]
+        # Calculate the pulse width for the channel pair 'ab'. The readout time is the
+        # gap left between consecutive triggers; see set_readout_times().
+        burst_pulse_width = exp_time - readouts["ab"]
+        logger.info(
+            f"DDG {self.name} exposure window: period {exp_time}s, gap {readouts['ab']}s, "
+            f"pulse width {burst_pulse_width}s"
+        )
 
         # Trigger detectors with delay 0, and pulse width = exp_time - readout_time
         self.set_delay_pairs(channel="ab", delay=0, width=burst_pulse_width)

csaxs_bec/devices/jungfraujoch/eiger.py

@@ -8,10 +8,21 @@ A few notes on setup and operation of the Eiger detectors through the JungfrauJo
 - Changes in energy may take time,  good to implement logic that only resets energy if needed.
 - For the Eiger, the frame_time_us in DetectorSettings is ignored, only the frame_time_us in
   the DatasetSettings is relevant
-- The bit_depth will be adjusted automatically based on the exp_time. Here, we need to ensure
-  that subsequent triggers properly consider the readout_time of the boards. For the Eiger detectors
-  at cSAXS, a readout time of 20us is configured through the JungfrauJoch deployment config. This
-  setting is sufficiently large for the detectors if they run in parallel mode.
+- The bit_depth will be adjusted automatically based on the exp_time (32 bit above 2622 us),
+  and in 32 bit the detector accumulates ~2.3 ms subframes. JungfrauJoch's own deployment config
+  budgets 20 us of board readout between frames, which is right for 16 bit parallel mode but far
+  smaller than the subframe granularity -- so a frame can outlast the trigger period without
+  anything reporting an error.
+- The Eiger is TRIGGERED rather than gated (DetectorTiming.TRIGGER), for stability: gating
+  it proved unreliable. It therefore ignores the gate width and runs its own timer from the
+  rising edge. The pulse train is deliberately shaped so that this timer coincides with the
+  gate -- the Eiger is meant to behave *as if* it were gated, so that every detector in the
+  scan integrates the same window. Sending it anything other than the gated window breaks
+  that, silently, and is only visible on a scope.
+- scan_info's exp_time is the trigger PERIOD, not the exposure. The delay generator gates for
+  exp_time - readout_time, and on_stage sends that same window as image_time_us so the detector
+  is not still acquiring when the next trigger arrives. The readout time must agree with DDG2's
+  DEFAULT_READOUT_TIMES["ab"]; a scan-level readout_time overrides both when larger.
 - beam_center and detector settings are required input arguments, thus, they may be set to wrong
   values for acquisitions to start. Please keep this in mind.
 
@@ -65,7 +76,19 @@ if TYPE_CHECKING:  # pragma no cover
 
 logger = bec_logger.logger
 
-EIGER_READOUT_TIME_US = 500e-6  # 500 microseconds in s
+# NOTE Two distinct numbers that used to be one. MIN_EXP_TIME is a validation floor
+# (its value is inherited from the former EIGER_READOUT_TIME_US and has no documented
+# provenance). EIGER_READOUT_TIME is the gap left between the end of one acquisition and
+# the next trigger; it must match the gap the delay generator leaves in the pulse train,
+# DDG2's DEFAULT_READOUT_TIMES["ab"], or the detector is told to acquire for longer than
+# the trigger period allows.
+MIN_EXP_TIME = 500e-6  # s
+EIGER_READOUT_TIME = 2e-4  # s -- matches DDG2 DEFAULT_READOUT_TIMES["ab"]
+# NOTE Each model declares its own value and passes it here; that duplication is
+# deliberate, since readout is a property of the detector (a 9M has more modules than a
+# 1.5M, and the Falcon needs 3 ms). They all happen to be 2e-4 today only because no
+# measured per-model number exists yet. This constant is the fallback for a model that
+# does not state one; deviceConfig's readout_time overrides it per deployment.
 
 
 class EigerError(Exception):
@@ -106,7 +129,7 @@ class Eiger(PSIDeviceBase):
         detector_distance: float = 100.0,
         beam_center: tuple[int, int] = (0, 0),
         scan_info: ScanInfo = None,
-        readout_time: float = EIGER_READOUT_TIME_US,
+        readout_time: float = EIGER_READOUT_TIME,
         device_manager=None,
         **kwargs,
     ):
@@ -291,6 +314,10 @@ class Eiger(PSIDeviceBase):
         # self.jfj_client.connect_and_initialise(timeout=10)
 
         # Setup Detector settings, here we may also set the energy already as this might be time consuming
+        # NOTE frame_time_us is required by the API but ignored for the Eiger ("For EIGER
+        # detector this is default frame time, not used otherwise"); the exposure comes
+        # from image_time_us in DatasetSettings. The 500 is a placeholder -- it is NOT the
+        # readout time, and it is in microseconds, unlike every other time in this module.
         settings = DetectorSettings(frame_time_us=int(500), timing=DetectorTiming.TRIGGER)
         self.jfj_client.set_detector_settings(settings, timeout=5)
 
@@ -344,9 +371,21 @@ class Eiger(PSIDeviceBase):
 
         # Setting up exp_time and num_triggers acquisition parameter
         exp_time = self.scan_parameters.exp_time
-        if exp_time <= self._readout_time:  # Exp_time must be at least the readout time
+        if exp_time <= MIN_EXP_TIME:
             raise ValueError(
-                f"Value error on device {self.name}: Exposure time {exp_time}s is less than readout time {self._readout_time}s."
+                f"Value error on device {self.name}: Exposure time {exp_time}s is below the "
+                f"minimum exposure time {MIN_EXP_TIME}s."
+            )
+        # NOTE exp_time is the trigger PERIOD, not the exposure. The delay generator gates
+        # for exp_time - readout_time, so the detector must acquire for the same window or
+        # it is still busy when the next trigger arrives. Scans default readout_time to 0,
+        # hence the max() against this device's configured value.
+        readout_time = max(self.scan_parameters.readout_time, self._readout_time)
+        acquisition_time = exp_time - readout_time
+        if acquisition_time <= 0:
+            raise ValueError(
+                f"Value error on device {self.name}: readout time {readout_time}s leaves no "
+                f"acquisition time within the trigger period {exp_time}s."
             )
         self._num_triggers = int(
             self.scan_parameters.num_points * self.scan_parameters.frames_per_trigger
@@ -369,7 +408,7 @@ class Eiger(PSIDeviceBase):
 
         # Create dataset settings for API call.
         data_settings = DatasetSettings(
-            image_time_us=int(exp_time * 1e6),
+            image_time_us=int(acquisition_time * 1e6),
             ntrigger=self._num_triggers,
             file_prefix=path,
             beam_x_pxl=float(self._beam_center[0]),
@@ -379,6 +418,10 @@ class Eiger(PSIDeviceBase):
         )
         logger.debug(f"Setting data_settings: {yaml.dump(data_settings.to_dict(), indent=4)}")
         prep_time = time.time()
+        logger.info(
+            f"{self.name} exposure window: period {exp_time}s, readout {readout_time}s, "
+            f"acquisition {acquisition_time}s"
+        )
         self.jfj_client.wait_for_idle(timeout=10)  # Ensure we are in IDLE state
         self.jfj_client.start(settings=data_settings)  # Takes around ~0.6s -> 6s currently.
 

csaxs_bec/devices/jungfraujoch/eiger_1_5m.py

@@ -11,7 +11,11 @@ from typing import TYPE_CHECKING
 
 from csaxs_bec.devices.jungfraujoch.eiger import Eiger
 
-EIGER1_5M_READOUT_TIME_US = 500e-6  # 500 microseconds in s
+# Gap this model needs between the end of an acquisition and the next trigger. Kept
+# per model on purpose -- readout is a detector property, not a beamline constant. The
+# delay generator's gap (DDG2 DEFAULT_READOUT_TIMES["ab"]) must be at least as large as
+# the largest value among the detectors in a scan, or the slowest one overruns.
+EIGER1_5M_READOUT_TIME = 2e-4  # s -- placeholder until a measured value exists
 DETECTOR_NAME = "EIGER 1.5M"
 
 
@@ -40,12 +44,13 @@ class Eiger1_5M(Eiger):
         beam_center: tuple[float, float] = (0.0, 0.0),
         scan_info: ScanInfo = None,
         device_manager: DeviceManagerDS = None,
+        readout_time: float = EIGER1_5M_READOUT_TIME,
         **kwargs,
     ) -> None:
         super().__init__(
             name=name,
             detector_name=DETECTOR_NAME,
-            readout_time=EIGER1_5M_READOUT_TIME_US,
+            readout_time=readout_time,
             detector_distance=detector_distance,
             beam_center=beam_center,
             scan_info=scan_info,

csaxs_bec/devices/jungfraujoch/eiger_9m.py

@@ -20,7 +20,11 @@ if TYPE_CHECKING:  # pragma no cover
     from bec_lib.devicemanager import ScanInfo
     from bec_server.device_server.device_server import DeviceManagerDS
 
-EIGER9M_READOUT_TIME_US = 500e-6  # 500 microseconds in s
+# Gap this model needs between the end of an acquisition and the next trigger. Kept
+# per model on purpose -- readout is a detector property, not a beamline constant. The
+# delay generator's gap (DDG2 DEFAULT_READOUT_TIMES["ab"]) must be at least as large as
+# the largest value among the detectors in a scan, or the slowest one overruns.
+EIGER9M_READOUT_TIME = 2e-4  # s -- placeholder until a measured value exists
 DETECTOR_NAME = "EIGER 9M"  # "EIGER 9M""
 
 
@@ -44,12 +48,13 @@ class Eiger9M(Eiger):
         beam_center: tuple[float, float] = (0.0, 0.0),
         scan_info: ScanInfo = None,
         device_manager: DeviceManagerDS = None,
+        readout_time: float = EIGER9M_READOUT_TIME,
         **kwargs,
     ) -> None:
         super().__init__(
             name=name,
             detector_name=DETECTOR_NAME,
-            readout_time=EIGER9M_READOUT_TIME_US,
+            readout_time=readout_time,
             detector_distance=detector_distance,
             beam_center=beam_center,
             scan_info=scan_info,

tests/tests_devices/test_delay_generator_csaxs.py

@@ -613,3 +613,95 @@ def test_ddg2_on_stop(mock_ddg2: DDG2):
 #     mock_ddg2.burst_mode.put(1)  # Enable burst mode
 #     mock_ddg2.stop()
 #     assert mock_ddg2.burst_mode.get() == 0  # Burst mode is disabled
+
+
+def test_ddg2_readout_times_default(mock_ddg2: DDG2):
+    """Without an override the module defaults are in force."""
+    assert mock_ddg2.get_readout_times() == DDG2_DEFAULT_READOUT_TIMES
+    # A copy, so a caller cannot mutate the device's state by accident.
+    mock_ddg2.get_readout_times()["ab"] = 1.0
+    assert mock_ddg2.get_readout_times()["ab"] == DDG2_DEFAULT_READOUT_TIMES["ab"]
+
+
+def test_ddg2_set_readout_times_validates(mock_ddg2: DDG2):
+    """Unknown pairs and non-positive values are rejected before they reach a scan."""
+    with pytest.raises(ValueError):
+        mock_ddg2.set_readout_times(xy=1e-3)
+    with pytest.raises(ValueError):
+        mock_ddg2.set_readout_times(ab=0)
+    with pytest.raises(ValueError):
+        mock_ddg2.set_readout_times(ab=-1e-3)
+    assert mock_ddg2.get_readout_times() == DDG2_DEFAULT_READOUT_TIMES
+
+
+def test_ddg2_set_readout_times_widens_the_gap(mock_ddg2: DDG2):
+    """A raised readout time shortens the pulse; the burst period is unchanged."""
+    ddg = mock_ddg2
+    exp_time = 0.1
+    frames_per_trigger = 10
+    ddg.on_connected()
+    ddg.scan_info.msg.info["exp_time"] = exp_time
+    ddg.scan_info.msg.info["frames_per_trigger"] = frames_per_trigger
+
+    ddg.set_readout_times(ab=3e-3)
+    assert ddg.get_readout_times()["ab"] == 3e-3
+    # Other pairs are untouched.
+    assert ddg.get_readout_times()["cd"] == DDG2_DEFAULT_READOUT_TIMES["cd"]
+
+    ddg.stage()
+    assert np.isclose(ddg.ab.width.get(), exp_time - 3e-3)
+    assert ddg.burst_period.get() == exp_time  # frame rate unaffected
+    ddg.unstage()
+
+
+def test_ddg2_readout_times_from_device_config():
+    """deviceConfig can set it per deployment, merged over the defaults."""
+    with patched_device(
+        DDG2, name="ddg2", prefix="test_ddg2:", readout_times={"ab": 3e-3}, _mock_pv_initial_value=0
+    ) as ddg:
+        try:
+            assert ddg.get_readout_times()["ab"] == 3e-3
+            assert ddg.get_readout_times()["gh"] == DDG2_DEFAULT_READOUT_TIMES["gh"]
+        finally:
+            ddg.destroy()
+
+
+def test_ddg2_too_long_readout_time_is_caught_at_stage(mock_ddg2: DDG2):
+    """A readout time exceeding the exposure must fail loudly, not emit a negative pulse."""
+    ddg = mock_ddg2
+    ddg.on_connected()
+    ddg.scan_info.msg.info["exp_time"] = 1e-3
+    ddg.scan_info.msg.info["frames_per_trigger"] = 1
+    ddg.set_readout_times(ab=3e-3)
+    with pytest.raises(ValueError):
+        ddg.stage()
+
+
+def test_ddg2_effective_readout_uses_the_scan_value_when_larger(mock_ddg2: DDG2):
+    """A scan-level readout_time widens the gap; the configured value is the floor."""
+    ddg = mock_ddg2
+    exp_time = 0.1
+    ddg.on_connected()
+    ddg.scan_info.msg.info["exp_time"] = exp_time
+    ddg.scan_info.msg.info["frames_per_trigger"] = 1
+    ddg.scan_info.msg.info["readout_time"] = 3e-3
+
+    ddg.stage()
+    assert ddg.effective_readout_times()["ab"] == 3e-3
+    assert np.isclose(ddg.ab.width.get(), exp_time - 3e-3)
+    ddg.unstage()
+
+
+def test_ddg2_effective_readout_falls_back_to_the_configured_value(mock_ddg2: DDG2):
+    """Scans default readout_time to 0, so the device value must still apply."""
+    ddg = mock_ddg2
+    exp_time = 0.1
+    ddg.on_connected()
+    ddg.scan_info.msg.info["exp_time"] = exp_time
+    ddg.scan_info.msg.info["frames_per_trigger"] = 1
+    ddg.scan_info.msg.info["readout_time"] = 0.0
+
+    ddg.stage()
+    assert ddg.effective_readout_times()["ab"] == DDG2_DEFAULT_READOUT_TIMES["ab"]
+    assert np.isclose(ddg.ab.width.get(), exp_time - DDG2_DEFAULT_READOUT_TIMES["ab"])
+    ddg.unstage()

tests/tests_devices/test_eiger.py

@@ -1,4 +1,5 @@
 # pylint: skip-file
+import inspect
 import os
 import threading
 import time
@@ -22,8 +23,9 @@ from ophyd import Staged
 from ophyd_devices.utils.psi_device_base_utils import DeviceStatus
 
 from csaxs_bec.devices.jungfraujoch.eiger import EigerError
-from csaxs_bec.devices.jungfraujoch.eiger_1_5m import Eiger1_5M
-from csaxs_bec.devices.jungfraujoch.eiger_9m import Eiger9M
+from csaxs_bec.devices.jungfraujoch.eiger import EIGER_READOUT_TIME, MIN_EXP_TIME
+from csaxs_bec.devices.jungfraujoch.eiger_1_5m import EIGER1_5M_READOUT_TIME, Eiger1_5M
+from csaxs_bec.devices.jungfraujoch.eiger_9m import EIGER9M_READOUT_TIME, Eiger9M
 
 if TYPE_CHECKING:  # pragma no cover
     from bec_lib.messages import FileMessage
@@ -416,7 +418,8 @@ def test_eiger_on_stage(eiger_1_5m):
         assert file_msg.hinted_h5_entries == {"data": "entry/data/data"}
 
         data_settings = DatasetSettings(
-            image_time_us=int(scan_msg.scan_parameters["exp_time"] * 1e6),
+            # exp_time is the trigger period; the detector acquires for the gated window
+            image_time_us=int((scan_msg.scan_parameters["exp_time"] - EIGER_READOUT_TIME) * 1e6),
             ntrigger=int(scan_msg.num_points * scan_msg.scan_parameters["frames_per_trigger"]),
             file_prefix=os.path.relpath(eiger._full_path, start="/sls/x12sa/data").removesuffix(
                 "_master.h5"
@@ -460,3 +463,111 @@ def test_eiger_preview_callback(eiger_1_5m):
                 mock_preview_put.assert_called_once_with(msg["data"]["default"])
             else:
                 mock_preview_put.assert_not_called()
+
+
+def test_eiger_acquisition_matches_the_gated_window(eiger_1_5m):
+    """The detector must acquire for exp_time - readout, not for the whole period.
+
+    exp_time is the trigger period. The delay generator gates for exp_time - readout, so
+    sending the full period leaves the detector still acquiring when the next trigger
+    arrives -- 20 us of margin, which is what JungfrauJoch subtracts internally.
+    """
+    eiger = eiger_1_5m
+    exp_time = eiger.scan_info.msg.scan_parameters["exp_time"]
+
+    with (
+        mock.patch.object(eiger.jfj_client, "wait_for_idle", return_value=True),
+        mock.patch.object(eiger.jfj_client, "start") as mock_start,
+        mock.patch.object(
+            eiger.jfj_client.api, "status_get", return_value=BrokerStatus(state="Idle")
+        ),
+    ):
+        eiger.stage()
+
+    sent = mock_start.call_args.kwargs["settings"].image_time_us
+    assert sent == int((exp_time - EIGER_READOUT_TIME) * 1e6)
+    assert sent < int(exp_time * 1e6)
+
+
+def test_eiger_scan_readout_time_wins_when_larger(eiger_1_5m):
+    """A scan asking for a wider gap must widen the acquisition window too."""
+    eiger = eiger_1_5m
+    exp_time = eiger.scan_info.msg.scan_parameters["exp_time"]
+    eiger.scan_info.msg.info["readout_time"] = 3e-3  # > EIGER_READOUT_TIME
+
+    with (
+        mock.patch.object(eiger.jfj_client, "wait_for_idle", return_value=True),
+        mock.patch.object(eiger.jfj_client, "start") as mock_start,
+        mock.patch.object(
+            eiger.jfj_client.api, "status_get", return_value=BrokerStatus(state="Idle")
+        ),
+    ):
+        eiger.stage()
+
+    assert mock_start.call_args.kwargs["settings"].image_time_us == int((exp_time - 3e-3) * 1e6)
+
+
+def test_eiger_rejects_a_readout_that_fills_the_period(eiger_1_5m):
+    """A gap wider than the period leaves nothing to acquire; fail rather than send <= 0."""
+    eiger = eiger_1_5m
+    exp_time = eiger.scan_info.msg.scan_parameters["exp_time"]
+    eiger.scan_info.msg.info["readout_time"] = exp_time + 1e-3
+
+    with (
+        mock.patch.object(eiger.jfj_client, "wait_for_idle", return_value=True),
+        mock.patch.object(eiger.jfj_client, "start"),
+        mock.patch.object(
+            eiger.jfj_client.api, "status_get", return_value=BrokerStatus(state="Idle")
+        ),
+    ):
+        with pytest.raises(ValueError):
+            eiger.stage()
+
+
+def test_eiger_minimum_exposure_check_is_unchanged(eiger_1_5m):
+    """Splitting readout from the validation floor must not relax the minimum exposure."""
+    eiger = eiger_1_5m
+    eiger.scan_info.msg.info["exp_time"] = MIN_EXP_TIME / 2
+
+    with (
+        mock.patch.object(eiger.jfj_client, "wait_for_idle", return_value=True),
+        mock.patch.object(eiger.jfj_client, "start"),
+        mock.patch.object(
+            eiger.jfj_client.api, "status_get", return_value=BrokerStatus(state="Idle")
+        ),
+    ):
+        with pytest.raises(ValueError):
+            eiger.stage()
+
+
+def test_eiger_models_may_declare_their_own_readout_time(eiger_1_5m, eiger_9m):
+    """Readout is a detector property, so each model states its own value.
+
+    They coincide today, but nothing should force them to: a 9M has more modules
+    than a 1.5M, and the Falcon needs 3 ms.
+    """
+    assert eiger_1_5m._readout_time == EIGER1_5M_READOUT_TIME
+    assert eiger_9m._readout_time == EIGER9M_READOUT_TIME
+
+
+def test_eiger_readout_time_is_overridable_per_deployment(mock_scan_info):
+    """deviceConfig's readout_time wins over the model's default."""
+    dev = Eiger1_5M(
+        name="eiger_1_5m", beam_center=(256, 256), detector_distance=100.0, readout_time=1.5e-3
+    )
+    try:
+        assert dev._readout_time == 1.5e-3
+    finally:
+        dev.destroy()
+
+
+def test_eiger_subclasses_name_readout_time_for_the_device_server():
+    """deviceConfig keys reach a device only if the class signature names them.
+
+    bec_server intersects the config keys with the named parameters of the class
+    (device_server/devices/devicemanager.py), so a subclass that only forwards
+    **kwargs silently drops readout_time -- and passing it explicitly used to raise
+    'got multiple values for keyword argument'.
+    """
+    for cls in (Eiger1_5M, Eiger9M):
+        assert "readout_time" in inspect.signature(cls).parameters