NDDerivedSignal (DerivedSignal) can be extremely noisy when disconnected

[klauer]

ERROR    Subscription value callback exception (EpicsSignal(read_pv='IM3L0:PPM:CAM:IMAGE2:ArrayData', name='im3l0_detector_array_data', p
arent='im3l0_detector', value=array([0, 0, 1, ..., 0, 0, 0], dtype=int16), timestamp=1600987788.245194, auto_monitor=False, string=False,
 write_pv='IM3L0:PPM:CAM:IMAGE2:ArrayData', limits=False, put_complete=False))
Traceback (most recent call last):
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1043, in _get_with_timeout
    self.wait_for_connection(timeout=connection_timeout)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 997, in wait_for_connection
    self._ensure_connected(self._read_pv, timeout=timeout)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 976, in _ensure_connected
    raise TimeoutError(f"{pv.pvname} could not connect within "
TimeoutError: IM3L0:PPM:CAM:IMAGE2:NDimensions_RBV could not connect within 1.0-second timeout.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/ophydobj.py", line 462, in inner
    cb(*args, **kwargs)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 586, in _derived_value_callback
    value = self.inverse(value)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/areadetector/base.py", line 105, in inverse
    array_shape = self.derived_shape[:self.derived_ndims]
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/areadetector/base.py", line 96, in derived_ndims
    ndims = ndims.get()
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1096, in get
    self._read_pv, timeout, connection_timeout, as_string, form)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1047, in _get_with_timeout
    f"within {connection_timeout:.2} sec") from err
ophyd.signal.ConnectionTimeoutError: Failed to connect to IM3L0:PPM:CAM:IMAGE2:NDimensions_RBV within 1.0 sec
ERROR    Subscription value callback exception (EpicsSignal(read_pv='IM3L0:PPM:CAM:IMAGE2:ArrayData', name='im3l0_detector_array_data', p
arent='im3l0_detector', value=array([0, 0, 2, ..., 0, 0, 0], dtype=int16), timestamp=1600987788.396543, auto_monitor=False, string=False,
 write_pv='IM3L0:PPM:CAM:IMAGE2:ArrayData', limits=False, put_complete=False))
Traceback (most recent call last):
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1043, in _get_with_timeout
    self.wait_for_connection(timeout=connection_timeout)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 997, in wait_for_connection
    self._ensure_connected(self._read_pv, timeout=timeout)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 976, in _ensure_connected
    raise TimeoutError(f"{pv.pvname} could not connect within "
TimeoutError: IM3L0:PPM:CAM:IMAGE2:NDimensions_RBV could not connect within 1.0-second timeout.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/ophydobj.py", line 462, in inner
    cb(*args, **kwargs)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 586, in _derived_value_callback
    value = self.inverse(value)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/areadetector/base.py", line 105, in inverse
    array_shape = self.derived_shape[:self.derived_ndims]
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/areadetector/base.py", line 96, in derived_ndims
    ndims = ndims.get()
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1096, in get
    self._read_pv, timeout, connection_timeout, as_string, form)
  File "/u1/conda/pcds-3.3.4/lib/python3.6/site-packages/ophyd/signal.py", line 1047, in _get_with_timeout
    f"within {connection_timeout:.2} sec") from err
ophyd.signal.ConnectionTimeoutError: Failed to connect to IM3L0:PPM:CAM:IMAGE2:NDimensions_RBV within 1.0 sec