Signals

In EPICS, Signal maybe backed by a read-only PV, a single read-write PV, or a pair of read and write PVs, grouped together. In any of those cases, a single value is exposed to bluesky. For more complex hardware, for example a motor record, the relationships between the individual process variables needs to be encoded in a Device (a EpicsMotor class ships with ophyd for this case). This includes both what Signals are grouped together, but also how to manipulate them a coordinated fashion to achieve the high-level action (moving a motor, changing a temperature, opening a valve, or taking data). More complex devices, like a diffractometer or a Area Detector, can be assembled out of simpler component devices.

A Signal is much like a Device – they share almost the same interface – but a Signal has no sub-components. In ophyd’s hierarchical, tree-like representation of a complex piece of hardware, the signals are the leaves. Each one represents a single PV or a read–write pair of PVs.

kind

The kind attribute is the means to identify a signal that is relevant for handling by a callback. kind controls whether the signal’s parent Device will include it in read(), read_configuration(), and/or hints.fields. The first use of kind is to inform visualization callbacks about the independent and dependent display axes for plotting. A Component marked as hinted will return a dictionary with that component’s fields list.

The kind attribute takes string values of: config, hinted, normal, and omitted. These values are like bit flags, a signal could have multiple values.

The value may be set either when the Signal is created or programmatically. Use the kind attribute when creating a Signal or Component, such as:

from ophyd import Kind

camera.stats1.total.kind = Kind.hinted
camera.stats2.total.kind = Kind.hinted

or, as a convenient shortcut (eliminates the import)

camera.stats1.total.kind = 'hinted'
camera.stats2.total.kind = 'hinted'

With ophyd v1.2.0 or higher, use kind instead of setting the hints attribute of the Device.

labels

Signal and Device now accept a labels attribute. The value is a list of text strings — presumed but not (yet) forced to be strings — which the user can use for grouping and displaying available hardware or other ophyd constructs. The labels are accessible via an attribute _ophyd_labels_, so named to facilitate duck-typing across libraries. For example, the bluesky IPython “magics” use this to identify objects for the purpose of displaying them in labeled groups.

The IPython magic command wa (available if bluesky is installed as well as ophyd) groups items by labels. Here is an example:

m1 = EpicsMotor('prj:m1', name='m1', labels=("general",))
m2 = EpicsMotor('prj:m2', name='m2', labels=("general",))

class MyRig(Device):
        t = Component(EpicsMotor, "m5", labels=("rig",),)
        l = Component(EpicsMotor, "m6", labels=("rig",))
        b = Component(EpicsMotor, "m7", labels=("rig",))
        r = Component(EpicsMotor, "m8", labels=("rig",))


rig = MyRig("prj:", name="rig")

Then in an ipython session:

In [1]: wa
general
  Positioner                     Value       Low Limit   High Limit  Offset
  m1                             1.0         -100.0      100.0       0.0
  m2                             0.0         -100.0      100.0       0.0

  Local variable name                    Ophyd name (to be recorded as metadata)
  m1                                     m1
  m2                                     m2

rig
  Positioner                     Value       Low Limit   High Limit  Offset
  rig_b                          0.0         -100.0      100.0       0.0
  rig_l                          0.0         -100.0      100.0       0.0
  rig_r                          0.0         -100.0      100.0       0.0
  rig_t                          0.0         -100.0      100.0       0.0

  Local variable name                    Ophyd name (to be recorded as metadata)
  rig.b                                  rig_b
  rig.l                                  rig_l
  rig.r                                  rig_r
  rig.t                                  rig_t