Hardware Triggering

For simple devices, it is enough to let bluesky and ophyd handle triggering the detector. In our case, though, many detectors are to be triggered simultaneously using one piece of hardware.

An example is using the scaler to trigger multiple pieces of hardware. The SIS3820 multi-channel scaler can measure multiple channels of input with one trigger. If each detector is an ophyd Device object, then running a bluesky plan with multiple of these devices on the same scaler will result in the scaler being triggered multiple times (once for each device in the plan).

Additionally, the scaler presents the counting gate on one of its control output lines. This can be fed into the Xspress3 electronics that power many of our Vortex detectors. Bluesky by default will try to trigger the Xspress3 directly. The Device definition for the Vortex detector could trigger the scaler itself, but this creates yet another trigger signal to the scaler, as described above.

The solution is to use the ScalerTriggered mixin class. This adds a scaler_prefix argument to __init__ that expects a channel access PV path and points to the scaler that should be used to trigger this device. If multiple instances of ScalerTriggered with the same scaler_prefix are present in a bluesky plan, then the scaler is only triggered once for all the devices.

scaler_prefix can be omitted, in which case the prefix argument will be used for the scaler prefix.

from haven.instrument.scaler_triggered import ScalerTriggered
from ophyd import Device

class VortexDetector(ScalerTriggered, Device):
    ...

vortex = VortexDetector(prefix="vortex1ioc:vortex", scaler_prefix="25idcVME:scaler1")