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Problem: RE stalls when writing FileName PV in area detector

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Pete R Jemian edited this page Dec 2, 2021 · 2 revisions

Problem: RE stalls when writing FileName PV in area detector

This call to RE(bps.mv(file_name_signal, "text")) stalls with this message reported to the console:

In [6]: RE(bps.mv(ad_filename, filename))
/home/prjemian/.conda/envs/bluesky_2022_1/lib/python3.9/site-packages/ophyd/utils/epics_pvs.py:289: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  ret = (a == b)

The user must type ^C^C to stop the RE and get a command prompt. Then, usually RE.abort() to place the RunEngine into the idle state.

Every command after this event will see the same message reported to the console.

CONTENTS

Remedy

Use the string=True kwarg when creating an EpicsSignalBase (subclass) that connects with a waveform string PV.

Details

EPICS

EPICS area detector uses many waveform string PVs to allow text with len() > 40 characters. One example is the HDF File Writing Plugin FileName PV which is a waveform of 256 characters. Here is a local instance with an empty string value:

(base) prjemian@zap:~$ caget ad:HDF1:FileName
ad:HDF1:FileName 256 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

The caget command has a -S option to render this PV value as a string:

(base) prjemian@zap:~$ caget -S ad:HDF1:FileName
ad:HDF1:FileName

ophyd

In ophyd, we see the same:

In [1]: from ophyd import EpicsSignalWithRBV

In [2]: ad_filename = EpicsSignalWithRBV("ad:HDF1:FileName", name="ad_filename", string=False)

In [3]: ad_filename.get()
Out[3]: array([0], dtype=uint8)

In [4]: ad_filename.get(as_string=True)
Out[4]: ''

or, simpler

In [5]: ad_filename = EpicsSignalWithRBV("ad:HDF1:FileName", name="ad_filename", string=True)

In [6]: ad_filename.get()
Out[6]: ''

String comparison works without comment:

In [7]: ad_filename.get()
Out[7]: ''

In [8]: ad_filename.get() == ""
Out[8]: True

The .put() also completes without comment:

In [9]: filename = "/home/8ididata/2021-3/ZDT{:02d}_{:06d}.bin".format(1,2)

In [10]: filename
Out[10]: '/home/8ididata/2021-3/ZDT01_000002.bin'

In [11]: ad_filename.put(filename)

and the camonitor-S trace shows this worked properly:

(base) prjemian@zap:~$ camonitor -S ad:HDF1:FileName{,_RBV}
ad:HDF1:FileName               2021-12-02 11:06:04.239834 0  
ad:HDF1:FileName_RBV           2021-12-02 11:06:04.239829 0  
ad:HDF1:FileName_RBV 2021-12-02 11:13:01.404614 /home/8ididata/2021-3/ZDT01_000002.bin  
ad:HDF1:FileName 2021-12-02 11:13:01.404634 /home/8ididata/2021-3/ZDT01_000002.bin

So, no problem from the ophyd layer, yet the error report comes from this layer:

/home/prjemian/.conda/envs/bluesky_2022_1/lib/python3.9/site-packages/ophyd/utils/epics_pvs.py:289: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  ret = (a == b)

bluesky

Pivot now to that same .put() operation using the bluesky.RunEngine, using bps.mv():

In [1]: from ophyd import EpicsSignalWithRBV

In [2]: ad_filename = EpicsSignalWithRBV("ad:HDF1:FileName", name="ad_filename", string=True)

In [3]: filename = "/home/8ididata/2021-3/ZDT{:02d}_{:06d}.bin".format(1,2)

In [4]: from bluesky import RunEngine, plan_stubs as bps

In [5]: RE = RunEngine({})

In [6]: RE(bps.mv(ad_filename, filename))
Out[6]: ()

No problem observed. When the string=True kwarg is not given, that produces the problem.

In [1]: from ophyd import EpicsSignalWithRBV

In [2]: ad_filename = EpicsSignalWithRBV("ad:HDF1:FileName", name="ad_filename")

In [3]: filename = "/home/8ididata/2021-3/ZDT{:02d}_{:06d}.bin".format(1,2)

In [4]: from bluesky import RunEngine, plan_stubs as bps

In [5]: RE = RunEngine({})

In [6]: RE(bps.mv(ad_filename, filename))
/home/prjemian/.conda/envs/bluesky_2022_1/lib/python3.9/site-packages/ophyd/utils/epics_pvs.py:289: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  ret = (a == b)

At this point, RE has not returned and is stuck with a failed comparison.

Software Versions

This article was developed using these package versions:

package version
bluesky 1.8.1
epics 3.5.0
ophyd 1.6.3
python 3.9.7
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