Add device support for SRS570 from synApps

[prjemian]

• FIX SRS570 amplifier #541
• FIX generalize preamplifier Devices #544

[prjemian]

Compare with other preamplifiers and generalize: Keithley 428 and Femto (USAXS has both).

Might be suitable as ophyd.PVPositioner if we can control the values for the setpoint to match the values the readback can provide. This is controller-dependent.

[gfabbris]

@prjemian: I double checked all the "value" PVs in the SRS570, and you either get the state number or a string with the value.

[prjemian]

I'm seeing the same, thinking that is a legacy misdirection. Numerical values should be numbers, not strings. Can they be forced into numbers? Not directly, as shown:

(bluesky_2021_2) prjemian@zap:~/.../BCDA-APS/apstools$ ipython
Python 3.8.10 (default, May 19 2021, 18:05:58) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import ophyd

In [2]: !caput gp:gp:text1 "1.0"
Old : gp:gp:text1                    
New : gp:gp:text1                    1.0

In [3]: sens = ophyd.EpicsSignal("gp:gp:text1", name="sens", string=False)

In [4]: type(sens.get())
Out[4]: str

[prjemian]

Can we subclass to force these to numbers? thinking: EpicsSignalNumber()

[prjemian]

On closer examination, sens_num is defined as a mbbo EPICS record type. There is a confusing overlap between the enumeration value and the sensitivity number which can take these values: 1 2 5 10 20 50 100 200 500

The mapping between mbbo index of 5 (sensitivity value of 50) and a sensitivity value of 5 (index of 2) is an ambiguity that cannot be resolved unless string values for the sensitivity are defined.

[prjemian]

Conclusion

To make this a subclass of ophyd.PVPositioner, would need to have gain (for setpoint and readback) be a number, not a string. Is it really worth it?

[prjemian]

Since other synApps hardware device support is located under _devices, relocate this new support with the others.

[prjemian]

Also, let pint handle the units conversions. Example:

In [57]: for units in amp.sensitivity_unit.enum_strs:
    ...:     print(units, pint.Quantity(200, units).to("A/V").magnitude)
    ...:
pA/V 2e-10
nA/V 2.0000000000000002e-07
uA/V 0.00019999999999999998

Note: this will make pint a new requirement for apstools

[prjemian]

Example:

In [59]: amp = SRS570_PreAmplifier("8idi:A1", name="amp")

In [60]: amp.gain
Out[60]: 0.00019999999999999998

[prjemian]

If the gain was an ophyd.Signal (or subclass), then it could be recorded in a run's baseline.

[prjemian]

Setting a precedent for current amplifiers (pre-amps) here that gain is a Component (either RO or R/W) with name shortcut to the parent Device. Per #544

[prjemian]

@gfabbris @mdwyman This is ready for review. Merge this week unless there are objections?