EPICS for Dummies
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What is EPICS?
EPICS (Experimental Physics and Industrial Control System) is a distributed control system, which means that the IOC (software) can run on a different computer as the user interfaces.
What is an IOC?
IOC (Input/Output Controller) is a piece of software running on a computer:
- serves something called process variables (PVs) that you can read and write to
- used to run on VMEs, but nowadays it runs on anything.
- usually talks to hardware, but that isn’t a requirement.
What is a PV?
PV (Process Variable) is a the field from a record:
PV = record_name + . + field_name
Most of the interactive parts of user displays have PVs associated with them.
What is a record?
Records are the building blocks of EPICS:
- There is a lot of different types of record, and each type is designed to perform a specific function
- Records are customizable via fields that allow users to configure how they behave and monitor their state.
- Records vary significantly in complexity (see #More about records)
- It is rare for new records to be created, now that asyn device support exists. add link to asyn?
Where do records come from?
Typically, records at the APS comes from either:
- EPICS base: main core of EPICS
- synApps: a collection of EPICS modules that are commonly used at the APS
- module: similar to a python package; it contains EPICS support and looks like an IOCs but it does not have a binary (i.e. executable)
| Module | Records | Module | Records |
|---|---|---|---|
| alive | alive | motor | motor |
| asyn | asyn | optics | table |
| busy | busy | scaler | scaler |
| calc | acalcout, scalcout, sseq, swait, transform | sscan | sscan |
| camac | camac | std | epid, timestamp, throttle |
| lua | luascript | vac | digitel, vs |
| mca | mca | vme | vme |
| allenBradley | ab1771IFE, ab1771N, ab1771IX, ... | allenBradley | ..., ab1791, abDcm |
If a record isn't listed here, it probably comes from EPICS base.
The advantage to using synApps over a collection of modules one assembles on their own is that some amount of BCDA testing has been done.
What is a record field?
- A controllable or informative property of a record
- Noteworthy fields:
- VAL - the default field (used if no field is specified in the vast majority of cases)
- SCAN - determines when a record processes
- NAME - the record name (helpful if records are aliased)
- DESC - record description
- RTYP - the record type
- DTYP - the device support type (not meaningful for all record types)
- DISP - disable puts (ca = channel access & pva = pv access) from outside the IOC (when non-zero); e.g. disable a motor from the motorx_all screen
- DISV/DISA/SDIS - disable record processing internally
- Record processing is disabled when DISA equals DISV (e.g. uses SDIS field to read in a value/mode, which will disable some control if a certain mode is active)
- DISA is set to the value of the PV in the SDIS field, if it isn't empty
- TPRO - trace processing: when set to 1 prints out all the records that get processed after the record is processed; used only for troubleshooting
- Note: some of the fields of a record are actually record attributes (they return the same value for all instances of the record type; see https://epics.anl.gov/)
How to learn more about record fields?
To find documentation for a specific field you need to know/find answers to the following questions:
- Is the field common to all or input/output record types?
- If not, does the record come from EPICS base or a different EPICS module (synApps)?
- The EPICS base record reference documentation is the best place to find information about record types that come from EPICS base; Field documentation appears in multiple locations:
- Fields common to all record types
- Fields common to input record types
- Fields common to output record types
- Record-specific documentation
- synApps record reference documentation
- In each module's documentation directory
- https://github.com/epics-modules
- /APSshare/epics/synApps_X_Y/support
- In each module's documentation directory
- The EPICS base record reference documentation is the best place to find information about record types that come from EPICS base; Field documentation appears in multiple locations:
What is a device?
More about records
- Records vary significantly in complexity; one way to classify them would be (*):
- Simple records are associated with individual data types
- bi, bo, mbbi, mbbo, longin, longout, ai, ao, stringin, stringout, waveform
- Intermediate records often perform calculations or link multiple records
- calcout, scalcout, acalcout, swait, transform, fanout, dfanout
- Complex records are associated with devices or higher-level functionality
- motor, scaler, mca, sscan, table, digitel, vs
- Flexible records allow custom functionality to be implemented
- sub, aSub, luascript
(*) this is not an official classification, nor an exhaustive list of records.
Note that:
- areaDetector databases are composed of mostly simple record.